Nuclear Magnetic Resonance : Volume 30 [1 ed.] 9781847553867, 9780854043323

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Nuclear Magnetic Resonance Volume 30

SPECIALIST PERIODICAL REPORTS Systematicand detailed review coverage in major areas of chemical research. A unique service for the active research chemist with annual or biennial,in-depth accounts of progress in particular fields of chemistry, in print and online.

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A Specialist Periodical Report

Nuclear Magnetic Resonance Volume 30 A Review of the Literature Published between June 1999 and May 2000 Senior Reporter G.A. Webb, Department of Chemistry, University of Surrey, Guildford, UK Reporters A.E. Aliev, University College, London, UK S. Ando, Tokyo Institute of Technology, Tokyo, Japan T. Baba, Kitami lnstitute of Technology, Kitami, Japan 1. Barsukov, University of Leicester, UK A.C. de Dios, Georgetown University, Washington, DC, USA H. Fukui, Kitami Institute of Technology, Kitami, Japan E.F. Hounsell, Birkbeck College, London, UK C.J. Jameson, University of Illinois at Chicago, USA K. Kamienska-Trela, Polish Academy of Sciences, Warszawa, Poland A. Khan, University of Lund, Sweden S. Kuroki, Tokyo Institute of Technology, Tokyo, Japan H. Kurosu, Nara Women’s University, Nara City, Japan R.V. Law, Imperial College of Science, Technology and Medicine, London, UK R. Ludwig, Universitat Dortmund, Germany S.J. Matthews, Imperial College of Science, Technology and Medicine, London, UK M.J.W. Prior, University of Nottingham, UK W. Schilf, Polish Academy of Sciences, Warszawa, Poland R.R. Sharp, University of Michigan, USA T. Watanabe, Tokyo University of Fisheries, Tokyo, Japan J. Wojcik, Polish Academy of Sciences, Warszawa, Poland T. Yamanobe, University of Gunma, Japan

RSeC ROYAL SOCIETY OF CHEMISTRY

ISBN 0-85404-332-2 ISSN 0305-9804

Copyright

The Royal Society of Chemistry 2001

All Rights Reserved Apart from m yfair dealing for the purposes of reseurch or private study, or criticism or review as permitted under the terms of the UK Copyright, Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royul Society of Chemistry, or in the case of reprogruphic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the VK, or in accordance with the terms of the licences issued by the appropriute Reproduction Rights Orgunizution outside the UK Enquiries concerning reproduction outside the terms stuted here should be sent to The Royul Society of Chemistry at the uddress printed on this puge. Published by The Royal Society of Chemistry Thomas Graham House, Science Park, Milton Road, Cambridge CB4 OWF, UK Registered Charity Number 207890 For further information see our web site at www.rsc.org Typeset by Cornputape (Pickering) Ltd, Pickering, North Yorkshire, U K Printed by Athenaeum Press Ltd, Gateshead, Tyne and Wear, UK

Volume 30 of the Specialist Periodical Reports on NMR consists of topical accounts which evince the power of NMR in the many areas which will be familiar to regular readers of this series. It is a great pleasure for me to welcome five new members to the reporting team. They are Drs. S. Ando and S. Kuroki who cover Applications of Nuclear Shielding, Drs. R. V. Law and A. E. Aliev who report on Solid State NMR and Dr. S. Matthews who deals with NMR of Proteins and Nucleic Acids. I wish to express my thanks to them, and to all members of the reporting team, for their determined efforts to provide comprehensive and readable accounts of their areas of expertise. I am also grateful to the production staff at the Royal Society of Chemistry for their assistance in the production of this volume. University of Surrey Guildford October 2000

G.A. Webb

Contents

Chapter 1

Chapter 2

Chapter 3

NMR Books and Reviews By W. Schilf 1 Books 2 Regular Reviews Series 3 Edited Books and Symposia 4 Reviews in Periodicals 5 Reviews and Books in Foreign Languages

1

1 1 7 16 38

Theoretical and Physical Aspects of Nuclear Shielding By C.J. Jameson and A. C.de Dios 1 Theoretical Aspects of Nuclear Shielding 1.1 General Theory 1.2 A b initio Calculations 1.3 Semiempirical Calculations 2 Physical Aspects of Nuclear Shielding 2.1 Anisotropy of the Shielding Tensor 2.2 Shielding Surfaces and Rovibrational Averaging 2.3 Isotope Shifts 2.4 Intermolecular Effects on Nuclear Shielding 2.5 Absolute Shielding Scales 3 References

46

Applications of Nuclear Shielding By S. Ando and S. Kuroki 1 Introduction 2 Various Chemical and Physical Influences on Nuclear Shielding 2.1 Substituent Effects 2.2 Conformation Effects 2.3 Intermolecular and Hydrogen Bonding Effects 2.4 Solvent Effects 2.5 Isotope Effects 3 Shielding of Particular Nuclear Species 3.1 Group 1 ('H, 2H, 3H, 6*7Li,23Na,87Rb, '33Cs) 3.1.1 Hydrogen (IH) (I= ' 1 2 ) 3.1.2 Deuterium (2H) (I = 1) 3.1.3 Lithium (677Li)(I = 1, 3/2) 3.1.4 Sodium (23Na)(I = 3/2)

68

Nuclear Magnetic Resonance, Volume 30 0The Royal Society of Chemistry, 2001

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46 46 52 54 54 54

57 59 60 62 63

68 68 68 69 70 73 73 74 74 74 75 75 75

...

Contents

Vlll

3.2

3.3

3.4 3.5

3.6

3.7

3.8

3.9

3.10 3.1 1

3.12

3.13

3.14

3.15

3.1.5 Rubidium (87Rb)(I= 3/2) 3.1.6 Cesium (133Cs)(I = 7/2) Group 2 (9Be, 25Mg, 137Ba) 3.2.1 Beryllium (9Be) (I= 3/2) 3.2.2 Magnesium (25Mg)(I= ' 1 2 ) 3.3.3 Barium ('37Ba) (I= 3/2) Group 3 and Lanthanoids (45Sc,"Y, 139La,1 5 3 E ~ ) 3.3.1 Scandium (45 Sc) (I= 7/2) 3.3.2 Yttrium (89Y) (I = 3.3.3 Lanthanum ('39La) ( I = 7/2) 3.3.4 Europium ( 1 5 3 E (I ~ )= 5/2) Group 4 (47-49Ti) 3.4.1 Titanium (47349Ti) (I = 5 / 2 , 7/2) Group 5 (51N,93Nb) 3.5.1 Vanadium (5'V) (I= 7/2) 3.5.2 Niobium (93Nb) (I = 9 / 2 ) Group 6 (95M0, 183W) 3.6.1 Molybdenum (95M0)(I = '/2) 3.6.2 Tungsten (183W)(I= Group 7 (55Mn,99Tc) 3.7.1 Manganese(55Mn)(I= 5/2) 3.7.2 Technetium (99Tc)(I = 3/2) Group 8 (57Fe,99Ru) 3.8.1 Iron (57Fe)(I= 3.8.2 Ruthenium (99Ru) ( I = 3/2) Group 9 (59C0,lo3Rh) 3.9.1 Cobalt (59C0)( I = 7/2) 3.9.2 Rhodium (Io3Rh)( I = 1/2) Group 10 (195Pf) 3.10. I Platinum ( 1 9 5 ~(It )= Group 11 (63Cu,107y109Ag) 3.1 I .I Copper ( 6 3 ~ u(I) = 3/2) 3.11.2 Silver (1073109Ag) (I = Group 12 (67Zn,111.113Cd, 199Hg) 3.12.1 Zinc ( 6 7 ~ n( I) = 5/2) 3.12.2 Cadmium (111-113Cd) (I = 3.12.3 Mercury (L99Hg)(I= l/2) Group 13 ( I 'B, 27Al,71Ga,203,205Tl) 3.13.1 Boron ("B)(I= 33/2) 3.13.2 Aluminium (27Al)(I = 5 / 2 ) 3.13.3 Gallium (71Ga)(I= 3/2) 3.13.4 Thallium (203T205Tl) (I = Group 4 (13C, 29Si,73Ge, 117*119Sn, 207Pb) 3.14.1 Carbon (13C) (I = '/2) 3.14.2 Silicon (29Si)(I = 3.14.3 Germanium (73Ge)(I= 9 /2 ) 3.14.4 Tin ("771'9Sn) (I = 3.14.5 Lead (207Pb)(I = Group 15 (14,15N, 31P) 3.15.1 Nitrogen (l4?''N)(I=1, '/2)

76 76 76 76 77 77 77 77 77 78 78 78 78 78 78 78 79 79 79 80 80 80 80 80 80 80 80 81 81 81 81 81 82 82 82 82 83 84 84 84 85 86 86 86 87 87 87 89 90 90

Contents

ix

4 Chapter 4

Chapter 5

Chapter 6

3.15.2 Phosphorus (3'P) ( I = 3.16 Group 16 (I7O, 33S,77Se, '25Te) 3.16.1 Oxygen ( I7O) ( I = 5 / 2 ) 3.16.2 Sulfur (33S)( I = 3/2) 3.16.3 Selenium (77Se)( I = '/2) 3.16.4 Tellurium ('25Te) ( I = '/*) 3.17 Group 17 (I9F, 35,37~1) 3.17.1 Fluorine (I9F) ( I = l/2) 3.17.2 Chlorine (35737CI)(I = 3/2, 3/2) 3.18 Group 18 (3He, '29Xe) 3.18.1 Helium (3He) ( I = 3.18.2 Xenon ( '29Xe)( I = References

Theoretical Aspects of Spin-Spin Couplings By H. Fukui and T. Baba 1 Introduction 2 Relativistic Calculation of Spin-Spin Couplings 3 Anisotropies in Spin-Spin Coupling Tensors 4 Basis Set and Correlation Dependences of Spin-Spin Couplings 5 Vicinal Proton-Proton Coupling Constants 6 Geometry Dependence of Spin-Spin Coupling Constants 7 Intermolecular Effects on Spin-Spin Couplings 8 Spin-Spin Coupling Constants in Hydrogen-Bonded Complexes 9 Through-Space Couplings 10 Conformational Analysis 11 References Applications of Spin-Spin Couplings By K. Kamienska-Trela and J. Wbjcik 1 Introduction 2 New Methods 3 One-Bond Couplings to Hydrogen 4 One-Bond Couplings not Involving Hydrogen 5 Two-Bond Couplings to Hydrogen 6 Two-Bond Couplings not Involving Hydrogen 7 Three-Bond Hydrogen-Hydrogen Couplings 8 Three-Bond Couplings between Hydrogen and Heteronuclei 9 Three-Bond Couplings not Involving Hydrogen 10 Couplings over More than Three Bonds 11 Couplings across the Hydrogen Bond and through Space 12 References Nuclear Spin Relaxation in Liquids and Gases By R. Ludwig 1 Introduction 2 General, Physical and Experimental Aspects of Nuclear Spin Relaxation

90 91 91 92 92 93 93 93 94 94 94 94 95 109

109 109 114

117 118 120 122 123 126 126 127 132

132 134 136 139 147 148 150 157 160 162 163 166 181

181 183

Contenls

X

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

3

4 5 6 Chapter 7

General Aspects Experimental Aspects Relaxation in Coupled Spin Systems Dipolar Couplings and Distance Information Exchange Spectroscopy Radiation Dumping Quadrupolar Interactions Intermolecular Dipolar Interaction in Diamagnetic and Paramagnetic Solution 2.9 Slow Motions in Glasses 2.10 Models for Molecular Dynamics Selected Applications of Nuclear Spin Relaxation 3.1 Pure Liquids 3.2 Non-Electrolyte Solutions 3.3 Electrolyte Solutions 3.4 Transition Metal Complexes 3.5 Molten Salts Nuckear Spin Relaxation in Gases Self-Diffusion in Liquids 5.1 Experimental and Theoretical Aspects 5.2 Selected Examples References

183 184 185

187 189 190 191 192 194 196 197 197 197 199 199 200 20 1 202 202 203 204

214 Solid State NMR Spectroscopy By A. E. Aliev and R. V. Law 214 1 Introduction 214 2 Reviews 214 3 Theory 224 4 Experimental Aspects 4.1 New Technique Developments 224 4.2 NMR Parameters: Experimental and Quantum Mechanical Studies 230 4.2.1 Spin-'/Z Nuclei: Isotropic Shifts and CS Tensors 230 4.2.2 Quadrupolar Nuclei: Isotropic Shifts, CS and EFG Tensors 234 4.2.3 Multinuclear Studies 238 4.2.4 Indirect and Residual Dipolar Interactions 240 4.3 Other Experimental Aspect 24 1 4.3.1 Single Crystal Studies 24 1 4.3.2 Relaxation, Spin Diusion and Polarization Studies 243 4.3.3 New (Less Studied) NMR Phenomena 244 4.3.4 Distance and Angle Measurements 246 4.3.5 Exotic and Troublesome Nuclei 249 4.3.6 Quantification Aspects 25 1 4.3.7 Novel Applications 252 5 Structural Applications 254 5.1 Organic Studies 259 5.2 Organometallics and Coordination Compounds 255 5.3 Natural Products 256

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Contents

6

7 8 9

Chapter 8

5.4 Biochemical, Medical and Pharmaceutical Applications 5.4.1 Proteins 5.4.2 Peptides 5.4.3 Lipids and Membranes 5.4.4 Biomedical Applications 5.4.5 Pharmaceutical Compounds 5.5 Coal, Soil Organic Matter and Other Related Materials 5.5.1 Coals and Related Materials 5.5.2 Soils 5.6 Polymers 5.7 Glasses and Amorphous Solids 5.8 Cements 5.9 Micro- and Meso-porous Solids 5.10 Surface Science and Catalysis 5.1 1 Inorganic and Other Related Materials Molecular Dynamics Studies 6.1 Organic Solids 6.2 Organometallicsand Coordination Compounds 6.3 Biological and Biomedical Applications 6.4 Polymers 6.5 Microporous Solids and Related Materials 6.6 Other Materials Phase Transitions In sicu Reactions 8.1 Polymerisation 8.2 Heterogeneous Catalysis References

Multiple Pulse NMR By I. Barsukov 1 Introduction 2 Shaped Radiofrequency Pulses and Solvent Suppression 2.1 Adiabatic Pulses 2.2 Solvent Suppression 3 NOE, Chemical Exchange and Relaxation 3.1 NOE and Chemical Exchange 3.2 Relaxation Time Measurements 3.3 Cross-Correlated Relaxation Experiments 4 Coupling Constants Measurements 4.1 Scalar Couplings 4.2 Couplings across Hydrogen Bonds 4.3 Residual Dipolar Couplings 5 Inverse Proton Detected Correlation Spectroscopy 5.1 General 5.2 Isotope Filtered Experiments 5.3 Isotope Edited Experiments 5.4 Heteronuclear Double Resonance Experiments 5.4.1 HSQCIHMQC 5.4.2 TROSY 5.5 Heteronuclear Triple Resonance Experiments

257 257 259 259 260 260 26 1 26 1 262 262 266 268 269 274 279 282 282 283 283 283 284 285 285 287 287 288 288 311

31 1 31 1 31 1 3 12 312 312 313 314 3 14 317 317 3 19 32 1 32 1 322 322 322 322 323 324

xii

Contents 6 References

330

Chapter 9

NMR of Proteins and Nucleic Acids 334 By S. J. Matthew 1 Introduction 334 2 New NMR Methodology 335 2.1 Dipolar Couplings 335 2.2 Transverse Relaxation Optimized Spectroscopy (TROSY) 337 340 2.3 General Methodology 34 1 3 Solution Structures 34 1 3.1 Free Proteins 343 3.2 Protein-Small Molecule Complexes 344 3.3 Protein-Metal Complexes 345 3.4 Protein-Protein Complexes 346 3.5 Protein-Nucleic Acid Complexes 349 4 Protein Relaxation and Dynamics 350 5 Protein Folding 354 6 References

Chapter 10

NMR of Carbohydrates, Lipids and Membranes By E. F. Hounsell 1 Introduction 2 Polysaccharides and Cyclodextrins 2.1 Polysaccharides 2.2 Cyclodextrins 3 Carbohydrate Based Natural Products 3.1 Newly Identified Lipids and Saponins 3.2 Lichens 4 Membrane Studies 4.1 Metal Complexes 4.2 Surfactants 5 Metabolism 6 Glyco biology 7 References

Chapter 11

Chapter 12

Synthetic Macromolecules By H. Kurosu and T. Yarnanobe 1 Introduction 2 Liquid Crystalline Polymers 3 Primary Structure 4 Characterization of Synthetic Macromolecules 5 Dynamics of Synthetic Macromolecules 6 Gels and Crosslinking Macromolecules 7 Polymer Blend and Diffusion of Synthetic Macromolecules 8 References NMR in Living Systems By M.J. W. Prior 1 General Applications and New Methodologies 1.1 Applications and Methodologies

360 360 360 360 36 1 362 364 364 365 366 366 367 368 37 1

377 377 378 378 378 392 394 395 397 412

412 412

...

Xlll

Contents

Spectral Editing Localisation Spectral Analysis Data Analysis Instrumentation 1.7 Intracellular Ions 1.8 Cellular Metabolites 1.9 pH Cells 2.1 Bacteria 2.2 Blood 2.3 Cultured and Tumour Cells 2.4 Reproductive 2.5 Yeast and Fungi Plants and Algae 3.1 Cells 3.2 Seeds 3.3 Tissues Animal Tissues 4.1 Brain 4.2 Heart 4.3 Kidney 4.4 Liver 4.5 Muscle 4.6 Reproductive 4.7 Tumour 4.8 Whole Animal Clinical Studies 5.1 Brain 5.2 Heart 5.3 Liver 5.4 Muscle 5.5 Turnour References

1.2 1.3 1.4 1.5 1.6

2

3

4

5

6

Chapter 13

Nuclear Magnetic Resonance Imaging By T. Watanabe 1 Introduction 2 Organic Aspects and Reviews 3 Instruments 4 Pulse Sequences and Data Processing 4.1 Pulse Sequences 4.2 improvement, Optimization of Parameters 4.3 Detection of Physical Parameters 4.4 Radical Detection 4.5 Others 4.6 Data Processing 5 Solid State NMR Imaging 5.1 Physical Phenomena (inorganic Compounds) 5.2 Teeth, Bone and Apatite

412 413 414 415 415 416 416 417 417 417 418 420 420 42 1 42 1 42 1 422 422 423 423 425 432 432 434 435 435 436 437 437 44 1 44 1 442 444 444 453 453 454 4 56 457 457 457 458 458 459 459 460 460 460

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Contents

6 Other Nuclei 6.1 Hyperpolarized Noble Gases 6.2 Other Nuclei 6.3 Other Nuclei; In vivo 7 Diffusion, Flow and Velocity Imaging 7.1 Diffusion 7.2 Flow, Velocity, Rheometry 7.3 Mass Transport, Absorption, Swelling, Drying and Sedimentation 8 Porosity, Fluid Assisted Imaging 9 Polymers 9.1 Foams 9.2 Elastomers 9.3 Gels 10 Plants, Trees and Roots 1 1 Food and Food Processing 1 1.1 Food; Storage and Maturity 1 1.2 Food Processing, Texture, and Glass Transition 12 In vivo and Ex vivo 12.1 In vivo Experiments 12.1.1 High Field or High Field Gradient 12.1.2 Growth, Progress or Cell Invasion 12.1.3 Metabolites and Homeostasis 12.1.4 Characterization, Assessment 12.2 Ex vivo Measurement 13 Miscellaneous 13.1 Water, Gas Hydrates 13.2 Geology 13.3 Human Images 14 References

Chapter 14

Paramagnetic N M R By R.R. Sharp 1 Introduction 2 Advances in Theory 2.1 ZFS Effects on Spin Relaxation 2.2 Chemical Shifts 2.3 Hyperpolarized Noble Gases 3 Lanthanide Ions 3.1 Chemical Shift Reagents 3.2 Use of Gadolinium Complexes as NMR Relaxation and Contrast Agents 3.3 Novel Lanthanide-containing Complexes 3.4 Solution Equilibria of Lanthanide Complexes 4 D-Block Ions 4.1 Polynuclear Metal Complexes 4.2 Mononuclear Complexes 4.3 Complexes of Pharmacological Interest 5 Porphyrins 5.1 Effect of Axial Ligation on Spin State

46 1 46 1 462 462 463 463 463 464 465 465 465 466 466 466 467 467 468 468 468 469 469 469 469 470 470 470 470 47 1 47 1 477

477 477 477 480 480 480 48 1 482 490 49 1 49 1 49 1 493 494 495 496

Contents

xv 6 Spin Labels and Spin Probes 6.1 Probes of Surface Exposure 6.2 Distance Constraints 6.3 Solid-state NMR 6.4 Novel Spin Labels 6.5 Correlated NMRIESR Spectroscopy 7 Reaction Kinetics 7.1 Chemical Exchange of Solvent Coordinated to Actinide Ions 7.2 Electron Self-exchange Reactions 7.3 Proton Transfer Reactions 7.4 Ligand Dynamics 8 Biological Systems 8.1 Copper Proteins 8.2 Fe/S Proteins 8.3 Heme Proteins 8.4 Mn(I1)-containing Proteins 8.5 Advances in Methodology Related to Paramagnetic NMR of Proteins 9 Chemically-Induced Dynamic Nuclear Polarization (CIDNP) 9.1 Dynamic Nuclear Polarization (DNP) 9.2 Overhauser Magnetic Resonance Imaging (OMRI) 10 Heterogeneous Media 10.1 Porous Media 11 References

Chapter 15

NMR of Liquid Crystals and Micellar Solutions By A. Khan 1 Introduction 2 Reviews 3 Models and Methods 4 Lyotropic Polymorphism 4.1 Phase Diagrams and Phase Structures 5 Isotropic Micellar Solution Phases 5.1 Normal Micelles 5.2 Mixed Micelles and Solubilization 5.3 Reversed Micelles and Microemulsions 6 Lyotropic Mesophases 6.1 Vesicles 6.2 Liquid Crystals 7 Surfactant-Protein and Surfactant-Polymer Systems 8 Water and Counterions 9 Thermotropic Mesomorphism 9.1 Relaxation Studies 9.2 Bandshapes-Order Parameters 10 Synthesis 1 1 References

499 499 500 503 503 504 504 504 505 506 507 507 508 509 509 510 51 1 5 14

516 517 517 517 518

527

527 528 529 530 53 I 534 534 535 537 538 539 54 1 543 546 547 547 548 550 551

Symbols and Abbreviations

These lists contain the symbols and abbreviations most frequently used in this volume, but they are not expected to be exhaustive. Some specialized notation is only defined in the relevant chapter. An attempt has been made to standardize usage throughout the volume as far as is feasible, but it must be borne in mind that the original research literature certainly is not standardized in this way, and some difficulties may arise from this fact. Trivial use of subscripts etc. is not always mentioned in the symbols listed below. Some of the other symbols used in the text, e.g. for physical constants such as h or T , or for the thermodynamic quantities such as H or S, are not included in the list since they are considered to follow completely accepted usage.

Symbols hyperline (electron-nucleus) interaction constant (i) hyperfine (electron-nucleus) interaction constant fii) parameter relating to electric field effects on nuclear shielding (i) magnetic induction field (magnetic flux density) (ii) parameter relating to electric field effects on nuclear shielding static magnetic field of N M R or ESR spectrometer r.f. magnetic fields associated with U I ,v2 spin-rotation coupling constant of nucleus X (used sometimes in tensor form):

c2='/3(Ci

+ 2c3.

components of C parallel and perpendicular to a molecular symmetry axis (i) self-diffusion coefficient (ii) zero-field splitting constant rotational diffusion tensor components of D parallel and perpendicular to a molecular symmetry axis internal diffusion coefficient overall isotropic diffusion coefficient electric field eigenvalue of k ( o r a contribution to 2) nuclear or electronic g-factor magnetic field gradient element of matrix representation of 2 Hamiltonian operator-subscripts indicate its nature nuclear spin operator for nucleus i components of I, (i) ionization potential (ii) moment of inertia nuclear spin-spin coupling constant through n bonds (in Hz). Further information may be given by subscripts or in brackets. Brackets are used for indicating the species of nuclei coupled, e.g. J(13C, 'H) or additionally, the coupling path, e.g. J(P0CF) reduced splitting observed in a double resonance experiment rotational quantum number reduced nuclear spin-spin coupling constant (see the notes concerning J ) eigenvalue of I,, (magnetic component quantum number) equilibrium macroscopic magnetization of a spin system in the presence of Bo components of macroscopic magnetization

xvi

Symbols and Abbreviations

xvii

the number of average mol. wt. valencep orbital of atom A fractional population (or rotamers etc.) element of bond-order, charge-density matrix electric field gradient (i) nuclear quadrupole moment (ii) quality factor for an r.f. coil valence s-orbital of atom A electron density in SA at nuclear A (i) singlet state (ii) electron (or, occasionally, nuclear spin) cf: I (iii) ordering parameter for oriented systems (iv) overlap integral between molecular orbitals elapsed time (i) temperature (ii) triplet state coalescence temperature for an N M R spectrum the glass transition temperature (of a polymer) spin-lattice relaxation time of the X nuclei (further subscripts refer to the relaxation mechanism) spin-spin relaxation time of the X nucleus (further subscripts refer to the relaxation mechanism) inhomogeneity contribution to dephasing time for M , or M total dephasing time for M.i or My; ( T ; ) - *= TT*+ ( T i ) -rv decay time following 9 0 0 - ~ - 9 0pulse ~ sequences spin-lattice and spin-spin relaxation time of the X nuclei in the frame of reference rotating with BI dipolar spin-lattice relaxation time mole fraction of compound atomic number of atom A (i) nuclear spin wavefunction (eigenfunction of 1:) for a spin - ‘ / 2 nucleus (ii) polarizability nuclear spin wavefunction (eigenfunction of 1-1 for a spin - ‘/2 nucleus magnetogyric ratio of nucleus X chemical shift of a nucleus of element X (positive when the sample resonates to high frequency of the reference). Usually in p.p.m. Kronecker delta ( = I if i=j,and = 0 otherwise) Dirac delta operator (i) time between field gradient pulses (ii) spectral width anisotropy in J (A J = J I I- J l , for axial symmetry) population difference between nuclear states change of difference in S full width (in Hz) of a resonance line at half-height (i) anisotropy in a(Aa = UJI- 01, for axial symmetry) (ii) differences in o for two different situations (i) susceptibility anisotropy (A, = ~ 1 -1 xl,for axial symmetry (ii) differences in electronegativities relative permittivity permittivity of a vacuum (i) nuclear Overhauser effect (ii) asymmetry factor (e.g. in e2qQ/h) (iii) refractive index (iv) viscosity magnetic dipole moment permeability of a vacuum Bohr magneton nuclear magneton Larmor precession frequency of nucleus i (in Hz) (i) spectrometer operating frequency (ii) Larmor precession frequency (general, or of bare nucleus)

xviii

3’

u1

up 7-

Symbols and Abbreviations frequency of ‘observing’r.f. magnetic field frequency of ‘irradiating’ r. f. magnetic field shielding parameter of nucleus i (used sometimes in tensor form). Usually in p.p.m, Subscripts may alternatively indicate contributions to u. components of u parallel and perpendicular to a molecular symmetry axis diagrammatic contribution to u paramagnetic contribution to u (i) pre-exchange lifetime of molecular species (ii) time between r.f. pulses (general symbol) correlation time mean time between molecular collisions in the liquid state angular momentum correlation time pulse duration translational magnetic relaxation correlation time (i) magnetic susceptibility (ii) electronegativity (iii) nuclear quadrupole coupling constant (= $qQ/h) carrier frequency in rad s-I as for Ujy ~ 0 u1, , u2 but in rdd s-’ modulation angular frequency (in rad S-I) sample rotation (rad s-’)

Abbreviations (a) Physicalproperties a.f. audiofrequency a.u. atomic unit a.m. amplitude modulation b.c.c. body-centred cubic c.m.c. critical micelle concentration e.d. electron diffraction e.f.g. electric field gradient f.c.c. face-centred cubic f.m. frequency modulation h.c.p. hexagonal close-packed h.f. hyperfine i.d. inside diameter i.f. intermediate frequency I.C. liquid crystalline mol. wt. molecular weight ad. outside diameter p.p.m. parts per million r.f. radiofrequency r.m.s. root mean square s.h.f. super-high frequency u.h.f. ult ra-high frequency analogue-to-digital converter ADC AEE average excitation energy approximation acquire AQ ARP adiabatic rapid passage BIRD bilinear rotation decoupling CCPPA coupled cluster polarization propagator approximation CH-COSY carbon-hydrogen correlation spectroscopy CHESS chemical shift selection CHF coupled Hartree-Fock molecular orbital calculations ClDEP chemicatly induced dynamic electron polarization

Symbols and Abbreviations CIDNP COSY CP CPMG CSA CSI

cw

DAC DD DEPT DLB DNP DQ DQF ECOSY EHT ENDOR EOM ESR

EXSY FC FID FLASH FET

FT GIAO HMQ HOHAHA HRPA IDESS IGLO INADEQUATE INDO INDO/S INDOR INEPT IR ISIS LIS LORG LSR MASS MBPT MEM MIND0 MQ MQC MQF NMR NOE NOESY NQCC NQR PFG PRE QF QPD REX ROESY RPA

chemically induced dynamic nuclear polarization correlation spectroscopy cross polarization Cam-Purcell pulse sequence. Meiboom-Gill modification chemical shielding anisotropy chemical shift imaging continuous wave digital-to-analogueconverter dipole-dipole(interaction or relaxation mechanism) distortionless enhancement by polarization transfer differential line broadening dynamic nuclear polarization double quantum double quantum filter exclusive correlation spectroscopy extended Hiickel molecular orbital theory electron-nucleus double resonance equations of motion electron spin resonance exchange spectroscopy Fermi contact free induction decay fast low angle shot finite perturbation theory Fourier transform gauge included atomic orbitals heteronuclear multiquantum homonuclear Hartman-Hahn higher random phased approximation improved depth selective single surface coil spectroscopy individual gauge for different localized orbitals incredible natural abundance double quantum transfer experiment intermediate neglect of'differential overlap intermediate neglect of differential overlap calculations for spectroscopy internuclear double resonance insensitivenuclei enhanced by polarization transfer infrared image selected in vivo spectroscopy lanthanide induced shift local origin lanthanide shift reagent magic angle sample spinning many body perturbation theory maximum entropy method modified INDO multiple quantum multiple quantum coherence multiple quantum filter nuclear magnetic resonance nuclear Overhauser enhancement nuclear Overhauser enhancement spectroscopy nuclear quadrupole coupling constant nuclear quadrupole resonance pulsed field gradient proton relaxation enhancement quadrupole moment/field gradient quadrature phase detection relativistically extended Hiickel molecular orbital theory rotating frame Overhauser enhancement spectroscopy random phase approximation

xix

xx

Symbols and Abbreviations

self consistent perturbation theory SCPT spin dipolar SD spin echo correlation spectroscopy SECSY spin echo Fourier transform SEFT SLITDRESS slice interleaved depth resolved surface coil spectroscopy second order polarization propagator approach SOPPA selective population inversion SPI selective population transfer SPT spin rotation (interaction or relaxation mechanism) SR tip angle reduced T I imaging TART total correlation spectroscopy TOCSY ultraviolet uv Waugh, Huber and Haberlen (cycle of pulses) WAHUHA zero quantum ZQ zero quantum coherence ZQC (b) Chemicnl species* acetylacetonato acac adrenocorticotropic hormone (corticotropin) ACTH adenosine diphosphate ADP adenosine monophosphate AMP adenosine triphosphate ATP bovine serum albumin BSA cytidine monophosphate CMP cyclopentadienyl CP dodecylammonium propionate DAP 1 ,Zdimethoxyethane DME dimethylformamide DMF dimyristoyl-lecithin DML dimethylsiloxane DMS dimethyl sulfoxide DMSO deoxyribonucleic acid DNA 2,3-diphosphoglycerate DPG dipalmitoyl-lecithin DPI dipivalo ylmet hana to dPm diphenylpicrylhydrazyl DPPH 2,2-dimethyl-2-silapentane-5-sulfonate (usually as the sodium salt) DSS di-t-butyl nitroxide DTBN N-(p-ethoxybenzy1idene)pbutylaniline EBBA ethylenediaminetetra-acetic acid EDTA ethylene-vinyl acetate EVA 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dionato fod 4,4’-bis(hepty1)azoxybenzene HAB hexamethylphosphoramide HMPA p-n-heptyloxyazoxy benzene HOAB inositolhexaphosphate IHP potassium dihydrogen phosphate KDP N-(p-methoxybenzy 1idene)-p-butylaniline MBBA nicotinamide adenine dinucleotide (phosphate) NADH(P) N-meth ylformamide NMF p-azoxyanisole PAA pyrene butyric acid PBA poly(L-benzyl p-glutamate) PBLG phosphatidyl choline (lecithin) PC polychlorinated biphenyl PCB polydimethylsiloxane PDMS poly(methacry1icacid) PMA poly(methy1 methacrylate) PMMA poly(oxymethy1ene) POM phospha tidylserine PS * Lower case initials are used when the species is a ligand

xxi

Symbols and Abbreviations PTFE PVC PVF PVP RNA SDS TAB TCNQ TFA THF TMS UTP

polytetrafluoroethylene poly(viny1 chloride) poly(viny1 fluoride) poly(viny1 pyrrolidone) ribonucleic acid (t RNA, transfer RNA) sodium dodecyl sulfate trimethylammonium bromide tetracyanoquinodimethane trifluoroacetic acid tet rah ydrofuran tetramethybilane uridine triphosphate

A mino-adic residues Ala alanine Arg arginine Asn asparagine Asp aspartic acid cysteine CYS gI u tami ne Gln Glu glutamic acid GlY glycine His histidine HY P hydroxyproline 1le isoleucine

Leu LYS Met Phe Pro Ser Thr TrP TY r Val

leucine lysine methionine phen ylalanine proline serine threonin tryptophan tyrosine valine

xxi

Symbols and Abbreviations PTFE PVC PVF PVP RNA SDS TAB TCNQ TFA THF TMS UTP

polytetrafluoroethylene poly(viny1 chloride) poly(viny1 fluoride) poly(viny1 pyrrolidone) ribonucleic acid (t RNA, transfer RNA) sodium dodecyl sulfate trimethylammonium bromide tetracyanoquinodimethane trifluoroacetic acid tet rah ydrofuran tetramethybilane uridine triphosphate

A mino-adic residues Ala alanine Arg arginine Asn asparagine Asp aspartic acid cysteine CYS gI u tami ne Gln Glu glutamic acid GlY glycine His histidine HY P hydroxyproline 1le isoleucine

Leu LYS Met Phe Pro Ser Thr TrP TY r Val

leucine lysine methionine phen ylalanine proline serine threonin tryptophan tyrosine valine

1 NMR Books and Reviews BY W. SCHILF

1

Books

No citation in this section 2

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R3 R4 R5 R6 R7 R8 R9

Regular Reviews Series Accounts of Chemical Research, vol. 32, 1999, J. Klinowski and T. L. Barr, ‘NMR and ESCA Chemical Shifts in Aluminosilicates:A Critical Discussion’, p. 633 J. P. Marino, H. Schwalbe and C. Griesinger, ‘J-Coupling Restraints in RNA Structure Determination’, p. 614 Annual Reports on N M R Spectroscopy, vol. 38, ed. G. A. Webb, Academic Press Ltd. London, 1999 J. C. Lindon, J. K. Nicholson and J. R. Everett, ‘NMR Spectroscopy of Biofluids’, p. 1 J. F. Hinton, ‘NMR Studies of Ion-Transporting Biological Channels’, p. a9 D. Gudat, ‘Application of Heteronuclear X, Y-Correlation Spectroscopy in Organometallic and Organoelement Chemistry’, p. 139 B. Wrackmeyer, ‘Application of ”Sn NMR Parameters’, p. 203 J. Malito, ‘Copper-63 NMR Spectroscopy’,p. 265 W. S. Price, ‘Water Signal Suppression in NMR Spectroscopy’, p. 289 T. Baba and Y. Ono, ‘Variable Temperature ‘H MAS NMR: A Powerful Tool for the Investigation of Dynamic Properties of Acidic Protons in Zeolites and Heteropoly Compounds’, p. 355

’

Annual Review of Biophysics and Biomolecular Structure, vol. 28, 1999 R10 R. Fu and T. A. Cross, ‘Solid-state Nuclear Magnetic Resonance Investigation of Protein and Polypeptide Structure’, p. 235 R11 C. A. Bush, M. Martin-Pastor and A. Imberty, ‘Structure and Conformation of Complex Carbohydrates of Glycoproteins, Glycolipids, and Bacterial Polysaccharides’, p. 269 Nuclear Magnetic Resonance, Volume 30

0The Royal Society of Chemistry, 2001 I

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Nuclear Magnetic Resonance

Annual Review on Physisal Chemistry, vol. 50, 1999 M. Greubele, ‘The Fast Protein Folding Problem’, p. 485 Biological Magnetic Resonance, vol. 15 ‘In Vivo Carbon-13 NMR’, ed. L. J . Berliner and P.-M. Robitaille, Kluwer Academic/Plenum Publ., New York, N.Y., 1998

vol. 16 ‘Modern Techniques in Protein NMR’, ed. N. R. Krishna and L. J. Berliner, Kluwer Academic/Plenum Publ., New York, N.Y., 1998 vol. 17 ‘Structure Computation and Dynamics in Protein NMR ’, ed. N. R. Krishna and L. J. Berliner, Kluwer Academic/Plenum Publ., New York, N.Y., 1999 Chemical Reviews, vol. 99, 1999 M. J. Wirth, ‘Chemical Analysis in Small Domains: Introduction’, p. 2843 M. E. Lacey, R. Subramanian, D. L. Olson, A. G. Webb and J. V. Sweedler, ‘High-Resolution NMR Spectroscopy of Sample Volumes from 1nL to IOpL’, p. 3133

vol. 100,2000 C. A. Reed and R. D. Bolskar, ‘Discrete Fulleride Anions and Fullerenium Cations’, p. 1075

Coordination Chemistry Reviews, vol. 184, 1999 R19 C. Bazzicalupi, A. Bencini, A. Bianchi, C. Giorgi and B. Valtancoli, ‘Pd(11) Complexes of Aliphatic Polyamine Ligands in Aqueous Solution: Thermodynamic and Structural Features’, p. 243 R20 M. C. Aragoni, M. Arca, F. A. Devillanova, A. Garau, F. Isaia, V. Lippolis and G. Verani, ‘Charge-Transfer Adducts between Donors Containing Chalcogens (S and Se) and Di-Iodine: Solution Studies’, p. 271 R2 1 M. Formica, V. Fusi, M. Micheloni, R. Pontellini and P. Romani, ‘Cryptand Ligands for Selective Lithium Coordination’, p. 347

R22 R23

V O ~185-186, . 1999 S. Aime, M. Botta, M. Fasano, C. S. Geninatti and E. Terreno, “H and 70-NMR Relaxometric Investigations of Paramagnetic Contrast Agents for MRI. Clues for Higher Relaxivities’, p. 321 V. Comblin, D. Gilsoul, M. Hermann, V. Humblet, V. Jacques, M.

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3

Mesbahi, C. Sauvage and J. F. Desreux, ‘Designing New MRI Contrast Agents: A Coordination Chemistry Challenge’, p. 45 1 R24 F. A. Walker, ‘Magnetic Spectroscopic (EPR, ESEEM, Mossbauer, MCD and NMR) Studies of Low-Spin Ferriheme Centers and their Corresponding Heme Proteins’, p. 47 1 R25 R. H. Fish, ‘Bio-Organometallic Chemistry: Synthesis, Structure, and Molecular Recognition Chemistry of (q5-Pentamethylcyclopentadieny1)rhodium-DNNRNA Complexes in Water’, p. 569 R26 S. J. Berners-Price, R. J. Bowen, P. Galettis, P. C . Healy and M. J. McKeage, ‘Structural and Solution Chemistry of Gold(1) and Silver(1) Complexes of Bidentate Pyridyl Phosphines: Selective Antitumour Agents’, p. 823

R27

vol. 188, 1999 L. P. Kazansky and B. R. McGarvey, ‘NMR and EPRSpectroscopies and Electron Density Distribution in Polyoxoanions’, p. 157

R28

vol. 189, 1999 P. J. Shapiro, ‘The Syntheses, Structures, Bonding, and Reactivity of Cyclopentadienylaluminium(III) Derivatives’, p. 1

vol. 190-192, 1999 L. Latos-Grazynski, K. Rachlewicz and J. Wojaczynski, ‘Novel Routes for the Modification of Iron Porphyrins’, p. 109 R30 H. M. Margues and K. L. Brown, ‘The Structure of Cobalt Corrinoids Based on Molecular Mechanics and NOE-Restrained Molecular Mechanics and Dynamics Simulations’, p. 127 R3 1 P. Tsiveriotis and N. Hadjiliadis, ‘Studies on the Interaction of Histidyl Containing Peptides with Palladium(I1) and Platinum(I1) Complex Ions’, p. 171 R32 A. F. Danil de Namor, ‘Solution Thermodynamics of Pyridinocalix(4)Arenes and Monovalent Cations’, p. 283 R29

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V O ~193-195, . 1999 A. Antinolo, F. Carrillo-Hermosilla, M. Fajardo, J. Fernandez-Baeza, S. Garcia-Yuste and A. Otero, ‘Advances in the Chemistry of BisCyclopentadienyl Hydride Derivatives of Niobium and Tantalum’, p. 43 vol. 197, 2000 M. Gerken and G. J. Schrobilgen, ‘The Impact of Multi-NMR Spectroscopy on the Development of Noble-Gas Chemistry’, p. 335

Current Medical Chemistry, vol. 6, 1999

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Nuclear Magnetic Resonance

R. P. Mason, ‘Transmembrane pH Gradients in Vivo: Measurements Using Fluorinated Vitamin B6 Derivatives’, p. 48 1 K. Gademann, T. Hintermann and J. V. Schreiber, ‘P-Peptides: Twisting and Turning’, p. 905 vol. 7,2000 H. Rudiger, H.-C. Siebert, D. Solis, J. Jimenez-Barbero, A. Romero, C.-W. Von der Lieth, T. Diaz-Maurino and H.-J. Gabius, ‘Medical Chemistry Based on the Sugar Code: Fundamentals of Lectinology and Experimental Strategies with Lectins as Targets’, p. 389 Current Opinion on Biotechnology, vol. 10, 1999 J. Varner and D. Ramkrishna, ‘Mathematical Models of Metabolic Pathways’, p. 146 R. Bott and R. Boelens, ‘The Role of High-Resolution Structural Studies in the Development of Commercial Enzymes’, p. 391 Current Opinion on Chemistry and Biology, vol. 3, 1999 J. Reedijk, ‘Medicinal Application of Heavy-Metal Compounds’, p. 236 G. Siegal, J. Van Duynhoven and M. Baldus, ‘Biomolecular NMR: Recent Advances in Liquids, Solids and Screening’, p. 530 W. G. Scott, ‘RNA Structure, Metal Ions, and Catalysis’, p. 705 Current Opinion in Drug Discovery & Development, vol. 2, 1999 M. J. Shapiro and J. R. Wareing, ‘High Resolution NMR for Screening Ligand/Protein Binding’, p. 396

vol. 3,2000 D. G. Robertson and S. J. Bulera, ‘High-Throughput Toxicology: Practical Considerations’, p. 42 E. Holmes and J. P. Shockcor, ‘Accelerated Toxicity Screening Using NMR and Pattern Recognition-Based Methods’, p. 72 Current Opinion on Structural Biology, vol. 9, 1999 L. McFail-Isom, C. C. Sines and L. D. Williams, ‘DNA Structure: Cations in Charge?’, p. 298 J. Jimenez-Barbero, J. L. Asensio, F. J. Canada and A. Poveda, ‘Free and Protein-Bound Carbohydrate Structures’, p. 549 D. A. Brant, ‘Novel Approaches to the Analysis of Polysaccharide Structures’, p. 556 G. Wider and K. Wuthrich, ‘NMR Spectroscopy of Large Molecules and Multimolecular Assemblies in Solution’, p. 594

I : N M R Books and Reviews

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H. N. B. Moseley and G. T. Montelione, ‘Automated Analysis of NMR Assignments and Structures for Proteins’, p. 635 vol. 10,2000 G. S. Jackson and A. R. Clarke, ‘Mammalian Prion Proteins’, p. 69 K. S. Gajiwala and S. K. Burley, ‘Winged Helix Proteins’, p. 110 Curyen t Organic Chemistry, vol. 3, 1999 G. Lippens, M. Bourdonneau, C. Dhalluin, R. Warrass, T. Richert, C. Seetharaman, C. Boutillon and M. Piotto, ‘Study of Compounds Attached to Solid Supports Using High Resolution Magic Angle Spinning NMR ’, p. 147 W. Zhang, W. Xie, J. Wang, X. Chen, J. Fang, Y. Chen, J. Li, L. Yu, D. Chen and P. G. Wang, ‘Recent Progress in Glyco-Chemistry and Green Chemistry’, p. 241

vol. 4,2000 N. Suryaprakash, ‘Liquid Crystals as Solvents in NMR Spectroscopy: Current Developments in Structure Determination’, p. 85 NATO Advanced Study Institute Series, Series B, vol. 371, 1998 D. Pines, ‘The Spin Fluctuation Model for High Temperature Superconductivity: Progress and Prospects’, p. 111 S. Kramer and M. Mehring, ‘Evidence for Gap Asymmetry and Spin Fluctuations from Nuclear Magnetic Resonance (NMR)’, p. 309 Series C, vol. 499, 1997 A. Kuki, D. Anglos, J. D. Angspurger, G. Basu, V. A. Bindra, M. Kubasik and A. Pettijohn, ‘Molecular Optical Rails Based on Aib. Modular Chemistry with Unusually Reliable Peptide Helices’, p. 503

vol. 526, 1999 S. J. Cantrill, M. C. T. Fyfe, F. M. Raymo and J. F. Stoddart, ‘Probing Self-Assembly by NMR’, p. 1 R60 N. Bampos, Z. Clyde-Watson, J. C. Hawley, C. C. Mak, A. VidalFerran, S. J. Webb and J. K. M. Sanders, ‘NMR Studies of Molecular Recognition by Metalloporphyrins’, p. 37 R6 1 V. Bohmer, 0. Mogck, M. Pons and E. F. Paulus, ‘Reversible Dimerization of Tetraureas Derived from Calix[4]arenes’, p. 45 R62 M. Pons, M. A. Molins, 0. Millet, V. Bohmer, P. Prados, J. De Mendoza, J. Veciana and J. Sedo, ‘Symmetry: Friend or Foe?’, p. 67 R63 C. F. G. C. Geraldes, ‘Paramagnetic NMR Effects of Lanthanide Ions as Structural Reporters of Supramolecular Complexes’, p. 133

R59

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Nuclear Mugnetic Resonance

B. D. Rao, ‘Characterization of Reaction Complex Structures of ATPUtilizing Enzymes by High Resolution NMR’, p. 155 R65 G. Wagner, H. Matsuo, H. Li, C. M. Flether, A. M. McGuire, A.-C. Gingras and N. Sonenberg ‘Intermolecular Interaction of Proteins Involved in the Control of Gene Expression’, p. 247 R66 H. Kessler and G. Gemmecker, ‘NMR-Based Modeling of ProteinProtein Complexes and Interaction of Peptides and Proteins with Anisotropic Solvents’, p. 255 R67 J. Feeney, ‘NMR Studies of Protein-Ligand and Protein-Protein Interaction Involving Proteins of Therapeutic Interest’, p. 28 1 R68 Y. Cohen, 0. Mayzel, A. Gafni, M. Greenwald, D. Wessely, L. Frish and Y. Assaf, ‘NMR Diffusion Measurements in Chemical and Biological Supramolecular Systems’, p. 301 R69 A. Casnati, J. De Mendoza, D. N. Reinhoudt and R. Ungaro, ‘Determination of Calixarene Conformations by Means of NMR Techniques’, p. 307 R64

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vol. 527, 1999 D. N. Reinhoudt, P. Timmerman, F. Cardullo and M. Crego-Calama, ‘Synthesis and Characterization of Hydrogen-Bonded Assemblies: Toward the Generation of Binding Site Diversity’, p. 181 vol. 539, 1999 J. A. Ripmeester and C. I. Ratcliffe, ‘Applications of Solid State NMR Spectroscopy to the Study of Crystalline Materials’, p. 25 1 vol. 543, 1999 A. Putnis and V. Vinograd, ‘Principles of Solid State NMR Spectroscopy and Applications to Determining Local Order in Minerals’, p. 389 Series E vol. 358, 1999 J. Jonas, ‘High-Resolution NMR Spectroscopy at High Pressure’, p. 231 vol. 362, 1999 W. H. Casey, J. Nordin, B. L. Phillips and S. Nordin, ‘Metal Detachments from (Hydr)Oxide Mineral Surfaces. A Molecular View’, p. 384 Series 3, vol. 62, 1999 K. Luders, W. Hoffmann and M. Baenitz, ‘Nuclear Resonance and Susceptibility Investigations of Hg-Based High-T, Superconductors’, 1999,62, 177

1: N M R Books and Reviews

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Progress in N M R Spectroscopy, vol. 34, ed. J. W. Emsley, J. Feeney and L. H. Sutcliffe, 1999, R76 M . Sattler, J. Schleucher and C. Griesinger, ‘Heteronuclear Multidimensional NMR Experiments for the Structure Determination of Proteins in Solution Employing Pulsed Field Gradients’, p. 93 R77 C. S. Johnson, Jr., ‘Diffusion Ordered Nuclear Resonance Spectroscopy: Principles and Applications’, p. 203 R78 P. Koehl, ‘Linear Prediction Spectral Analysis of NMR Data’, p. 257 R79 S. Williams, ‘Cerebral Amino Acids Studied by Nuclear Magnetic Resonance Spectroscopy in Vivo’,p. 301 R80 G. W. Buchanan, ‘Nuclear Magnetic Resonance Studies of Crown Ethers’, p. 327 vol. 35, 1999 R8 1 J. H. Davies and M. Auger, ‘Static and Magic Angle Spinning NMR of Membrane Peptides and Proteins’, p. 1 R82 R. J. Abraham, ‘A Model for the Calculation of Proton Chemical Shifts in Non-Conjugated Organic Compounds’, p. 85 R83 M. J. Shapiro and J. S. Gounarides, ‘NMR Methods Utilized in Combinatorial Chemistry Research’, p. 153 R84 0. N. Antzutkin, ‘Sideband Manipulation in Magic-Angle-Spinning Nuclear Magnetic Resonance’, p. 203 R85 G. Vlahov, ‘Application of NMR to the Study of Olive Oils’, p. 341 R86 A. J. Horsewill, ‘Quantum Tunneling Aspects of Methyl Group Rotation Studied by NMR’, p. 359

3

Edited Books and Symposia

ACS Symposia Series vol. 717, 1999, R87 J. J. Fitzgerald and S. M. DePaul, ‘Solid-state NMR Spectroscopy of Inorganic Materials: An Overview’, p. 2 R88 A. Medek, L. Marinelli and L. Frydman, ‘Multiple-Quantum MagicAngle Spinning NMR of Half-Integer Quadrupolar Nuclei’, p. 136 R89 J. A. Reimer and S. B. Adler, ‘Oxygen Motion in Simple and Complex Oxides’, p. 156 R90 J. J. Fitzgerald, ‘Solid-state ‘H, 170,and 27Al NMR Studies of the Surface Chemistry of Alumina Materials’, p. 182 R9 1 G. E. Maciel, ‘Siloxane-Based Solid Networks, from Silicas to Silicones’, p. 326

R92

vol. 719, 1999 A. K . Ghose, V. N. Viswanadhan and J. J. Wendoloski, ‘Adapting Structure-Based Drug Design in the Paradigm of Combinatorial Chemistry and High-Throughput Screening: An Overview and New Examples

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Nuclear Magnetic Resonance

with Important Caveats for Newcomers to Combinatorial Library Design Using Pharmacophore Models or Multiple Copy Simultaneous Search Fragments’, p. 226

R93

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R95 R96 R97 R98 R99 RlOO

vol. 727, 1999 J. A. Glass Jr., S. S. Kher, Y. Tan and J. T. Spencer, ‘The Chemical Vapor Deposition of Metal Boride Thin Films from Polyhedral Cluster Species’, p. 130 vol. 732, 1999 C. M. Szabo, L. K. Sanders, W. Arnold, J. S. Grimley, N. Godbout, M. T. McMahon, B. Moreno and E. Oldfield, ‘NMR and Quantum Chemistry of Proteins and Model Systems’, p. 40 J. C. Facelli, J. 2. Hu, M. S. Solum, R. J. Pugmire and D. M. Grant, ‘Modeling of the 15Nand I3CChemical Shift Tensors in Purine’, p. 162 Y. Wei and A. E. McDermott, ‘Effects of Hydrogen Bonding on ‘H Chemical Shifts’, p. 177 A. C. De Dios, J. L. Roach and A. E. Walling, ‘The NMR Chemical Shift: Local Geometry Effects’, p. 220 M. Buhl, ‘Correlation between Transition-Metal NMR Chemical Shifts and Reactivities’, p. 240 J. A. Tossell, ‘Local and Long-Range Effects on NMR Shieldings in Main-Group Metal Oxides and Nitrides’, p. 304 C. J. Jameson, A. K. Jameson, A. C. De Dios, R. E. Gerald, H-M. Lim and P. Kostikin, ‘Application of Nuclear Shielding Surfaces to the Fundamental Understanding of Adsorption and Diffusion in Microporous Solids’, p. 335

vol. 740,2000 RlOl B. K. Lavine, S. Hendayan, W. T. Cooper and Y. He, ‘Selectivity in Micellar Liquid Chromatography. Surfactant-Bonded Phase Associations in Micellar Reversed Phase Liquid Chromatography’, p. 290 vol. 746,2000 R102 F. Gyenes, A. Kornilov and J. T. Welch, ‘The Synthesis of a Series of Fluorinated Tribactams’, p. 182

Advances in Conjugated Linoleic Acid Research, vol. I , ed. M. P. Yurawecz, AOCS Press, Champaign, Ill, 1999 R103 A. L. Davis, G. P. McNeil and D. C . Caswell, ‘Identification and Quantification of Conjugated Linoleic Acid Isomers in Fatty Acid Mixtures by 13CNMR Spectroscopy’, p. 164 Advances in BioChirality, 1998, ed. G. Palyi, C . Zucchi and L. Caglioti, Elsevier Science, Oxford, U.K., 1999

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R104 B. Noszal, ‘Chiral Genetics of Drugs and Related Compounds’, p. 347 Advances in High Pressure Bioscience and Biotechnology, [Proceedings of International Conference of High Pressure Bioscience and Biotechnology, Heidelberg, I9981, ed. H. Ludwig, Springer-Verlag, Berlin, Germany, 1999 R105 J. Jonas, ‘High-Resolution NMR Studies of Proteins Under High Pressure’, p. 191

R106 R107

R 108 R109

Rl 10 Rlll

Rl12 R113 Rl14 R115 R116 R117

Advances in Lignocellulosics Characterization, ed. D. S. Argyropoulos, TAPPI Press, Atlanta, Ga., 1999 T. J. Tamminen and B. R. Hortling, ‘Isolation and Characterization of Residual Lignin’, p. 1 J. Ralph, J. M. Marita, S. A. Ralph, R. D. Hatfield, F. Lu, R. M. Ede, J. Peng, S. Quideau, R. F. Helm, J. H. Grabber, H. Kim, G. JimenezMonteon, Y. Zhang, H-J. G. Jung, L. L. Landucci, J. J. MacKay, R. R. Sederoff, C. Chapple and A. M. Boudet, ‘Solution-State NMR of Lignins’, p. 55 D. S. Argyropoulos, ‘Applications of Quantitative Phosphorus-3 1 NMR to Pulping, Bleaching and Yellowing’, p. 109 E. Billa, D. S. Argyropoulos and E. G. Koukios, ‘Recent Advances in Residual Kraft Lignins Characterization Combining Phosphorus-3 1 NMR and Fluorescence Spectroscopy by Chemometrics’, p. 131 C. E. Frazier, J. Ni and R. G. Schmidt, ‘Applications of NMR Spectroscopy to the Analysis of Wood/Adhesive Bondlines’, p. 145 R. W. Hemingway, P. J. Steynberg, J. P. Steynberg and T. Hatano, ‘NMR Studies on the Conformation of Polyflavanoids and their Association with Proteins’, p. 157 Advances in Magnetic Resonance in Food Science, [Special Publication of Royal Society of Chemistry, No. 2311, ed. P. S. Belton, B. P. Hills and G. A. Webb, Royal Society of Chemistry, UK, 1999 D. C. P. Jardim, J. R. Mitchell, W. Derbyshire, J. M. V. Blanshard and J. A. G. Areas, ‘Molecular Mobility of a System: Waxy Maize, Glycerol and Water, Studied by NMR’, p. 63 S. J. Schmidt, ‘Probing the Physical and Sensory Properties of Food Systems Using NMR Spectroscopy’, p. 79 P. S. Belton, ‘NMR of Food Biopolymers’, p. 115 R. H. Newman, ‘Editing the Information in Solid-state Carbon-13 NMR Spectra of Food’, p. 144 I. E. Bechmann, H. T. Pedersen, L. Norgaard and S. B. Engelsen, ‘Comparative Chemometric Analysis of Transverse Low-Field I H NMR Relaxation Data’, p. 217 I. A. Farhat, J. M. V. Blanshard and J. R. Mitchell, ‘Time Domain ‘H

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NMR: Its Relevance to the Processing and Storage of Starch Systems’, p. 280 Advances in Process Measurements for the Ceramics Industry, ed. A. Jillavankatesa and G. Y. Onoda, American Ceramic Society, Westerville, Ohio, 1999 R118 P. S. Wang and G. Onoda, ‘An Overview of Moisture Measurement Techniques for Ceramic Processing’, p. 263 Allergic Contact Dermatitis, ed. J.-P. Lepoittevin, D. A. Basketter, A. Goossens and A.-T. Karlberg, Springer, Berlin, Germany, 1998 R119 E. Gafvert and A.-T. Karlberg, ‘Isolation and Identification of Contact Allergens’, p. 43 Annual Reports in Combinatorial Chemistry & Molecular Diversity, vol. 2 , ed. M. R. Pavia and W. H. Moos, Kluwer Academic Publishers, Dordrecht, 1999 R 120 W. L. Fitch, ‘Analytical Methods for Quality Control of Combinatorial Libraries’, p. 33 Biologically Active Natural Products: Agrochemicals, ed. H. G. Cutler and S. J. Cutler, CRC: Boca Raton, FI., 1999 R121 G. R. Waller, ‘Recent Advances in Saponins Used in Foods, Agriculture, and Medicine’, p. 243 Bioorganic Chemistry: Carbohydrates., ed. S. M. Hecht, Oxford University Press, Inc., New York, NY, 1999 R 122 A. S. Serianni, ‘Carbohydrate Structure, Conformation, and Reactivity: NMR Studies with Stable Isotopes’, pp. 244-312,575-579 Calcium as a Cellular Regulator, ed. E. Carafoli and C. Klee, Oxford University Press, New York, NY, 1999 R123 N. Tjandra, A. Bax, A. Crivici and M. Ikura, ‘Calmodulin Structure and Target Interaction’, p. 152 Catalysis and Zeolites. Fundamentals and Applications, ed. J. Weitkamp and L. Puppe, Springer, Berlin, Germany, 1999 R124 H. G. Karge, M. Hunger and H. K. Beyer, ‘Characterization of Zeolites - Infrared and Nuclear Magnetic Resonance Spectroscopy and X-Ray Diffraction’, p. 198 Chemicals from Plants, ed. N. J. Walton and D. E. Brown, Imperial College Press, London, UK, 1999 R125 G. Massiot, C. Lavaud and J.-M. Nuzillard, ‘Structure Elucidation of Plant Secondary Products’, p. 187

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Chemistry and Biochemistry of Biz, ed. R. Banerjee, John Wiley & Sons, Inc., New York, N. Y., 1999 R126 H. P. C. Hogenkamp, ‘B12: 1948-1998’, p. 3 R127 K. L. Brown, ‘NMR Spectroscopy of B I ~ ’p., 197 Chemistry of‘Hypervalent Compounds, ed. K. Akiba, Wiley-VCH, New York, NY, 1999 R 128 K. Yamada and T. Okuda, ‘Characteristic Properties of Hypervalent Compounds: Static and Dynamic Structures of Sb(III), Sn(II), and Ge(I1) Halides’, p. 49 Chemical Synthesis Using Supercritical Fluids, ed. P. G. Jessop and W. Leitner, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999 R129 J. W. Rathke, R. J. Klingler, R. E. Gerald, D. E. Fremgen, K. Woelk, S. Gaemers and C. J. Elsevier, ‘NMR Spectroscopy in Supercritical Fluids’, p. 165 Cisplatin: Chemistry and Biochemistry of a Leading Anticancer Drug, ed. B. Lippert, Verlag Helvetica Chimica Acta, Zurich, Switzerland, 1999 R130 J.-M. Malinge and M. Leng, ‘Interstrand Cross-Links in Cisplatin- or Transplatin-Modified DNA’, p. 159 R131 S. 0. Ano, Z. Kuklenyik and L. G. Marzilli, ‘Structure and Dynamics of Pt Anticancer Drug Adducts from Nucleotides to Oligonucleotides as Revealed by NMR Methods’, p. 247 R132 Y. Chen, Z . Guo and P. J. Sadler, ‘195Pt-and ”N-NMR Spectroscopic Studies of Cisplatin Reactions with Biomolecules’, p. 293 Colloid-Polymer Interactions: From Fundamentals to Practice, ed. R. S. Farinato and P. L. Dubin, Wiley, New York, NY, 1999 R133 F. D. Blum, ‘Nuclear Magnetic Resonance of Surface Polymers’, p. 207 Dynamic Spin in Chemistry, ed. S. Nagakura, H. Hayashi and T. Azumi, Kodansha, Tokyo, Japan, 1998 R 134 T. Azumi, ‘Spin Spectroscopy Focusing on Nuclear Spin Polarization’, p. 153 R135 N. Hirota and S. Yamauchi, ‘Chemically Induced Dynamic Electron Polarization (CIDEP) Studies in Photochemical Reactions’, p. 187 R136 M. Okazaki, ‘Reaction-Yield-Detected ESR and its Application to Control Chemical Reactions’, p. 249 Essentials of Glycobiology, ed. A. Varki, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, 1999 R137 0. Hindsgal and R. D. Cummings, ‘Protein-Glycan Interactions’, p. 41 R138 A. E. Manzi and H. van Halbeek, ‘Principles of Structural Analysis and Sequencing of Glycans’, p. 581

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Experimental Methods in Polymer Science, ed. T. Tanaka, Academic Press, San Diego, Calif., 2000 R139 I. Ando, M. Kobayashi, M. Kanekiyo, S. Kuroki, S. Ando, S. Matsukawa, H. Kurosu, H. Yasanaga and S. Amiya, ‘NMR Spectrosopy in Polymer Science’, p. 26 1, Flavor Chemistry 30 Years of Progress, ed. R. Teranishi, E. L. Wick and I. Hornstein, Kluwer Academic/Plenum Publishers, New York, N.Y ., 1999 R140 G. J. Martin and M. L. Martin, ‘Thirty Years of Flavor NMR ’, p. 18 Grignard Reagents: New Developments, ed. H. G. Richey Jr., John Wiley & Sons Ltd., Chichester UK, 2000 R141 T. S. Ertel and H. Bertagnolli, ‘X-Ray Absorption Spectroscopy and Large Angle X-Ray Scattering of Grignard Compounds’, p. 329 Handbook of Electron Spin Resonance, vol. 2 , ed. C. P. Poole Jr. and H. A. Farach, AIP Press, Secaucus, NJ, 1999 R142 G. R. Eaton and S. S. Eaton, ‘ESR Imaging’, p. 327 High Temperature Superconductors, ed. K. B. Garg and S. M. Bose, Narosa, New Delhi, 1998 R 143 R. G. Sharma, ‘Superconducting Magnet System Development at NPL’, p. 242 Marihuana and Medicine, [Con$ 1, 1998, ed. G. G. Nahas, Humana, Totowa, NJ, 1999 R144 R. Mechoulam, W. A. Devane and R. Glaser, ‘Cannabinoid Geometry and Biological Activity’, p. 65 Mesoscopic Materials and Clusters, ed. T. Arai, K. Mihama, K. Yamamoto and S. Sugano, Springer, Berlin, Germany, 1999 R145 S.-I. Kobayashi, H. Goto and S. Katsumoto, ‘Kubo Effects in Small Particles of Metals’, p. 113 R146 K. Kimura, ‘Electronic State of Ultrafine Particles Suspended in Liquid Media’, p. 153 R147 Y. Achiba, H. Shiromaru, T. Wakabayashi and S. Suzuki, ‘Structure and Stability of Large Carbon Clusters’, p. 379 Metal Clusters in Chemistry, vol. 2, ed. P. Braunstein, L. A. Oro and P. R. Raithby, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999 R148 L. J. Farrugia and A. G. Orpen, ‘Structure and Dynamics in Metal Carbonyl Clusters: NMR, EXAFS and Crystallographic Studies’, p. 1001

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R 149 P. J. Dyson, ‘Reversible Skeletal Rearrangements in Transition Metal Clusters’, p. 1028 Natural Product Analysis, Chromatography - Spectroscopy - Biological Testing, ed. P. Schreier, M. Herderich, H.-U. Humpf and W. Schwab, Vieweg, Wiesbaden, Germany, 1998 R150 B. Schneider, D. Holscher, A. Kolbe and T. Blitzke, ‘HPLC-NMR in Phytochemical Analysis’, p. 137

New Aspects in Bioorganic Chemistry, ed. U. Diederichsen, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999 R 151 G. Gemmecker, ‘Structural Studies of Intermolecular Interactions by NMR Spectroscopy’, p. 379 R152 S. J. Glaser, ‘Tailor-Made Experimental Building Blocks for NMR Studies of Bioorganic Compounds’, p. 388 Organosilicon Chemistry IV: From Molecules to Materials, ed. N. Auner and J. Weis, Wiley-VCH Verlag GmbH, Weinheim, Germany, 2000 R153 P. E. Ritzenhoff and H. C. Marsmann, ‘Unconventional Solvents for Hydrolysis of Tetramethoxysilane’, p. 42 1 Phosphorus Biogeochemistry in Subtropical Ecosystems, ed. K. R. Reddy, G. A. O’Connor and C. L. Schelske, Boca Raton, Fla., 1999 R154 S. Newman and J. S. Robinson, ‘Forms of Organic Phosphorus in Water, Soils, and Sediments’, p. 207 Polymer Surfaces, ed. H. Hommel, Research Signpost, Trivandrum, India, 1998 R155 J. P. Cohen Addad, ‘Permanent or Temporary Networks as Observed from NMR ’, p. 257 Polymer Surfaces and Interfaces III, ed. R. W. Richards and S. K. Peace, Wiley, Chichester, UK, 1999 R156 P. J. McDonald and D. M. Lane, ‘Magnetic Resonance Relaxation and Imaging’, p. 237 Proceedings of 6th Asian Conference, [Solid State Ionics], ed. B. V. R. Chowdari, World Scientific, Singapore, Singapore, 1998 R157 D. Brinkmann, ‘New Aspects of NMR Studies in Solid Electrolytes’, p. 93 R 158 A. Magistris, P. Mustarelli and C. Tomasi, ‘Transport-Structure Correlations in AgI-Based Silver Oxysalt Glasses’, p. 557 Proceedings of 8th International Conference, Executive Conference Management, Plymouth, Mich., 1999

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R 159 E. P. Giannelis, ‘Polymer-Layered Silicate Nanocomposites: Emerging Scientific and Commercial Opportunities’, Paper 1/1

R160

R161 R162 R163 R 164

Proceedings of 4th International Conference on Magnetic Resonance Microscopy, Albuquerque, New Mexico, 1997, [Spatially Resolved Magnetic Resonance] ed. P. Bluemler, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1998 M. Heidenreich, A. Spyros, W. Kockenberger, N. Chanrakumar, R. Bowtell and R. Kimmich, ‘CYCLCROP Mapping of 13C Labelled Compounds: Perspectives in Polymer Science and Plant Physiology’, p. 21 W. E. Maas, L. H. Merwin and D. G. Cory, ‘Materials Imaging with Examples from Solid Rocked Propellants’, p. 141 A. Guthausen, G. Zimmer, R. Eymael, U. Schmitz, P. Blumler and B. Blumich, ‘Soft-Matter Relaxation by the NMR-MOUSE’, p. 195 R. L. Kleinberg and C. Flaum, ‘Review: NMR Detection and Characterization of Hydrocarbons in Subsurface Earth Formations’, p. 555 Y. Xia, ‘Introduction to Magnetic Resonance’, p. 7 13

Proceedings of International School of Physics, “Enrico Fermi”, 10s Press, 1998 R 165 M. Randeria, ‘Precursor Pairing Correlations and Pseudogaps’, p. 53 Proceedings of the International Symposium on Asphaltenes at the Fine Particle Society Meeting in Chicago, 1995 and the Symposium on Asphaltene and Resin Characterization in Sun Francisco, 1998, [Structures and Dynamics of Asphaltenes], ed. 0. C. Mullins and E. Y. Sheu, Plenum Publishing Corporation, New York, NY, 1998 R166 J. Sjoblom, 0. Saether, 0. Midttun, M.-H. Ese, 0. Urdahl and H. Fordedal, ‘Asphaltene and Resin Stabilized Crude Oil Emulsions: Experimental Characterization and Destabilization’, p. 337 Proceedings of the 5th International Symposium, London 1997, [Innovation & Perspectives in Solid Phase Synthesis & Combinatorial Libraries 19981, ed. R. Epton., Mayflower Scientific Ltd., Kingswinford, UK, 1999 R167 J. L. Lauer and G. B. Fields, ‘Protein-Like Molecular Structure: Synthesis and Application for Inducing Cellular Receptor Binding and Signal Transduction’, p. 185 R168 U. Gotz, M. Raitza, K. Albert and E. Bayer, ‘Suspension 31P-NMRas a New Method to Characterize Antisense Oligonucleotides’, p. 305 Proceedings of IOCDICYTED Symposium, 1997, [Chemistry, Biological and Pharmacological Properties of Medicinal Plants from the Americas], ed. K. Hostettmann, M. P. Gupta and A. Marston, Harwood, Amsterdam, Netherlands, 1999

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R169 K. Hostettmann and J-L. Wolfender, ‘Application of LC/MS and LCI NMR in the Search for New Bioactive Compounds from Plants of the Americas’, p. 19 Proceedings of the PaclJic Symposium, [Biocomputing ’991, ed. R. B. Altman, World Scientific, Singapore, Singapore, 1999 R170 S. 1. Mathieson, C. J. Penkett and L. J. Smith, ‘Characterization of Side-Chain Conformational Preferences in a Biologically Active but Unfolded Protein’, p. 542 Proceedings of SPE International Symposium on Oilfield Chemistry, Society of Petroleum Engineers, Richardson, Tex., 1999 R171 D. N. Harry, D. E. Putzig, R. Moorhouse, T. DelPesco and P. Jernakoff, ‘Chemical Structures of Group 4 Metal Crosslinkers for Polygalactomannans’, p. 297 Proteome and Protein Analysis, ed. R. M. Kamp, D. Kyriakidis and T. Choli-Papadopoulou, Springer-Verlag, Berlin, Germany, 2000 R172 B. Berch, M. Blackledge, B. Brutscher, F. Cordier, J. Chr. Hus and D. Marion, ‘Protein Structure and Dynamics by NMR in Solution’, p. 91 R173 P. M. Rudd, M. R. Wormald and R. A. Dwek, ‘Exploring Functions for Glycosylation in Host Defence Using novel Oligosaccharide Sequencing Technology’, p. 321 Spectral Properties of Lipids, ed. R. J. Hamilton and J. Cast, Sheffield Academic Press, Sheffield, UK, 1999 R174 W. L. J. Meeussen, ‘Pulse-NMR in the Food Science Laboratory’, p. 123 Spectroscopic Properties of Inorganic and Organometallic Compounds. vol. 32, ed. G. Davidson, Royal Society of Chemistry, Cambridge, UK, 1999 R175 B. E. Mann, ‘Nuclear Magnetic Resonance Spectroscopy, p. 1 R 176 K. B. Dillon, ‘Nuclear Quadrupole Resonance Spectroscopy’, p. 204 Structure-Function Analysis of G Protein-Coupled Receptors., ed. J. Wess, Wiley-Liss, New York, NY, 1999 R177 P. L. Yeagle, ‘Use of Nuclear Magnetic Resonance Techniques to Study G Protein-Coupled Receptor Structure’, p. 355 The Biology - Chemistry Interface, ed. R. Cooper and J. K. Snyder, Dekker, New York, NY, 1999 R178 K. Hostettmann, M. Hostettmann, S. Rodriguez and J.-L. Wolfender, ‘LC-Hyphenated Techniques in the Search for New Bioactive Plant Constituents’, p. 65

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R179 T. Kusumi and I. I. Ohtani, ‘Determination of the Absolute Configuration of Biologically Active Compounds by the Modified Mosher’s Method’, p. 103 Transreactions in Condensation Polymers, ed. S. Fakirov, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999 R180 H. R. Kricheldorf and Z. Denchev, ‘Interchange Reactions in Condensation Polymers and their Analysis by NMR Spectroscopy’, p. 1 R181 R. N. James, S. S. Mahajan and S. Sivaram., ‘Inhibition of Transreactions in Condensation Polymers’, p. 219 R182 K. L. L. Eersels, A. M. Aerdts and G. Groeninckx, ‘Reactive Melt Processing of Aliphatic/Aromatic Polyamide Blends: Effect on Molecular Structure, Semicrystalline Morphology, and Thermal Properties’, p. 267 Triple Helix Forming Oligonucleotides, ed. C. Malvy, A. Harel-Bellan and L. L. Pritchard, Kluwer, Boston, Mass., 1999 R 183 H. Porumb, ‘Triple-Helix Structure. The Triple-Helix-Forming Oligonucleotide’, p. 17

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R184 D. Adams, B. Adeva, E. Arik, A. Arvidson, B. Badelek, M. K. Ballintijn, G. Bardin, G. Baum, P. Berglund, L. Betev, I. G. Bird, R. Birsa, P. Bjorkholm, B. E. Bonner, N. De Botton, M. Boutemeur, F. Bradamante, A. Bravar, A. Bressan, S. Bultmann, E. Burtin, C. Cavata, D. Crabb, J. Cranshaw, T. Cuhadar, S. D. Torre, R. Van Dantzig, B. Derro, A. Deshpande, S. Dhawan, C. Dulya, A. Dyring, S. Eichblatt, J. C. Faivre, D. Fasching, F. Feinstein, C . Fernandez, S. Forthmann, B. Frois, A. Gallas, J. A. Garzon, T. Gaussiran, H. Gilly, M. Giorgi, E. Von Goeler, S. Goertz, G. Gracia, N. De Groot, M. G. Perdekamp, E. Gulmez, K. Haft, D. Von Harrach, T. Hasegava, P. Hautle, N. Hayashi, C. A. Heusch, N. Horikawa, V. W. Hughes, G. Igo, S. Ishimoto, T. Iwata, E. M. Kabuss, T. Kageya, et al., ‘The Polarized Double Cell Target of the SMC’, Nucl. Instrum. Methods Phys. Res., Sect. A , 1999,437,23 R185 S. F. Akber, ‘The Efficacy of Paramagnetic Ions on Spin Lattice Relaxation Time in Biological Systems’, Physiol. Chem. Phys. Med. NMR, 1998,30, 175 R186 T. M. Alam, M. Celina, D. R. Wheeler, R. A. Assink, R. L. Clough and K. T. Gillen, ‘Using I7O NMR Spectroscopy to Investigate Polymer Degradation’, Polym. News, 1999,24, 186 R 187 K. Albert, ‘Liquid Chromatography - Nuclear Magnetic Resonance Spectroscopy’, J. Chromatogr., A , 1999,856, 199 R 188 V. E. Anderson, ‘Vibrational Analysis of Enzyme Bound Substrates:

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R 190 R191 R192 R193

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R197 R198 R199 R200 R20 1

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The Importance of Electrostatic Interactions’, Biomed. Health Res., 1999,27,85 E. F. Annison and W. L. Bryden, ‘Perspectives on Ruminant and Nutrition and Metabolism. 11. Metabolism in Ruminant Tissues’, Nutr. Res. Rev., 1999, 12, 147 Anonymous (UK), ‘IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN) and Nomenclature Committee of IUBMB (NCIUBMB), Eur. J. Biochem., 1999,264,607 K. Asahi, ‘Nuclear and Fundamental Physics with Spin Polarized Nuclei’, Genshikaku Kenkyu, 2000,44,45 R. H. Atalla and D. L. Van der Hart, ‘The Role of Solid-state Carbon13 NMR Spectroscopy in Studies of the Nature of Native Celluloses’, Solid State Nucl. Magn. Reson., 1999, 15, 1 S. P. Babailov and Yu. G. Krieger, ‘NMR Methods for Molecular Structure Studies of Paramagnetic Lanthanide Complexes in Solutions. Applications to Crown Ether Complexes’, J. Struct. Chem., 1998,39,580 A. D. Bain, ‘On Combination Lines in the NMR Spectra of Strongly Coupled Spin Systems’, Can. J. Chem., 1999,77, 1810 K. K. Baldridge and J. S. Siegel, ‘Correlation of Empirical G(TMS) and Absolute NMR Chemical Shifts Predicted by ab Initio Computations’, J. Phys. Chem. A, 1999,103,4038 U. Bandyopadhyay, S. Adak and R. K. Banerjee, ‘Role of Active Site Residues in Peroxidase Catalysis: Studies on Horseradish Peroxidase’, PINSA-B: Proc. Indian Natl. Sci. Acad., Part B, 1999,65, 315 E. Barbar, ‘NMR Characterization of Partially Folded and Unfolded Conformational Ensembles of Proteins’, Biopolymers, 1999,51, 191 T. J. Bastow, ‘47*49TiNMR in Metals, Inorganics, and Gels’, 2. Naturforsch., A: Phys. Sci., 2000, 55,291 H. Beall and D. F. Gaines, ‘Mechanistic Aspects of Boron Hydride Reactions’, Inorg. Chim. Acta, 1999, 289, 1 H. Beall and D. F. Gaines, ‘Chemistry of Pentaborane(9). A Review’, Collect. Czech. Chem. Commun., 1999,64,747 B. Bechinger, ‘The Structure, Dynamics and Orientation of Antimicrobial Peptides in Membranes by Multidimensional Solid-state NMR Spectroscopy’, Biochim. Biophys. Acta, 1999,1462, 157 B. Bechinger, R. Kinder, M. Helmle, T. C. B. Vogt, U. Harzer and S. Schinzel, ‘Peptide Structural Analysis by Solid-state NMR Spectroscopy’, Biopolymers, 1999,51, 174 A. T. Bell, ‘NMR Applied to Zeolite Synthesis’, Colloids Surf:, A , 1999, 158,221 M. R. Bendal and T. E. Skinner, ‘Comparison and Use of Vector and Quantum Representations of J-Coupled Spin Evolution in an IS Spin System During RF Irradiation of One Spin’, J. Mag. Reson., 2000, 143, 329 I. Bertini, C. Luchinat and A. Rosato, ‘NMR Spectra of Iron-Sulfur Proteins’, Adv. Inorg. Chem., 1999,47,251

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R206 A. K. Bhuyan and J. B. Udgaonkar, ‘Real-Time NMR Measurements of Protein Folding and Hydrogen Exchange Dynamics’, Curr. Sci., 1999,77,942 R207 E. Biala, 0. Botta, E. Moyroud and P. Strazewski, ‘Ribonucleosides, Aminoacyl-, and Peptidyl-RNA. Synthesis, Thermodynamics, and Structure’, Chimia, 1999,53, 226 R208 R. Blinc, ‘The Impact of NMR on the Field of Liquid Crystals’, Liq. Cryst., 1999, 26, 1295 R209 B. Blumich, P. Blumler, L. Gasper, A. Guthausen, V. Gobbels, S. Laukemper-Ostendorf, K. Unseld and G. Zimmer, ‘Spatially Resolved NMR in Polymer Science’, Macromol. Symp., 1999,141,83 R210 B. Blumich, D. E. Demco, S. Stapf, U. Goerke, C. Chwatinski, L. Gasper, R. Giesen, R. Haken and S. Han, ‘Functional NMR Imaging of Materials’, Polym. Mater. Sci.Eng., 2000,82, 148 R211 P. R. Bodart, J. P. Amoureux, M. Pruski, A. Bailly and C. Fernandez, ‘Applications of 27Al Multiple Quantum Magic Angle Spinning NMR to Aluminophosphate Molecular Sieves’, Magn. Reson. Chem., 1999, 37, S69 R212 J.-L. Bonardet, J. Fraissard, A. Gedeon and M.-A. Springuel-Huet, ‘Nuclear Magnetic Resonance of Physisorbed ‘29XeUsed as a Probe to Investigate Porous Solids’, Catal. Rev. -Sci. Eng, 1999,41, 115 R213 M. Botta, ‘Second Coordination Sphere Water Molecules and Relaxivity of Gadolinium(II1) Complexes: Implications for MRI Contrast Agents’, Eur. J. Inorg. Chem., 2000, 3, 399 R214 S. A. Bourne, X. Y. Mbianda, A. M. Modro, T. A. Modro, L. R. Nassimbeni, P. H. Vanrooyen and H. Wan, ‘Structural, Spectroscopic and Reactivity Studies on Phosphoric Amides’, Phosphorus, Sulfur Silicon Relat. Elem., 1999, 144-146, 693 R215 N. 0. Brace, ‘Syntheses with Perfluoroalkyl Iodides. Part 11. Addition to Non-Conjugated Alkadienes; Cyclization of 1,6-Heptadiene, and of 4-Substituted 1,6-Heptadienoic Compounds: Bis-Ally1 Ether, Ethyl Diallylmalonate, N,N’-Diallylamine and n-Substituted Diallylamines; and Additions to Homologous Exo- and Endocyclic Alkenes, and to Bicyclic Alkenes’, J. Fluorine Chem., 1999,96, 101 R2 16 C. Breen, ‘The Characterization and Use of Polycation-Exchanged Bentonites’, Appl. Clay Sci., 1999, 15, 187 R217 D. Brinkmann, ‘Quadrupolar Relaxation - What Would We Do without it in High-T, Superconductor Studies?’, 2. Naturforsch., A: Phys. Sci., 2000,55, 323 R218 H. G. Brittain, ‘Methods for the Characterization of Polymorphs and Solvates’, Drugs Pharm. Sci., 1999,95,227 R2 19 B. Brodsky, ‘Hydrogen Bonding in the Triple-Helix’, Proc. -Indian Acad. Sci., Chem. Sci., 1999, 111, 13 R220 H. B. Brom, ‘NMR in High-Temperature Superconductors’, Proc. Int. Sch. Phys. “Enrico Fermi”, 1998, 136, 77 R221 B. Brown, ‘Cooking with Qubits’, New Sci., 1999, 163, 38

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R222 M. F. Brown and A. A. Nevzorov, ‘2H-NMR in Liquid Crystals and Membranes’, Colloids SurJ.’,A , 1999,158,281 R223 S. Broyde and B. E. Hingerty, ‘Effective Computational Strategies for Determining Structures of Carcinogen-Damaged DNA’, J. Comput. Phys., 1999,151,313 R224 A. T. Brunger, P. D. Adams and L. M. Rice, ‘Annealing in Crystallography: A Powerful Optimization Tool’, Prog. Biophys. Mol. Biol., 1999,72, 135 R225 E. Brunner, ‘Applications of Laser-Polarized 29Xe under Continuous Flow’, Magn. Reson. Chem., 1999,37, S14 R226 E. Brunner, ‘Enhancement of Surface and Biological Magnetic Resonance Using Laser-Polarized Noble Gases’, Concepts Magn. Reson., 1999,11,313 R227 A. I. Brusilovets, V. Bdjhola T. Lis and A. Savitsky, ‘1,3,2,4-0xaazaphosphatitanetidine. Structure and Properties’, Phosphorus, Su@r Silicon Relat. Elem., 1999, 144-146, 729 R228 M. Buck, ‘Trifluoroethanol and Colleagues: Cosolvents Come of Age. Recent Studies with Peptides and Proteins’, Q. Rev. Biophys., 1998, 31, 297 R229 B. M. Burkhard, R. M. Gassman, D. A. Langs, W. A. Pangborn, W. L. Duax and V. Platnev, ‘Gramicidin D Conformation, Dymamics and Membrane Ion Transport’, Biopolymers, 1999,51, 129 R230 V. M. Buznik, ‘Nuclear Spectroscopy of Glasses: A Review’, Glass Phys. Chem., 2000,26, 1 R23 1 P. Carbonero, I. Diaz, J. Vicente-Carbajosa, J. Alfonso-Rubi, K. Gaddour and P. Lara, ‘Cereal Amylase/Trepsin Inhibitors and Transgenic Insect Resistance’, Dev. Plant Breed., 1999, 8, 147 R232 W. E. Carlos, ‘Magnetic Studies of GaN-Based Materials and Devices’, EMIS Datarev. Ser., 1999,23, 104 R233 W. R. Carper, ‘Recent Developments in Multinuclear NMR Relaxation Studies of Viscous Solutions’, Recent Res. Dev. Phys. Chern., 1997, 1, 73 R234 W. R. Carper, ‘Direct Determination of Quadrupolar and Dipolar NMR Correlation Times from Spin-Lattice and Spin-Spin Relaxation Rates’, Concepts Magn. Reson., 1999,11, 51 R235 J. A. Carver, ‘Probing the Structure and Interactions of Crystalline Proteins by NMR Spectroscopy’, Prog. Retinal Eye Res., 1999,18,431 R236 R. K. Castellano and J. Rebec, Jr. ‘Reversible-Formed Polymeric Capsules Based on Calixarene Tetraureas’, Polym. Mater. Sci. Eng., 1999,80, 16 R237 B. Casu and G. Torri, ‘Structural Characterization of Low Molecular Weight Heparins’, Sernin. Thromb. Hernostasis, 1999,25, 17 R238 K. G. Caulton, ‘Direct Observation of a Transition Metal Alkane Complex, CpRe(C0)2(Cyclopentane), Using NMR Spectroscopy’, Chemtracts, 1999,12, 343 R239 J. C. C. Chan, ‘Spin Echoes in Half-Integer Quadrupole Systems’, Concepts Magn. Reson., 1999, 11, 363

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R240 B. Chankvetadze and G. Blaschke, ‘Selector-Selectand Interactions in Chiral Capillary Electrophoresis’, Electrophoresis, 1999,20, 2592 R241 A. Chen and M. J. Shapiro, ‘Affinity NMR’, Anal. Chem., 1999, 71, 669A R242 K. Y. Chen, ‘Study of Polyphosphate Metabolism in Intact Cells by 31P Nuclear Magnetic Resonance Spectroscopy’, Prog. Mol. Subcell. Biol., 1999,23,253 R243 D. B. Chesnut and L. D. Quin, ‘The Theoretical Determination of Phosphorus NMR Chemical Shielding’, Adv. Mol. Struct. Res., 1999, 5, 189 R244 W. L. A. K. Chiu and S. C. F. Au-Yeung, ‘Current Trends in Conformation Analysis of DNA Structure Using Nuclear Magnetic Resonance Spectroscopy’, Curr. Top. Anal. Chem., 1998,1, 1 17 R245 K-Y. Choi, M. H. Yi, ‘Soluble Polyimides Containing Alicyclic Structures’, Macromol. Symp., 1999,142, 193 R246 R. L. Cook and C. H. Langford, ‘Ramped Amplitude Cross Polarization Magic Angle Spinning NMR (Ramp-CP-MAS-NMR). A Technique for Quantitative Study of the Composition of Solid State Polymers’, Polym. News, 1999,24,6 R247 C. E. Cooper and J. S. Wyatt, ‘NMR Spectroscopy and Imaging of the Neonatal Brain’, Biochem. SOC.Trans., 2000’28, 121 R248 D. J. Craik, ‘Applications of NMR in Drug Design: Structure-Activity Relationships in Disulfide-Rich Peptides’, Protein Pep?. Lett., 1999, 6, 34 1 R249 F. Cruz and S. Cerdan, ‘Quantitative 13C NMR Studies of Metabolic Compartmentation in the Adult Mammalian Brain’, N M R Biomed., 1999,12,451 R250 V. L. Davidson and 2 . Zhu, ‘Reaction Products and Intermediates of Tryptophan Tryptophylquinone Enzymes’, J. Mol. Catal. B: Enzym., 2000,8,69 R251 G. Davies and E. A. Ghabbour, ‘Understanding Life after Death’, Chem. Ind. (London), 1999,11,426 R252 W. J. DeGrip, C. H. W. Claassen and P. H. M. Bovee-Geurts, ‘LargeScale Functional Expression of Visual Pigments: Towards High-Resolution Structural and Mechanistic Insight’, Biochem. SOC.Trans., 1999, 27,937 R253 R. C. Deka, ‘Acidity in Zeolites and their Characterization by Different Spectroscopic Methods’, Indian J. Chem. Technol., 1998’5, 109 R254 R. De Renzi, ‘pSR and NMR: Fundamental Concepts and Selected Examples’, Proc. Scott. Univ. Summer Sch. Phys., 1999,51,211 R255 S. Divakar, ‘Nuclear Magnetic Resonance Spectroscopy in Food Applications: A Critical Appraisal’, J. Food Sci. Technol., 1998,35,469 R256 A. M. Dixon and C. K. Larive, ‘NMR Spectroscopy with Spectral Editing for the Analysis of Complex Mixtures’, Appl. Spectrosc., 1999, 53,426A R257 S. Djordjevic and A. M. Stock, ‘Structural Analysis of Bacterial

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R258 R259

R260 R261 R262 R263 R264 R265 R266 R267 R268

R269 R270 R271

R272 R273

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Chemotaxis Proteins: Components of a Dynamic Signaling System’, J. Struct. Biol., 1998,124, 189 F. D. Doty, G. Entzminger and Y. A. Yang, ‘Magnetism in HighResolution NMR Probe Design. I: General Methods’, Concepts Magn. Reson., 1998, 10, 133 F. D. Doty, G . Entzminger and Y. A. Yang, ‘Magnetism in HighResolution NMR Probe Design. 11: HRMAS’, Concepts Magn. Reson., 1998,10,239 A. Dransfeld and D. B. Chesnut, ‘How Does the 31PNMR Chemical Shift Change with Phosphorus Bond Length? An Ab Initio NMR Study’, Phosphorus, Surfur Silicon Relat. Elem., 1999, 1 6 1 4 6 , 6 5 3 P. C. Driscoll and S. M. Kristensen, ‘NMR of Natural Macromolecules’, Nucl. Magn. Reson., 1999, 28, 309 W. W. Du Mont, L. Muller and F. Ruthe, ‘Trichlorosilylation of Trialkylgermanium and -Tin Moieties to Unexpected Heteronoepentanes’, Phosphorus, Sulfur Silicon Relat. Elem., 1999,150-151, 149 J. 0. DUUS,P. M. St.Hilaire, M. Meldal and K. Bock, ‘Carbohydrate Chemistry: Synthetic and Structural Challenges towards the End of the 20th Century’, Pure Appl. Chem., 1999,71,755 C. Dybowski and S. Bai, ‘Solid-state Nuclear Magnetic Resonance’, Anal. Chem., 2000,72, 1 L. Eggeling and H. Sahm, ‘L-Glutamate and L-Lysine: Traditional Products with Impetuous Developments’, Appl. Microbiol. Biotechnol., 1999,52, 146 A. Ejchart, ‘Scalar Couplings in Structure Determination of Proteins’, Bull. Pol. Acad. Sci., Chem., 1999,47, 1 M. E. Elyashberg, ‘Expert Systems for Structure Elucidation of Organic Molecules by Spectral Methods’, Russ. Chem. Rev., 1999,68, 525 S. V. Evans and R. C. MacKenzie, ‘Characterization of ProteinGlycolipid Recognition at the Membrane Bilayer’, J. Mol. Recognit., 1999,12, 155 J. Feeney, ‘NMR Studies of Ligand Binding to Dihydrofolate Reductase’, Angew. Chem., Int. Ed., 2000’39,291 T. P. Fehlner, ‘Connections between “B NMR Chemical Shifts and Electronic Structure in Matallaboranes. A Precis’, Collect. Czech. Chem. Commun., 1999,64,767 E. Fernandez-Megia and S. V. Ley, ‘Recent Studies on the Use of MPA and MTPA in the Determination of the Absolute Configuration of Secondary Alcohols by ‘H NMR’, Cherntracts, 1999, 12, 539 G . T-K. Fey and D-L. Huang, ‘Synthesis, Characterization and Cell Performance of Inverse Spinel Electrode Materials for Lithium Secondary Batteries’, Electrochim. Acta, 1999,45,295 R. H. Fillingame and S. Divall, ‘Proton ATPases in Bacteria: Comparison to Escherichia Coli FIFO as the Prototype’, Novartis Found. Symp., 1999,221,218

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R274 R. H. Fillingame, W. Jiang and 0. Y. Dmitriev, ‘Coupling H+ Transport to Rotary Catalysis in F-Type ATP Synthases: Structure and Organization of the Transmembrane Rotary Motor’, J. Exp. Biol., 2000,203,9 R275 H-C. Flemming, J. Wingender, R. Moritz, W. Borchard and C. Mayer, ‘Physico-Chemical Properties of Biofilms - a Short Review’, Spec. Pub1.-R. SOC.Chem., 1999,242, 1 R276 P. Forgo and V. T. D’Souza, ‘The Use of High-Resolution NMR Spectroscopy in Supramolecular Systems’, Org. Lett., 1999,1, 1543 R277 D, Forkel-Wirth, ‘Exploring Solid State Physics Properties with Radioactive Isotopes’, Rep. Prog. Phys., 1999,62, 527 R278 D. Forkel-Wirth, ‘Solid State Physics with Radioactive Isotopes’, Acta Phys. Pol., B, 1999,30, 1587 R279 J. Fraissard, ‘NMR Studies of Supported Metal Catalysts’, Catal. Today, 1999,51,481 R280 J. Fraissard and P. Batamack, “H-NMR Study of Heterogeneous Adsorbent-Adsorbate (Water, Methanol) Equilibria at 4 K: Application to the Acid Strength of Solids’, Colloids Surf, A , 1999, 158,211 R281 A. D. Frankel, ‘If the Loop Fits...’, Nut. Struct. Biol., 1999, 6, 1081 R282 R. Freeman, ‘Pioneers of High-Resolution NMR’, Concepts Magn. Reson., 1999, 11,61 R283 H. Fujii, S. Kazama and L. J. Berliner, ‘Nitroso, Nitroxyl and NO: InVivo ESR’, Curr. Top. Biophys., 1999,23, 11 R284 T. Fujii and T. Itaya, ‘Systematic Tables of Mono- and Poly-NMethylated Adenines: Acids Dissociation Constants and UV and NMR Spectral Data’, Heterocycles, 1999,51,2255 R285 Y. S. Fung, ‘Room Temperature Molten Salt as Medium for Lithium Battery and Alloy Electrodeposition - Fundamental and Application’, Trends Inorg. Chem., 1998,5, 117 R286 H.-J. Gabius, ‘Escherichia Coli P-Galactosidase Recognizes a HighEnergy Conformation of C-Lactose, a Nonhydrolyzable Substrate Analogue. NMR and Modeling Studies of the Molecular Complex’, Chemtracts, 1999, 12,762 R287 L. E. Garcia-Ayuso and M. D. L. De Castro, ‘A Comparative Study between Chromatography and Other Techniques for Quantification of Triglycerides and Fatty Acids in Dairy Products’, Semin. Food Anal., 1999,4,39 R288 C. A. Geiger, ‘Thermodynamics of (Fe2+, Mn2+, Mg, Ca)3-A12Si3012 Garnet: A Review and Analysis’, Mineral. Petrol., 1999,66,271 R289 A. Gelasco and S. J. Lippard, ‘Anticancer Activity of Cisplatin and Related Complexes’, Top. Biol. Inorg. Chem., 1999, 1, 1 R290 R. Ghose, T. R. Eykyn and G. Bodenhausen, ‘Average Liouvillian Theory Revisited: Cross-Correlated Relaxation between Chemical Shift Anisotropy and Dipolar Couplings in the Rotating Frame in Nuclear Magnetic Resonance’, Mol. Phys., 1999,96, 128I

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R29 1 R. S. Glass, ‘Aspects of Organoselenium Chemistry’, Phosphorus, Sulfur Silicon Relat. Elem., 1998, 136-138, 159 R292 M. Goez, ‘CIDNP Investigations of PET-Induced Reactions of Organic Substrates’, J. In$ Rec., 1998,24,9 R293 Y-D. Gong and S-E. Yoo, ‘Combinatorial Chemistry 11: Spectrscopic Analytical Tool for the Solid-Phase Chemistry’, Korean J. Med. Chem., 1999,9,26 R294 B. M. Goodson, ‘Using Injectable Carriers of Laser-Polarized Noble Gases for Enhancing NMR and MRI’, Concepts Magn. Reson., 1999, 11,203 R295 C. B. Gorman, ‘Molecular Structure Property Relationships for Electron Transfer Rate Attenuation in Redox Active Core Dendrimers’, Polym. Prepr. (Am. Chem. SOC.,Div. Polym. Chem.), 2000,41,564 R296 R. J. Gorte, ‘What Do We Know About the Acidity of Solid Acids?’, Catal. Lett., 1999,62, 1 R297 R. J. Gorte and D. White, ‘Measuring Sorption Effects at Zeolite Acid Sites: Pursuing Ideas from W. 0. Haag’, Microporous Mesoporous Mater., 2000,35-36,447 R298 P. Granger, ‘Selenium and Tellurium Solid State NMR’, Phosphorus, Sulfur Silicon Relat. Elem., 1998,136,137&138, 373 R299 J. Greaves, P. Hobbs, D. Chadwick and P. Haygarth, ‘Prospects for the Recovery of Phosphorus from Animal Manures: A Review’, Environ. Technol., 1999,20,697 R300 0, H. Griffith and M. Ryan, ‘Bacterial Phosphatidylinositol-Specific Phosphalipase C: Structure, Function, and Interaction with Lipids’, Biochim. Biophys. Acta, 1999, 1441,237 R30 1 J. Grobelny, ‘Structural Investigations of Unsaturated and CrossLinked Polyesters by Nuclear Magnetic Resonance Spectroscopy’, J. Macromol. Sci., Rev. Macromol. Chem. Phys., 1999, C39,405 R302 S. Gronowitz, ‘Selenophene, a Twin-Brother of Thiophene?’, Phosphorus, Sulfur Silicon Relat. Elem., 1998, 136138, 59 R303 R. Gruetter, J. Pfeuffer, I. Tkac, G. Damberg and E. R. Seaquist, ‘NMR Studies of in Vivo Brain Glucose Concentration and Transport’, Alfred Benzon Symp., 1999,45,128 R304 S. Grzesiek, ‘Structural Characterization of Proteins by NMR ’, Dev. Biol. Stand., 1998, 96, 37 R305 T. Gullion, ‘Introduction to Rotational-Echo Double-Resonance NMR’, Concepts Magn. Reson., 1998, 10,277 R306 F. D. Gunstone, ‘High-Resolution NMR Spectroscopy as an Analytical Tool’, Lipid Technol., 1999, 11, 39 R307 H. Gunther, ‘Selected Topics from Recent NMR Studies of Organolithium Compounds’, J. Braz. Chem. SOC.,1999, 10,241 R308 S. R. Gupta and V. Malik, ‘Measurement of Leaf Litter Decomposition’, Mod. Methods Plant Anal., 1999, 20, 181 R309 N. A. Guzman, S. A. Park, D. Schaufelberger, L. Hernandez, X. Paez, P. Rada, A. J. Tomlinson and S. Naylor, ‘New Approaches in Clinical

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R3 10 R311 R312 R3 13 R314 R315 R316 R3 17 R318 R319 R320 R321

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Chemistry: Online Analyte Concentration and Microreaction Capillary Electrophoresis for the Determination of Drugs, Metabolic Intermediates, and Biopolymers in Biological Fluids’, J. Chromatogr., B: Biomed. Sci. Appl., 1997,697, 37 G. Hagele, ‘Application of Efficient Program Systems for Spectral Analysis of Phosphorus Containing Compounds’, Phosphorus, Sulfur Silicon Relat. Elem., 1999, 144-146, 659 Kh. H. Haider and W. H. Stimson, ‘Cardiac Myofibrillar Proteins: Biochemical Markers to Estimate Myocardial Injury’, Mol. Cell. Biochem., 1999,194, 31 B. Halle and V. P. Denisov, ‘Water and Monovalent Ions in the Minor Groove of B-DNA Oligonucleotides as Seen by NMR’, Biopolymers, 1998,48,210 I. Hannus, ‘Adsorption and Transformation of Halogenated Hydrocarbons over Zeolites’, Appl. Catal., 1999, 189,263 P. E. Hansen, ‘Isotope Effects on Chemical Shifts of Proteins and Peptides’, Magn. Reson. Chem., 2000, 38, 1 P. I. Haris, ‘Synthetic Peptide Fragments as Probes for Structure Determination of Potassium Ion-Channel Proteins’, Biosci. Rep., 1998, 18,299 R. K. Harris and P. R. Bodart, ‘Applications of Magic-Angle Spinning NMR to Nitrogen-Containing Ceramics’, Muter. Sci. Forum, 2000, 325-326,305 I. Hasegawa, ‘Organic-Silica Porous Hybrids Consisting of the Cubeoctameric Silicate Structure as a Building Block’, Recent Res. Dev. Pure Appl. Chem., 1998,2,573 K. Hatada, T. Kitayama, K. Ute, and T. Nishiura, ‘Structurally Controlled Polymers with Ultimate Precision. Synthesis, Characterization, and Properties’, Macromol. Symp.’ 1999, 143, 111 A. J. R. Heck, ‘Ligand Fishing by Mass Spectrometry’, Spectrosc. Eur., 1999,11, 12 N. H. H. Heegaard and R. T. Kennedy, ‘Identification, Quantitation, and Characterization of Biomolecules by Capillary Electrophoretic Analysis of Binding Interactions’, Electrophoresis, 1999, 20, 3122 W. Heil, J. Dreyer, D. Hofmann, H. Humblot, E. Lelievre-Berna and F. Tasset, ‘3He Neutron Spin-Filter’, Physica B (Amsterdam), 1999, 267-268,328 P. Heitjans, ‘NM and P-NMR Relaxation by Diffusion in InterfaceDominated and Disordered Solids’, Schr. Forschungszent. Juelich, Muter. Muter., 2000,3, B12. 1 L. Henriksen and N. Stuhr-Hansen, ‘Electrophilic Organoselenium Reagents; Approaches to the Synthon Phenylselenium ion (PhSe+)’, Phosphorus, Sulfur Silicon Relat. Elem., 1998,136,137&138, 175 M. Herm and T. Schrader, ‘Synthetic Adrenaline Receptors Based on Bisphosphanates’, Phosphorus, Sulfur Silicon Relat. Elem., 1999, 144146,749

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R325 S. Hermanek, ‘NMR as a Tool for Elucidation of Structures and Estimation of Electron Distribution in Boranes and their Derivatives’, Inorg. Chim. Acta, 1999,289,20 R326 A. J. Hill, ‘Physical and Chemical AgeindDegradation of Polymers and Composites as Detected by Positron Annihilation Lifetime Spectroscopy’, AIP Conf Proc., 1999,4!#7,699 R327 M. Hillgartner, H. Zimmermann, S. Mimietz, A. Jork, F. Thurmer, H. Schneider, U. Noth, C. Hasse, A. Haase, G. Fuhr, M. Rothmund and U. Zimmermann, ‘Immunoisolation of Transplants by Entrapment in lgF-Labelled Alginate Gels. Production, Biocompatibility, Stability, and Long-Term Monitoring of Functional Integrity’, Materialwiss. Werkstofftech., 1999,30,783 R328 B. P. Hills, C. E. Manning and J. Godward, ‘A Multistate Theory of Water Relations in Biopolymer Systems’, Spec. Publ. - R. SOC.Chern, 1999,231,45 R329 K. M. Holtz and E. R. Kantrowitz, ‘The Mechanism of the Alkaline Phosphatase Reaction: Insights from NMR Crystallography and SiteSpecific Mutagenesis’, FEBS Lett., 1999,462,7 R330 F. Horii, ‘Solid-state NMR Analyses of the Structure Formation and Dynamics of Crystalline and Liquid Crystalline Polymers’, Polym. Muter. Sci. Eng., 2000,82, 139 R331 M. A. T. Horst, ‘Quadrupolar Spin Relaxation in the Gas Phase: Experiments and Calculations’, Magn. Reson. Rev., 1998, 17, 195 R332 W.-G. Hu and K. Schmidt-Rohr, ‘Polymer Ultradrawability. The Crucial Role of Relaxation Chain Mobility in the Crystallites’, Acta Polym., 1999,50,27 1 R333 P. L. Huyskens, T. Zeegers-Huyskens and 2. Pawelka, ‘Influence of Proton Transfer on the Formation of Hydrogen-Bonded Complexes of Higher Stoichiometry and on their Dissociation into Free Ions’, J. Solution Chem., 1999,28,915 R334 J. S. Ingwall, K. Saupe and M. Spindler, ‘31PNMR Spectroscopy of the Mouse Heart’, Dev. Cardiovasc. Med., 1998,210, 183 R335 P. K. Isbester, L. Kaune and I. J. Munson, ‘Magic-Angle Spinning NMR: A Window into Flow Catalytic Reactors’, CHEMTECH, 1999, 29,40 R336 Y. Ito, ‘Application of Multi-Dimensional NMR to Structure Analysis of Protein-Protein Interactions: Recognition of Ubiquitin by Yeast Ubiquitin Hydrolase’, RIKEN Rev., 1999,24, 59 R337 I. I. Ivanova, ‘Application of in Situ MAS NMR for Elucidation of Reaction Mechanisms in Heterogeneous Catalysis’, Colloids SurJ, A , 1999,158, 189 R338 R. Iwamoto and J. Grimblot, ‘Influence of Phosphorus on the Properties of Alumina-Based Hydrotreating Catalysts’, Adv. Catal., 1999, 44, 417 R339 J. B. Jackson, S. J. Peake and S. A. White, ‘Structure and Mechanism of Proton-Translocating Transhydrogenase’, FABS Lett., 1999,464, 1

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R340 S. Jacob and J. P. Kennedy, ‘Synthesis, Characterization and Properties of Octa-Arm Polyisobutylene-Based Star Polymers’, Adv. Polym. Sci., 1999,146, 1 R341 C. Jager, P. Hartmann, R. Witter and M. Braun, ‘New 2D NMR Experiments for Determining the Structure of Phosphate Glasses: A Review’, J. Non-Cryst. Solids, 2000,263 & 264, 61 R342 M. Jaspars, ‘Computer Assisted Structure Elucidation of Natural Products Using Two-Dimensional NMR Spectroscopy’, Nut. Prod. Rep., 1999, 16, 241 R343 Z. Jedlinski, M. Kowalczuk, G. Adamus, W. Sikorska and J. Rydz, ‘Novel Synthesis of Functionalized Poly( 3-hydroxybutanoic Acid) and its Copolymers’, Int. J. Biol. Macromol., 1999, 25, 247 R344 R. B. Jennings and C. Steenbergen, ‘Ca2+ Ion Shifts in Vivo in Reversible and Irreversible Ischemic Injury’, Prog. Exp. Cardiol., 1998, 1, 151 R345 D. C. Johnston, ‘Heavy Fermion Behaviors in LiV204’, Physica B (Amsterdam),2000,281 &282,2 1 R346 W.-I. Jung and G. J. Dietze, ‘31P Nuclear Magnetic Resonance Spectroscopy: A Noninvasive Tool to Monitor Metabolic Abnormalities in Left Ventricular Hypertrophy in Humans’, Am. J. Cardiol., 1999, 83, 19H R347 M. Kakihana and M. Yoshimura, ‘Synthesis and Characteristics of Complex Multicomponent Oxides Prepared by Polymer Complex Method’, Bull. Chem. SOC.Jpn., 1999,72, 1427 R348 H. Kano, N. Ishida and M. Koizumi, ‘Physical States of Water in Plant Tissues, Possible Probes for Non-Destructive Quality Estimation of Agricultural Products and Foods by NMR’, Recent Res. Dev. Agric. Biol. Chem., 1997, 1, 125 R349 C . Kapusta and P. C. Riedi, ‘NMR Spectroscopy in Mixed Valence Manganites’, J. Magn. Magn. Muter., 1999, 196197,446 R350 0. Karlsson and D. Sundberg, ‘Recent Developments in Emulsion Polymers - Controlling and Predicting Latex Particle Morphology’, Recent Res. Dev. Macromol. Res., 1998,3, 325 R351 B. A. Keay and I. R. Hunt, ‘Aspects of the Intramolecular Diels-Alder Reaction of Furan Dienes Leading to the Formation of Epoxydecalin Systems’, Adv. Cycloaddit., 1999,6, 173 R352 W. R. Kem, M. W. Pennington and R. S. Norton, ‘Sea Anemone Toxins as Templates for the Design of Immunosuppressant Drugs’, Perspect. Drug Discovery Des., 1999, 15116, 111 R353 I. S. Kim, K . D. Barrow and P. L. Rogers, ‘Application of Nuclear Magnetic Resonance Spectroscopy to Analysis of Ethanol Fermantation Kinetics in Yeasts and Bacteria’, Biotechnol. Lett., 1999,21, 839 R354 P. B. Kingsley, ‘Methods of Measuring Spin-Lattice (TI) Relaxation Times: An Annotated Bibliography’, Concepts Magn. Reson., 1999, 11, 243 R355 B. Kinkead, E. B. Binder and C. B. Nemeroff, ‘Does Neurotensin

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R363 R364 R365 R366

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Mediate the Effects of Antipsychotic Drugs?’, Biol. Psychiatry, 1999, 46,340 S. T. Kinsey and T. S. Moerland, ‘The Use of Nuclear Magnetic Resonance for Examining pH in Living Systems’, SOC.Exp. Biol. Semin. Ser., 1999,68,45 Y. Kitaoka, K. Ishida, H. Mukuda, Z. Q . Mao, S. Ikeda, S. Nishizaki, Y. Maeno, R. Kanno and M. Takano, ‘NMR Probe of Magnetism and Superconductivity in Ruthenate Oxides’, Mater. Sci. Eng., B, 1999, B63, 83 G. Knothe, ‘Reaction of Monounsaturated Fatty Compounds with Selenium Dioxide-Based Hydroxylation Systems’, Recent Res. Dev. Oil Chem., 1997,1,11 S. -I. Kobayashi, ‘Nuclear Magnetic Resonance in Small Particles of Metals’, Philos. Mag. B, 1999,79, 1263 H. Koller, G. Engelhardt and R. A. Van Santen, ‘The Dynamics of Hydrogen Bond and Proton Transfer in Zeolites - Joint Vistas from Solid-state NMR and Quantum Chemistry’, Top. Catal., 1999,9, 163 D. V. Konarev and R. N. Lyubovskaya, ‘Donor-Acceptor Complexes and Radical Ionic Salts Based on Fullerenes’, Russ. Chem. Rev., 1999, 68, 19 S. N. Konchenko, A. V. Virovets, S. V. Tkachev, N. V. Podberezskaya and V. A. Varnek, ‘Iron Chalcogenide Carbonyl Clusters and their Heterometallic Derivatives’, J. Struct. Chem., 1998,39, 728 R. Konrat, M. Tollinger, G. Kontaxis and B. Krautler, ‘NMR Techniques to Study Hydrogen Bonding in Aqueous Solution’, Monatsh. Chem., 1999,130,961 A. Kovac, H. Scheib, J. Pleiss, R. D. Schmid and F. Paltauf, ‘Molecular Basis of Lipase Stereoselectivity’, Eur. J. Lipid Sci. Technol., 2000, 102, 61 B. B. Kragelund, J. Knudsen and F. M. Poulsen, ‘Acyl-Coenzyme A Binding Protein (ACBP)’, Biochim. Biophys. Acta, 1999,1441, 150 E. A. Kravchenko and Y. A. Buslaev, ‘Electronic Effects of Substitution and Reconstruction of the Coordination Polyhedron in Adducts of Main Group IVA Element Halides’, Russ. Chem. Rev., 1999,68,709 H. Kumakura, ‘Development of Bi-2212 Conductors and Magnets for High-Magnetic-Field Generation’, Supercond. Sci. Technol., 2000, 13, 34 H. Kurosu and T. Yamanobe, ‘NMR of Synthetic Macromolecules’, Nucl. Magn. Reson., 1999,28, 364 K. K. Laali, ‘Building Novel Cations and Molecules with Phosphaalkynes, Tetraphosphacubanes, and Phosphirenes: A Progress Summary’, Adv. Stained Interesting Org. Mol., 1999, (Suppl. l), 223 M. Lalia-Kantouri, ‘Coordination Compounds of 2-Hydroxyaryloximes with Several Transition Metals’, Monogr. Ser. Int. Con$ Coord. Chem., 1999,4,97 M. A. Landau, ‘19Fand 31PNMR Study of the Structure of Phosphorus

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R374 R375 R376 R377 R378 R379 R380 R381 R382 R383 R384 R385

R386

Fluorides and Mechnisms of their Reactions’, Russ. J. Org. Chem., 1999,35,435 C. K. Larive, S. M. Lunte, M, Zhong, M. D. Perkins, G. S. Wilson, G. Gokulrangan, T. Williams, F., ,Afroz, C. Schoeneich, T. S. Derrick, C. R. Middaugh and S. Bogdanowich-Knipp, ‘Separation and Analysis of Peptides and Proteins’, Anal. Chem., 1999,71, 389R P. T. Larsson, E.-L. Hult, K. Wickholm, E. Pettersson and T. Iversen, ‘CP/MAS Carbon-13 NMR Spectroscopy Applied to Structure and Interaction Studies on Cellulose I’, Solid State Nucl. Magn. Reson., 1999,15,31 K. K. S. Lau and K. K. Gleason, ‘Low Dielectric Constant Fluorocarbon Films’, Muter. Res. SOC.Symp. Proc., 1999,544,209 J. T. J. Lecomte and C. J. Falzone, ‘Where U and I Meet’, Nut. Struct. Biol., 1999,6, 605 A. P. Legrand, H. Hommel and J. B. d’Espinose De la Caillierie, ‘On the Silica Edge, an NMR Point of View’, Colloids Surf.’, A, 1999, 158, 157 C. Legros, P. E. Bougis and M-F. Martin-Euclaire, ‘Molecular Biology of Scorpion Toxins Active on Potassium Channels’, Perspect. Drug Discovery Des., 1999, 15/16, 1 P. N. L. Lens and M. A. Hemminga, ‘Nuclear Magnetic Resonance in Environmental Engineering: Principles and Applications’, Biodegradation, 1998,9,393 W. D. Lilac and S. Lee, ‘Analysis of the Solid Phase Copolymerization Grafting Process’, Korean J. Chem. Eng., 1999,16,275 P. J. Lillford and S. Ablett, ‘From Solid-Liquid Ratios to Real Time Tomography - the Development of NMR in Food Applications’, Spec. Publ. - R. SOC.Chem, 1999,231,3 E. Lindner, T. Schneller, F. Auer and H. A. Mayer, ‘Chemistry in Interphases - a New Approach to Organometallic Syntheses and Catalysis’, Angew. Chem., Int. Ed., 1999,38,2155 J. C. Lindon, M. Liu and J. K. Nicholson, ‘Diffusion Coefficient Measurement by High Resolution NMR Spectroscopy: Biochemical and Pharmaceutical Applications’, Rev. Anal. Chem., 1999,18,23 L. Lomozik and R. Bregier-Jarzebowska, ‘Complexes of Cadmium(I1) and Mercury(I1) with Polyamines, Nucleosides and Nucleotides’, Pol. J. Chem., 1999,73,927 R. L. Longo, ‘Spin Density Properties from the Electron Propagator: Hyperfine and Nuclear Spin-Spin Couplings’, Adv. Quantum Chem., 1999,35,53 P. Lopez, M. Reilly and T. Oh, ‘1,8-Naphthalenediylbis(dichloroborane) and 1,8-Naphthalenediylbis(mercuricchloride) Compounds as Bidentate Lewis Acids: An NMR Investigation’, Recent Res. Dev. Org. Chem., 1999,3,297 0. V. Lounasmaa and E. Thuneberg, ‘Vortices in Rotation Superfluid 3He’, Proc. Natl. Acad. Sci. USA, 1999, 96, 7760 I

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R387 M. Ludgate, ‘Animal Models of Graves’ Disease’, Eur. J. Endocrinol., 2000,142, 1 R388 L. Luo and T. J. Marks, ‘Ziegler-Natta Catalyst Activation. Thermodynamic and Kinetic Aspects of Metallocenium Ion-Pair Formation, Dissociation, and Structural Reorganization’, Top. Catal., 1999,7,97 R389 D. Luthi, D. Gunzel and J. A. S. McGuigan, ‘Mg-ATP Binding: Its Modification by Spermine, the Relevance to Cytosolic Mg2+ Buffering, Changes in the Intracellular Ionized Mg2+ Concentration and the Estimation of Mg2+by 31P-NMR’, Exp. Physiol., 1999,84,231 R390 Y. Maeno, S. Nishizaki and Z. Q. Mao., ‘Experimental Evidence for Spin-Triplet Superconductivity in Sr2Ru04’, J. Supercond., 1999, 12, 535 R391 V. Magafa, G. ’Stavropoulos, A. Aaberg and M. Liakopoulou-Kyriakides, ‘Spectral Analysis of a Series of Partially Protected and Deprotected Tetrapeptides, Analogs of AS-I Phytotoxin’, Amino Acid& 1999, 16,403 R392 N. M. Mahoney and S. C. Almo, ‘Julius Marmur Award Lecture. The Role of Profilin in Site-Specific Actin Assembly’, Einstein Q. J. Biol. Med., 1998, 15, 126 R393 J. L. Male, D. A. Braden and D. R. Tyler, ‘CIDNP in the Photolysis of Coenzyme B12 Model Compounds Suggesting that C-Co Bond Homolysis Occurs from the Singlet State’, Chemtracts, 1997,10,908 R394 M. Malet-Martino, V. Gilard and R. Martino, ‘The Analysis of Cyclophosphamide and its Metabolites’, Curr. Pharm. Des., 1999, 5, 56 1 R395 J. P. G. Malthouse, ‘Using NMR as a Probe of Protein Structure and Function’, Biochem. SOC.Trans., 1999,27,701 R396 M. D. Manson, ‘Allele-SpecificSuppression as a Tool to Study ProteinProtein Interactions in Bacteria’, Methods (Orlando, Fla.), 2000,20, 18 R397 C. Marcolli and G. Calzaferri, ‘Monosubstituted Octasilasesquioxanes’, Appl. Organomet. Chem., 1999,13,2 13 R398 V. E. Marquez, K. Nacro, S. Benzaria, J. Lee, R. Sharma, K. Teng, G. W.A. Milne, B. Bienfait, S. Wang, N. E. Lewin and P. M. Blomberg, ‘The Transition from a Pharmacophore-Guided Approach to a Receptor-Guided Approach in the Design of Potent Protein Kinase C Ligands’, Pharrnacol. Ther., 1999,82,251 R399 G. E. Martin and C. E. Hadden, ‘Long-Range ‘H-15N Heteronuclear Shift Correlation at Natural Abundance’, J. Nat. Prod., 2000,63,543 R400 G. J. Martin, ‘Recent Advances in Site-Specific Natural Isotope Fractionation Studied by Nuclear Magnetic Resonance’, hot. Environ. Health Stud., 1998,34,53 R40 1 G. J. Martin, ‘Recent Advances in Site-Specific Natural Isotope Fractionation Studied by Nuclear Magnetic Resonance’, hot. Environ. Health Stud., 1998,34,233 R402 A. Martinez-Richa and J. C. Jauregui, ‘Application of Solid-state NMR in the Study of Materials’, Rev. Mex. Fis., 1999,45,66

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R403 L. Masaro and X. X. Zhu, ‘Physical Models of Diffusion for Polymer Solutions, Gels and Solids’, Prog. Polym. Sci., 1999, 24, 73 1 R404 K. Matsumoto, ‘Surface Analysis for Glass Coating Technology’, J. Surf: Anal., 1999, 5,298 R405 T. Matsushita, ‘Current High-Temperature Superconducting Coils and Applications in Japan’, Supercond. Sci. Technol., 2000, 13, 5 1 R406 V. J. McBrierty, S. J. Martin and F. E. Karasz, ‘Understanding Hydrated Polymers: The Perspective of NMR ’, J. Mol. Liq., 1999, 80, 179 R407 A. Medek and L. Frydman, ‘Multiple-Quantum Magic-Angle Spinning NMR: A New Technique for Probing Quadrupolar Nuclei in Solids’, J. Braz. Chem. SOC., 1999,10,263 R408 M. Mehring, ‘Concepts of Spin Quantum Computing’, Appl. Magn. Reson., 1999, 17, 141 R409 K. R. Metz, M. M. Lam and A. G. Webb, ‘Reference Deconvolution: A Simple and Effective Method for Resolution Enhancement in Nuclear Magnetic Resonance Spectroscopy’, Concepts Magn. Reson., 2000,12,2 1 R4 10 R. M. Metzger, ‘All About (N-Hexadecylquinolin-4-ium- 1-yl)-methylidenetricy anoquinodime thanide, a Unimolecular Rectifier of Electrical Current’, J. Mater. Chem., 2000, 10, 55 R411 H. Meyer, ‘Quantum Diffusion and Tunneling in the Solid Hydrogens (A Short Review), Low Temp. Phys., 1998,24,381 R412 S. Milosavljevic, V. Vajs, V. Bulatovic, D. Djokovic, I. Aljancic, N. Juranic and S. Macura, ‘Application of Two-Dimensional Nuclear Magnetic Resonance Methods for Structure Elucidation of Sesquiterpene Lactones (Guaianolides) from Anthemis Carpatica and Diterpenes (Kauranes) from Achillea Clypeolata’, Recent Res. Dev. Phytochem., 1998,2,383 R413 J. M. Moore, ‘NMR Techniques for Characterization of Ligand Binding: Utility for Lead Generation and Optimization in Drug Discovery’, Biopolymers, 1999,51, 22 1 R414 E. Morgado Jr., S. M. C. De Menezes and C. R. N. Pacheco, ‘NMR Studies of Colloidal Oxides’, Surfactant Sci. Ser., 1999,78, 125 R415 J. Munday, H. Floyd and P. R. Crocker, ‘Sialic Acid Binding Receptors (Siglecs) Expressed by Macrophages’, J. Leukocyte Biol., 1999,66, 705 R416 P. F. Mutolo, M. Witschas, G. Regelsky, J. S. auf der Guenne and H. Eckert, ‘Nuclear Magnetic Resonance (NMR) Studies of the Local Structure of Phosphorus Chalcogenide Glasses: An Overview’, J. NonCryst. Solids, 1999, 256 & 257, 63 R417 M. Nakahara, T. Yamaguchi and H. Ohtaki, ‘The Structure of Water and Aqueous Electrolyte Solutions under Extreme Conditions’, Recent Res. Dev. Phys. Chem., 1997, 1, 17 R418 R. J. Nelson, Y. Maguire, D. F. Caputo, G. Leu, Y. Kang, M. Pravia, D. Tuch, Y. S. Weinstein and D. G. Cory, ‘Counting Echoes: Application of a Complete Reciprocal-Space Description of NMR Spin Dynamics’, Concepts Magn. Reson., 1998, 10, 331

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R419 J. M. Newsam, ‘Framework and Non-Framework Cation Ordering Phenomena’, Zeoraito, 1999,16, 99 R420 J. K. Nicholson, J. C. Lindon and E. Holmes, “‘Metabonomics”: Understanding the Metabolic Responses of Living Systems to Pathophysiological Stimuli via Multivariate Statistical Analysis of Biological NMR Spectroscopic Data’, Xenobiotica, 1999,29, 1181 R421 C. J. Noren, J. Wang and F. B. Perler, ‘Dissecting the Chemistry of Protein Splicing and its Applications’, Angew. Chem., Int. Ed., 2000, 39, 450 R422 G. L. Northcott and K. C. Jones, ‘Experimental Approaches and Analytical Techniques for Determining Organic Compounds Bound Residues in Soil and Sediment’, Environ. Pollut., 2000, 108, 19 R423 M. Odelius and A. Laaksonen, ‘Combined MD Simulation - NMR Relaxation Studies of Molecular Motion and Intermolecular Interaction’, Theor. Comput. Chem., 1999,7,281 R424 M. Ota, Y. Shimizu, K. Tonosaki and Y. Ariyoshi, ‘Chemical Studies of Gurmarin, a Sweetness-Suppressing Polypeptide’, Recent Res. Dev. Agric. Biol. Chem., 1998, 2,445 R425 M. A. Palafox, ‘Empirical Correlations in Vibrational Spectroscopy’, Trends Appl. Spectrosc., 1998,2, 37 R426 A. M. Panich, ‘Nuclear Magnetic Resonance Study of Fluorine-Graphite Intercalation Compounds and Graphite Fluorides’, Synth. Met., 1999,100, 169 R427 C. G. Pantano, A. K. Singh and H. Zhang, ‘Silicon Oxycarbide Glasses’, J. Sol-Gel Sci. Technol., 1999, 14, 7 R428 E. Papirer and H. Balard, ‘Inverse Gas Chromatography: A Method for the Evaluation of the Interaction Potential of Solid Surfaces’, Surfactant Sci. Ser., 1999, 80, 145 R429 H. Pasch, ‘Analysis of Complex Polymers by Multidimensional Techniques. Invited Lecture’, Phys. Chem. Chem. Phys., 1999,1,3879 R430 M. Pellegrini and D. F. Mierke, ‘Structural Characterization of Peptide Hormone/Receptor Interactions by NMR Spectroscopy’, Biopolymers, 1999,51,208 R431 J. T. Pelton and L. R. McLean, ‘Spectroscopic Methods for Analysis of Protein Secondary Structure’, Anal. Biochem., 2000, 277, 167 R432 G. Pelzl, S. Diele and W. Weissflog, ‘Banana-Shaped Compounds. A New Field of Liquid Crystals’, Adv. Mater. ( Weinheim, Ger.), 1999, 11, 707 R433 Z.-Y. Peng, ‘Determinants of the Native-Like Tertiary Topology in the a-Lactalbumin Molten Globule’, Int. Congr. Ser., 1999, 1194, 145 R434 B. Pepin-Donat, A. Viallat, E. Rebourt and J. Bras, ‘New Class of Conducting Polymers: Fully Conjugated Gels of Poly(3-octylthiophene)’, Synth. Met., 1999, 102, 1506 R435 B. Perseghin, K. F. Petersen and G. I. Shulman, ‘NMR Studies on the Mechanism of Insulin Resistance’, Contemp. Endocrinol., 1999,12, 159 R436 H. Pfeifer, ‘A Short History of Nuclear Magnetic Resonance Spectro-

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scopy and of its Early Years in Germany’, Magn. Reson. Chem., 1999, 37, S154 M. Pfuhl and P. C. Driscoll, ‘Protein Nuclear Magnetic Resonance Spectroscopy in the New Millennium’, Philos. Trans. R. Soc. London, Ser. A , 2000,358,5 13 M. Pianca, E. Barchiesi, G. Esposto and S. Radice, ‘End Groups in Fluoropolymers’, J. Fluorine Chem., 1999,95, 7 1 T. Pietrass, ‘NMR of Molecules and Surfaces Using Laser-Polarized Xenon’, Colloids Surf.’,A, 1999,158, 5 1 I. S. Podkorytov, ‘Multipulse NMR. Part 111. INADEQATE Pulse Sequence’, Concepts Magn. Reson., 1999,11,97 L. D. Possani, B. Becerril, M. Delepierre and J. Tytgat, ‘Scorpion Toxins Specific for Na+-Channels’, Eur. J. Biochem., 1999, 264, 287 C. B. Post, B. S. Gaul, E. 2. Eisenmesser and M. L. Schneider, ‘NMR Structure of Phospho-Tyrosine Signaling Complexes’, Med. Res. Rev., 1999,19,295 M. J. Potrzebowski, ‘High Resolution Selenium Solid State NMR Spectroscopy as a Tool for Structural Studies of Organoselenium Compounds’, Phosphorus, Sulfur Silicon Relat. Elem., 1998, 136, 137&138,423 G. K. S. Prakash, G. Rasul, N. J. Head, A. Mitra and G. A. Olah, ‘Onium Ions of Cubane’, Adv. Strained Interesting Org. Mol., 1999, Supl. 1, 109 M. Pravia, E. Fortunato, Y. Weinstein, M. D. Price, G. Teklemariam, R. J. Nelson, Y. Sharf, S. Somaroo, C. H. Tseng, T. F. Have1 and D. G. Cory, ‘Observations of Quantum Dynamics by Solution-State NMR Spectroscopy’, Concepts Magn. Reson., 1999,11,225 W. Preetz and G. Peters, ‘The Hexahydro-Closo-Hexaborate Dianion [B6Hd2- and its Derivatives’, Eur. J. Znorg. Chem., 1999, 11, 1831 P. S. Pregosin and M. Valentini, ‘EnantioselectiveHomogeneous Catalysis: Transferring Chirality via Phosphine Complexes. A 2-D NMR Approach’, Enantiomer, 1999,4, 529 J. Preuschen, J. Rottstegge and H. W. Spiess, ‘Structure and Dynamics of Polymer Colloidal Systems from ‘H- and 19F-NMR’, Colloids Surf:, A , 1999,158, a9 W. S. Price, ‘Pulsed-Field Gradient Nuclear Magnetic Resonance as a Tool for Studying Translational Diffusion: Part 11. Experimental Aspects’, Concepts Magn. Reson., 1998, 10, 197 M. J. W. Prior, ‘Nuclear Magnetic Resonance Spectroscopy of Living Systems’, Nucl. Magn. Reson., 1999,28,432 M. Pursch, L. C. Sander and K. Albert, ‘Understanding ReversedPhase LC with Solid-state NMR’, Anal. Chem., l999,71,733A C . D. Putnam and J. A. Tainer, ‘The Food of Sweet and Bitter Fancy’, Nut. Struct. Biol., 2000,7, 17 G. K. Radda, ‘Of Mice and Men: From Early NMR Studies of the

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Heart to Physiological Genomics’, Biochem. Biophys. Res. Commun., 1999,266,723 J. D. F. Ramsay, ‘Characterization of the Pore Structure of Membranes’, MRS Bull., 1999,24, 36 B. D. N. Rao, ‘HOWFar Does the Itinerant Phosphoryl Group Move on a Phosphoryl-Transfer Enzyme?’, Phosphorus, Surfur Silicon Relat. Elem., 1999,144-146,309 R. G. Ratcliffe and A. Roscher, ‘Prospects for in Vivo NMR Methods in Xenobiotic Research in Plants’, Biodegradation, 1998,9,411 J-M. Reau and P. Hagenmuller, ‘Fast Ionic Conductivity of Fluorine Anions with Fluorite- or Tysonite-Type Structures’, Rev. Inorg. Chem., 1999,19,45 D. L. Reger, ‘Tris(pyrazo1yl)methaneLigands: The Neutral Analogs of Tris(pyrazoly1)borateLigands’, Comments Znorg. Chem., 1999’21, 1 G. Ricci, A. M. Caccuri, M. L. Bello, E. Di Iorio, M. Nicotra, M. Nuccetelli, L. Stella, G. Antonini, N. Rosato and G. Federici, ‘Enzymatic Mechanism and Molecular Dynamics of Glutathione Transferase P1-1 ’, Clin. Chem. Enzymol. Commun., 2000,8,213 P. A. Rice, ‘Holding Damaged DNA Together’, Nat. Struct. Biol., 1999, 6,805 P. C. Riedi, T. Thomson and G. J. Tomka, ‘NMR of Thin Magnetic Films and Superlattices’, Handb. Magn. Mater., 1999,12,97 J. A. Ripmeester and C. I. Ratcliffe, ‘The Contributions of NMR Spectroscopy to Clathrate Science’, J. Struct. Chem., 1999,40,654 M. A. Roberts and B. Quemener, ‘Measurement of Carrageenans in Food: Challenges, Progress, and Trends in Analysis’, Trends Food Sci. Technol., 1999,10, 169 E. Roeder, ‘Analysis of Pyrrolizidine Alkaloids’, Curr. Org. Chem., 1999,3,557 M. S. Rogalski, ‘Thin Film Nuclear Resonance Spectroscopy’, Nondestr. Test. Eval., 1998, 15, 15 A. Roodt, A. Abou-Hamdan, H. P. Engelbrecht and A. E. Merbach, ‘Substitution Studies of Second- and Third-Row Transition Metal 0 x 0 Complexes’, Adv. Inorg. Chem., 2000,49, 59 D. L. Rothman, N. R. Sibson, F. Hyder, J. Shen, K. L. Behar and R. G. Shulman, ‘In Vivo Nuclear Magnetic Resonance Spectroscopy Studies of the Relationship between the Glutamate-Glutamine Neurotransmitter Cycle and Functional Neuroenergetics’, Philos. Trans. R. SOC. London, Ser. B, 1999,354, 1165 C. Roumestand, C. Delay, J. A. Gavin and D. Canet, ‘A Practical Approach to the Implementation of Selectivity in Homonuclear Multidimensional NMR with Frequency Selective Filtering Techniques. Application to the Chemical Structure Elucidation of Complex Oligosaccharides’, Magn. Reson. Chem., 1999,37,451 P. M. Rudd, M. R. Wormald and R. A. Dwek, ‘Glycosylation and the Immune System’, Trends Glycosci. Glycotechnol., 1999,11, 1

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R470 P. M. Rudd, M. R. Wormald, R. L. Stanfield, M. Huang, N. Mattsson, J. A. Speir, J. A. DiGennaro, J. S. Fetrow, R. A. Dwek and I. A. Wilson, ‘Roles for Glycosylation of Cell Surface Receptors Involved in Cellular Immune Recognition’, J. Mol. Biol., 1999,293, 351 R471 M. C. Sacchi, A. Carvill, F. Forlini, I. Tritto and P. Locatelli, ‘Microstructure of Polypropene Samples Produced with Different Ansa-Zirconocene/Methylaluminoxane Catalysts: Clues on Hydrogen Activation Mechanism’, Recent Res. Dev. Macromol. Res., 1999,4,57 R472 B. C . Sales, L. A. Boatner and J. 0. Ramey, ‘Chromatographic Studies of the Structures of Amorphous Phosphates: A Review’, J. Nun-Cryst. Solids, 2000,263&264, 155 R473 B. A. Salvatore, ‘Physical Methods and Techniques. NMR Spectroscopy’, Annu. Rep. Prog. Chem., Sect. B: Org. Chem., 1999, 95, 395 R474 S. P. Samijlenko, I. M. Kolomiets, I. V. Kondratyuk and A. V. Stepanyugin, ‘Model Considerations on Physico-Chemical Nature of Protein-Nucleic Acids Contacts through Amino Acid Carboxylic Groups: Spectroscopic Data’, Biopolim. Kletka, 1998,14,47 R475 R. Sanchez and A. Sali, ‘Comparative Protein Structure Modeling in Genomics’, J. Comput. Phys., 1999,151, 388 R476 V. S. S. Sastry, K. Venu, S. U. Maheswari and R. K. Subramanian, ‘NQRStudy of Dynamics in Incommensurate Phases’, 2. Naturforsch., A: Phys. Sci., 2000,55,281 R477 A. Schenck, ‘Static Magnetic Properties of Metallic Systems Explored by pSR-Spectroscopy’, Proc. Scott. Univ. Summer Sch. Phys., 1999, 51, 39 R478 F. Schlunzen, I. Kolln, D. Janell, M. Gluhmann, I. Levin, A. Bashan, J. Harms, H. Bartels, T. Auerbach, M. Pioletti, H. Avila, K. Anagnostopoulos, H. A. S. Hansen, W. S. Bennett, I. Agmon, M. Kessler, A. Tocilj, S. Krumbholz, M. Peretz, S. Weinstein, F. Franceschi and A. Yonath, ‘The Identification of Selected Compounds in Electron Density Maps of Prokaryotic Ribosomes at 7 Resolution’, J. Synchrotron Radiat., 1999,6,928 R479 H. J. Schneider-Muntau, ‘Large Bore High Field Magnets’, Nucl. Phys. B, Proc. Suppl., 1999,72, 152 R480 R. Schroeder, C. Waldsich and H. Wank, ‘Modulation of RNA Function by Aminoglycoside Antibiotics’, EMBO J., 2000, 19, 1 R481 S. G. Sedgwick and S. J. Smerdon, ‘The Ankyrin Repeat: A Diversity of Interactions on a Common Structural Framework’, Trends Biochem. Sci., 1999,24, 31 1 R482 D. F. Shantz and R. F. Lobo, ‘Guest-Host Interactions in Zeolites as Studied by NMR Spectroscopy: Implications in Synthesis, Catalysis and Separations’, Top. Catal., 1999,9, 1 R483 A. L. Sherwin, ‘Neuroactive Amino Acids in Focally Epileptic Human Brain: A Review’, Neurochem. Res., 1999,24, 1385 R484 I. D. Shin, L. Huang and A. E. Tonelli, ‘NMR Observations of the Conformations and Motions of Polymers Confined to the Narrow

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Channels of their Inclusion Compounds’, Macromol. Symp., 1999, 138, 21 G. I. Shulman, ‘Cellular Mechanisms in Insulin Resistance in Humans’, Am. J. Cardiol., 1999,84(1A), 35 J. A. Sikorski, R. D. Sammons, K. J. Gruys and M. C. Walker, ‘Insights from 31P NMR Studies of Substrate and Inhibitor Complexex with EPSP Synthase’, Phosphorus, Sulfur Silicon Relat. Elem., 1999, 144-146, 293 P. A. Simonovov, S. Yu. Troitskii and V. A. Likholubov, ‘Preparation of the Pd/C Catalysts: A Molecular Level Study of Active Site Formation’, Kinet. Catal., 2000,41,255 T. Simonsson, H. R. Widlund and M. Kubista, ‘Structure-Function Correlation of the Insulin-Linked Polymorphic Region. Cytosine-Rich Strands of the Insulin Minisatellite Adopt Hairpins with Intercalated Cytosine+-Cytosine Pairs’, Chemtracs, 1999, 12,210 G. N. Sinyakov, A. M. Shul’ga, I. V. Filatov, K. Dzilinski, N. V. Ivashin, S. N. Terekhov and V. S. Chirvonyi, ‘Spectroscopy of the Anions of Tetrapyrrole Compounds’, J. Appl. Spectrosc., 1999,66, 528 C. P. Slichter, ‘The Knight Shift - a Powerful Probe of CondensedMatter Systems’, Philos. Mag. B, 1999,79, 1253 P. B. Smith, A. J. Pasztor Jr., M. L. McKelvy, D. M. Meunier, S. W. Froelicher and F. C.-Y. Wang, ‘Analysis of Synthetic Polymers and Rubbers’, Anal. Chem., 1999,71,61R J. C. Soares and R. B. Innis, ‘Neurochemical Brain Imaging Investigations of Schizophrenia’, Biol. Psychiatry, 1999,46,600 Y.-Q. Song, B. M. Goodson and A. Pines, ‘NMR and MRI Using Laser-Polarized Xenon’, Spectroscopy (Eugene, Oreg. , 1999,14,26 M.-A. Springuel-Huet, J.-L. Bonardet, A. Gedeon and J. Fraissard, ‘‘29XeNMR Overview of Xenon Physisorbed in Porous Solids’, Magn. Reson. Chem., 1999,37, S1 F. Stallmach and J. Karger, ‘The Potentials of Pulsed Field Gradient NMR for Investigation of Porous Media’, Adsorption, 1999,5, 117 A. G. Stepanov, ‘High-Resolution Solid-state NMR Spectroscopy in Studies of Conversions of Hydrocarbons and Alkohols on Zeolites’, Russ. Chem. Rev., 1999,68,563 D. Stoffler, M. 0. Steinmetz and U. Aebi, ‘Imaging Biological Matter across Dimensions: From Cells to Molecules and Atoms’, FASEB J., 1999,13, S195 H. B. Stuhrmann, ‘Neutron Contrast Variation and Dynamic Nuclear Polarization’, Physica B (Amsterdam), 1999,267-268,92 I. W. Sutherland, ‘Polysaccharases for Microbial Exopolysaccharides’, Carbohydr. Polym., 1999,38, 3 19 I. Svare, ‘Conductivity and NMR Relaxation from Ionic Motion in Disordered Glasses with Distributions of Barriers’, Solid State lonics, 1999,125,47 C. Szantay, Jr., ‘Evolution of Magnetization in a B1 Field. I. The J ,

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Impact of BdB1 Inhomogeneity and Fast Chemical Exchange in HighResolution NMR ’, Concepts Magn. Reson., 1999, 11, 343 T. Takabatake, ‘Kondo Semiconductors’,JJAP Ser., 1999,11,80 H. Takahashi, T. Nakanishi, K. Kami, Y. Arata and I. Shimada, ‘A Novel NMR Method for Determining the Interfaces of Large ProteinProtein Complexes’, Nat. Struct. Biol., 2000,7,220 N. Tan, J. Zhou and S. Zhao, ‘Advances in Structural Elucidation of Glucuronide Oleanate-Type Triterpene Carboxylic Acid 3, 28-0-Bisdesmosides (1962-1997)’, Phytochemistry, 1999,52, 153 F. P. Temme, ‘Cayleyan Sn-EncodedSU(2)*Sn/G Embeddings: Nuclear Spin Permutation Symmetries via Polyhedral Lattice-Point Models, from Modulo4 x(Ci(S,JG)) Combinatorial Invariance Sets’, Int. J. Quantum Chem., 2000,78,71 D. Thiebaut, ‘Detection in SFC’, Chromatogr.: Princ. Pract., 1999, 2, 149 N. Thirupathi, S. S. Krishnamurthy and M. Nethaji, ‘Cyclotriphosphazanes and Bicyclic Tetraphosphapentazanes’, Phosphorus, Suvur Silicon Relat. Elem., 1999, 144-146,265 C. H. Thompson and G. J. Kemp, ‘31PMagnetic Resonance Spectroscopy of Skeletal Muscle in Chronic Renal Failure’, Med. Biochem., 1999,1,97 C. Tilcock, ‘Delivery of Contrast Agents for Magnetic Resonance Imaging, Computed Tomography, Nuclear Medicine and Ultrasound’, Adv. Drug Delivery Rev., 1999,37,33 N. Tjandra, ‘Establishing a Degree of Order: Obtaining High-Resolution NMR Structures from Molecular Alignment’, Structure (London), 1999,7, R205 J. M. Torres-Valencia, C. M. Cerda-Garcia-Rojas and P. JosephNathan, ‘Stereochemical Assignment of 2,3-Epoxy-2-Methylbutanoate Esters in Natural Products’, Phytochem. Anal., 1999,10,221 V. V. Turov and R. Leboda, “H NMR Chemical Shifts of Adsorbed Molecules on the Carbon Surface’, Adsorpt. Sci. Technol., 1998,16, 837 B. G. Turrell, ‘Studies of Magnetism by Nuclear Orientation and NMR ON’, Hyperfine Interact., 1999,120/121, 13 N. J. Turro and I. V. Khudyakov, ‘Applications of Chemically Induced Dynamic Electron Polarization to Mechanistic Photochemistry’, Res. Chem. Intermed., 1999,25, 505 J. J. Van Der Klink, ‘NMR Spectroscopy as a Probe of Surfaces of Supported Metal Catalysts’, Adv. Catal., 1999,44, 1 R. A. van Santen, ‘Theory, Spectroscopy and Kinetics of Zeolite Catalyzed Reactions’, Catal. Today, 1999,50, 5 1 1 M. Vasak, ‘Application of ‘I3Cd NMR to Metallothioneins’, Biodegradation, 1998, 9, 501 I. Vekhter, J. P. Carbotte, P. J. Hirschfeld and E. J. Nicol, ‘Quasiparticle Properties of d-Wave Superconductors in the Vortex State’, AIP Con$ Proc., 1999,483, 341

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R519 J. Vervoort and M. Hefti, ‘NMR of Flavoproteins’, Methods Mol. Biol. (Totowa, N.J.), 1999,131, 139 R520 H. J. Vogel, P. Lundberg and K. Bagh, ‘Noninvasive NMR Studies of Metabolism in Cultured Catharanthus Roseus Cells’, In Vitro Cell. Dev. Biol.: Plant, 1999,35, 144 R521 L.-Q. Wang, G. J. Exarhos and J. Liu, ‘Nuclear Magnetic Resonance. Characterization of Self-Assembled Nanostructured Materials’, Adv. Mater. (Weinheim, Ger.), 1999,11, 1331 R522 S. B. Warner, M. Polk and K. Jacob, ‘Spider Dragline Silk’, J. Macromol. Sci., Rev. Macromol. Chem. Phys., 1999, C39,643 R523 H. Weinberg, ‘Disinfection Byproducts in Drinking Water’, Anal. Chem., 1999,71,80lA R524 D. Wemmer, ‘Solid-state Nuclear Magnetic Resonance of Protein Deposits’, Methods Enzymol., 1999,309,536 R525 P. A. Wender, K. W. Hinkle, M. F. T. Koehler and B. Lippa, ‘The Rational Design of Potential Chemotherapeutic Agents: Synthesis of Bryostatin Analogues’, Med. Res. Rev., 1999, 19, 388 R526 G. V. M. Williams, ‘NMR Studies of the Normal-State Pseudogap in High Temperature Superconducting Cuprates’, Stud. High Temp. Supercond., 1999,27, 113 R527 S. G. Withers, ‘Understanding and Exploiting Glycosidases’, Can. J. Chem., 1999,77,1 R528 W. Wolf, C. A. Presant and V. Waluch, ‘I9F-MRS Studies of Fluorinated Drugs in Humans’, A h . Drug Delivery Rev., 2000,41, 55 R529 J. Workman, Jr., D. J. Veltkamp, S. Doherty, B. B. Anderson, K. E. Creasy, M. Koch, J. F. Tatera, A. L. Robinson, L. Bond, L. W. Burgess, G. N. Bokerman, A. H. Ullman, G. P. Darsey, F. Mozayeni, J. A. Bamberger, G. Stautberg and M. Stautberg, ‘Process Analytical Chemistry’, Anal. Chem., 1999,71, 121R R530 R. Wu, M. E. Barr, G. Hernandez, C. C. Orji and L. A. Silks, ‘Recent Progress in the Synthesis and Application of Chiral Selones: Exploitation of the Selenocarbonyl Group’, Recent Res. Dev. Org. Bioorg. Chem., 1998,2,29 R53 1 K. Wuthrich, ‘Protein Structures in Solution: (1) Effects of Solvation; (2) Studies of Large Molecules’, AIP Con$ Proc., 1999,487,3 R532 K. Wuthrich, ‘Protein Recognition by NMR’, Nut. Struct. Biol., 2000, 7,188 R533 B. Xiang, ‘Coherence-Transfer Table: A Convenient Method for Driving Coherence-Transfer Pathways of an NMR Pulse Sequence. I. Coherence-Transfer Table in I,, I,,, and I, Systems’, Concepts Magn. Reson., 1999,11, 393 R534 I. Yamaguchi, K. Osakada and T. Yamamoto, ‘Main Chain and Side Chain Type Polyrotaxanes Based on Poly(Benzimidazole)s’, Recent Res. Dev. Polym. Sci., 1998,2, 547 R535 I. V. Yaminsky and A. M. Tishin, ‘Magnetic Force Microscopy’, Russ. Chem. Rev., 1999,68, 165

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R536 D. Yang and H. Brunengraber, ‘Glutamate, a Window on Liver Intermediary Metabolism’, J. Nutr., 2000, 130, 99 1S R537 X.-L. Yang and A. H.-J. Wang, ‘Structural Studies of Atom-Specific Anticancer Drugs Acting on DNA’, Pharmacol. Ther., 1999,83, 181 R538 S. Yokoyama, ‘Toward High-Throughput Determination of Protein Structures by Nuclear Magnetic Resonance’, RlKEN Rev., 1999,24,67 R539 C. Yuan and M. -D. Tsai., ‘Pancreatic Phospholipase AZ: New Views on Old Issues’, Biochim. Biophys. Acta, 1999, 1441,215 R540 K. Zangger and I. M. Armitage, ‘Silver and Gold NMR ’, Met. -Based Drugs, 1999,6,239 R54 1 G. Zanotti, ‘Muscle Fatty Acid-Binding Protein’, Biochim. Biophys. Acta, 1999, 1441,94 R542 C. Zune and R. Jerome, ‘Anionic Polymerization of Methacrylic Monomers: Characterization of the Propagating Species’, Prog. Polym. Sci., 1999,24,631 R543 N. V. Zyk, S. Z. Vatsadze, E. K. Beloglazkina, B. M. Musin and N. S. Zefirov, ‘Chemistry of Organic Derivatives of Sulfoxylic Acid. 11. Structure, Reactivity, and Application’, Russ. J. Org. Chem., 1998, 34, 1371

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Chinese R544 L. Cai, Z. Chen, P. Yang and Q. Wu, ‘Hyphenation of Capillary Electrophoresis with Nuclear Magnetic Resonance and its Applications’, Guangpuxue Yu Guangpu Fenxi, 1999,19,347 R545 J. Cui and C. Lin, ‘Application of NMR in the Analysis of Enantiomeric Purity and its Absolute-Configuration (I)’, Ghangxi Daxue Xuebao, Ziran Kexueban, 1999,24,238 R546 X.-Z. Deng, H. Li and H.-R. Yang, ‘NMR for Studying the Aromatic Electrophilic Substitution Reaction’, Guangdong Gongye Daxue Xuebao, 1999,16,67 R547 Z . Du, ‘Determination of Solution Structures of Proteins and Nucleic Acids by High Resolution NMR Spectroscopy’, Junshi Yixue Kexueyuan Yuankan, 1999,23,305 R548 G.-H. Gao and D.-C. Wange, ‘Progress in the Determination of Protein Solution Structure by NMR’, Shengwu Huaxue Yu Shengwu Wuli Jinzhan, 1999,26,228 R549 Y. Guo and J. Li, ‘Spectroscope Studies on Characteristics of Adsorbed Layer of Surfactants at Solid-Liquid Interface’, Huaxue Yanjiu Yu Yingyong, 1999, 11, 241 R550 J. Ma and H. Wu, ‘New Method to Discover High-Affinity Ligands for Bio-Macromolecules: SAR-by-NMR ’, Huaxue Jinzhan, 1999,11,265 R551 J. Qu, G. Luo and 2. Wu, ‘Advances in High Performance Liquid

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Chromatography-Nuclear Magnetic Resonance Technology’, Fenxi Huaxue, 1999,27,976 R552 K. M. Su, X. D. Jiang and S. H. Cai, ‘Determination of Enantiomeric Excess of Chiral Compounds by ‘H-NMR ’, Guangpu Shiyanshi, 1999, 16,604 R553 T. Wang and X. Mao, ‘Progress in 31PNMR Studies on Metabolism of Energy Substances in Vivo and in Vitro’, Bopuxue Zazhi, 1999,16,469 R554 D.-N. Yi, ‘Applications of Fourier Transform Methods to Organic Compound Spectrometry’, Zhonggua Yiyao Gongye Zazhi, 2000,31,39 Czech R555 J. Czernek and V. Sklenar, ‘Ab Initio Calculations of NMR Chemical Shifts in Biomolecules’, Chem. Listy, 2000,94,90 French R556 L. Charlet and M. L. Schlegel, ‘The Exchange Capacity of Soils. Structures and Charges at the Particlemater Interface’, C. R. Acad. Agric. Fr., 1999,85, 7 R557 P. Granger, J. Hirschinger, K. Elbayed, C. Sizun, P. Kempgens, J. Raya, J. Rose and P. Braunstein, ‘What Is Possible with a Quadrupolar Nuclei in NMR? 59C0 as an Example’, J. Chim. Phys. -Chim. Biol., 1999,96, 1479 R558 F. Guillaume, ‘Complementarity of NMR, IQNS, and Numerical Simulation Techniques for Studying Molecular Movements’, J. Phys. IV, 2000,10, 165 R559 J.-P.Haluk and M. Irmouli, ‘The Fixed Polymer Constituents in Cooperage Oak: Cellulose, Hemicelluloses and Lignin’, J. Sci. Tech. Tonnellerie, 1998,4, 1 German R560 K. Albert, ‘LC-NMR Coupling’, Anal. -Taschenb., 1999,20, 107 R561 J. L. Baumgarten, ‘Resolution Increase - without New Hardware’, Labor-Praxis, 1999,23,34 R562 L. Beyer, R. Richter and 0. Seidelman, ‘Ferrocene-Substituted 1,3Bidentate Ligands and their Heteronuclear Transition Metal Chelates’, J. Prakt. Chem. ( Weinheim, Ger.), 1999,341,704 R563 H.-J. Briegel, I. Cirac and P. Zoller, ‘Quantum Computer’, Phys. Bl., 1999,55,37 R564 R. Huber, ‘Structural Analysis of Large Protein Complexes’, Naturwiss. Rundsch., 1999,52, 85 R565 U. Katscher and S. Petersson, ‘Nuclear Magnetic Resonance Tomography Using the Overhauser Effect’, Phys. Bl., 2000,56,51 R566 H. Knicker, ‘13C- and ”N-NMR Spectroscopy in Soil Science’, Verbundvorhaben Biol. Sanierung Ruestungsaltlasten, Tagungsband Statussemin, 3rd, 1997, C - 1, Umweltbundesamt, Berlin, Germany R567 J. Kunze and H.-P. Fink, ‘Characterization of Cellulose and Cellulose

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Derivatives by High Resolution Solid State I3C-NMR Spectroscopy’, Papier (Bingen, Ger.), 1999,53,753 G. Schilling, ‘NMR Spectroscopy of Quadrupole Nuclei in Supercritical Liquids’, GIT Labor-Fachz., 1999,43, 1350 G. Schottner, S. Hofacker, J. Rattenmaier and R. Schwert, ‘Dye-Doped Inorganic-Organic Composites’, Werkstoffwoche ’98, Band VIZI: Symposium 10: Polymere. Symposium 14: Simulation Polymere, 1998, ed. W. Michaeli, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999, p. 53 F. Stallmach, ‘Opportunities and Limits of NMR for the Petrophysical Characterization of Reservoir Racks’, DGMK Tagungsber., 1999, 9901, 373 B. Wolter, G. Dobmann and U. Netzelmann, ‘Nondestructive Testing by NMR’, Werkstoffwoche’98, Band X: Symposium 13, Werkstoffpruej, 1998, ed. W. Muster, J. Ziebs and R. Link, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999

Greek R572 A. Kolocouris, T. Mavromoustakos, M. Zervou, P. Roumelioti, J. Matsoukas, M. Papadopoulos and S. Raptis, ‘Nuclear Magnetic Resonance and Computational Chemistry, Valuable Tools in the Design of Peptidomimetic Angiotensin I1 Receptor Antagonists as a New Generation of Antihypertensive Drugs’, Pharmakeutike, 1998, l l, 125 Hungarian R573 S. Nemes and J. Mozsa, ‘Empirical Calculation Methods of 13C Chemical Shifts for Structure Elucidation of Macromolecular Hydrocarbons’, Muanyag Gumi, 1999,36,235 Japanese R574 N. Abe, K. Yamamoto, T. Murata and A. Hirota, ‘Studies on Structure Elucidation and DPPH Radical Scavenging Activity of the SorbicilinRelated Compounds from Trichoderma sp. USF-2690’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1999,41,553 R575 K. Akasaka, ‘Determination of Absolute Configurations of Natural Products by Discrimination of Chiral Centers at Remote Positions’, Kagaku to Seibutsu, 2000,38,2 16 R576 I. Ando, C. Zhao, M. Kobayashi and S. Kuroki, ‘Structure and Dynamics of Polymer Gels Viewed by NMR Spectroscopy’, Nettowaku Porima, 1999,20, 122 R577 M. Asakawa, ‘Construction of Molecular Switches Based on Catenanes and Rotaxanes’, Kobunshi Ronbunshu, 1999,56,524 R578 K. Asayama and Y. Kitaoka, ‘NMR in Highly Correlated Superconductors’, Nippon Butsuri Gakkaishi, 1998,53,507 R579 N. Awazu, T. Kurome, K. Shimanaka. K. Ikai, I. Kato and K. Takesako, ‘Structure and Biological Activities of Antifungal Antibiotic

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R583 R584

R585 R586 R587 R588

R589 R590 R59 1

R592 R593 R594 R595 R596 R597

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TKR 1785 Having a Sphingosine-Like Lipid Moiety’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1999,41,577 J. Azuma, ‘Composite Formation of Cellulose with Hemicellulose’, Cellul. Commun., 1999,6, 12 T. Baba, ‘Variable Temperature ‘H MAS NMR Spectra of H+- and Ag+-ExchangedZeolites’, Shokubai, 1999,41, 572 A, Chiba, T. Aoyama, R. Suzuki, Y. Toyooka, M. Fujita, H. Tamegai, T. Eguchi, K. Kakinuma and T. Oshima, ‘Specific Inhibitors and Substrate Recognition of 3-Isopropylmalate Dehydrogenase’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1998,40,43 K. Endo, ‘Computation Chemistry and Analysis’, Kemikaru Enjiniyaringu, 1999,44,890 H. Fuchino, T. Satoh, H. Irino and N. Tanaka, ‘Studies on the Constituents of Betula Species in Japan’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1997,39,493 A. Goto and T. Shimizu, ‘Low Energy Magnetic Excitations in High-T, Cuprates Studied by NMR ’, Nippon Butsuri Gakkaishi, 2000,55,36 Y. Goto, ‘Analysis of P-Lactoglobulin Folding Reaction by NMR’, Seitai Bunshi Kaiseki Kenkyu Senta Dayori, 1999,20, 10 M. Hada, ‘Quantum Chemistry of NMR’, Kagaku (Kyoto), 1999, 54, 66 K. Hamayasu, K. Hara, K. Fujita, H. Hashimoto, K. Matsuda, Y. Okada, T. Tanimoto, K. Koizumi, H. Nakano and S. Kitahata, ‘Synthesis and Properties of Hetero-Branched Cyclodextrins’, J. Appl. Glycosci., 1999,46,217 S. Hayashi, ‘Orientation and Dynamics of Organic Molecules Confined in Nano-Space’, Shokubai, 1999,41, 590 T. Hinoe, ‘Spectroelectrochemistry’,Bunseki, 1999,7,580 Y. Ikeda, H. Naganawa, S. Kondo and K. Umezawa’, Apoptosis Inducers in Human Pancreatic Carcinoma, Polyoxypeptins and Chloptosin Produced by Streptomyces’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1999,41,235 Y. Ikushima, ‘Fundamental Properties of Supercritical Water’, Zairyo to Kankyo, 2000,49, 117 Y. Imizu, ‘Sulfated Zirconia-Supported Zirconocene Catalysts’, Shokubai, 1999,41,556 K. Inumaru, ‘In Situ Solid State NMR Analysis of Reaction Fields’, Shokubai, 1999,41,308 K. Ishida, H. Matsuaa, M. Murakami and K. Yamaguchi, ‘Biologically Active Compounds of the Cyanobacterium Microcystis Aeruginosa’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1997,39,667 K. Ishihara, ‘Recent High Pressure Kinetics of the Reactions of Metal Complexes in Solution’, Koatsuryoku no Kagaku to Gijutsu, 1999, 9, 22 1 K. Itabashi, ‘Characteristic Properties of Synthetic Mordenite and Discovery of SRing(2Al) Avoidance Rule’, Zeoraito, 1999,16, 104

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R598 Y. Ito, ‘Issues in Description of Spin Pseudogap and Fermi Liquids’, Bussei Kenkyu, 1999,72,450 R599 M. Iwaoka, ‘Quantitative Evaluation and Applications of Interatomic Weak Interactions Involving Group 16 Elements’, Kakaku to Kagyo (Tokyo),2000,53, 140 R600 Y. Kajihara, ‘Sialylation of Oligosaccharide on a Protein by Chemoenzymatic Method’, YukiGosei Kagaku Kyokaishi, 2000,58, 138 R601 K. Kawano and T. Aizawa, ‘Identification of Protein Adsorbing Site onto Solid Surface Using Hydrogen-Deuterium Exchange Labeling’, Seibutsu Butsuri, 1999, 39, 109 R602 T. Kiyoshi, ‘Development Trend of Extremely High Field Superconducting Magnets’, Kinzoku, 2000,70,257 R603 M. Kobayashi, M. Kanekiyo and I. Ando, ‘Clarification on Gelation Mechanism of PVA Aqueous Solution by NMR Method’, Kobunshi Kako, 1999,48,98 R604 S. Kobayashi, ‘Progress in Physical Property Measurements. 1. NMR, pSRand STM’, Maruzen Co., Ltd., Tokyo, Japan, 1997 R605 M. Kobayashi, W. Wang, K. Higuchi, M. Aoki, Y. Tsutsui, S. Aoki and N. Murakami, ‘Absolute Stereostructure of Callystatin A, a Potent Cytotoxic Polyketide from the Nagasakian Marine Sponge Callyspongia Truncata’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1997,39, 175 R606 J. Kohno, Y. Koguchi, M. Nishio, T. Ohnuki and S. Komatsubara, ‘Structures of TMC-95, Novel Proteasome Inhibitors from Apiospora Montagnei’, Tennen YukiKagobutsu Toronkai Koen Yoshishu, 1999,41, 57 1 R607 Y. Kojima, ‘Surface Modification of Nanosize Hydroxyapatite by Graft Polymerization Method’, Phosphorus Lett., 1999,35, 30 R608 K. Komiyama, S. Omura, T. Sunazuka and T. Hirose, ‘Novel IL-6 Inhibitors, Madindolines A and B Isolated from Streptomyces Nitrosporeus K93-07 1 1: Structure and Action Mechanism’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1999,41,36 1 R609 H. Kondo and S. Tsuda, ‘Molecular Structure Analysis of Biological Macromolecules which Regulate Freezing Point of Water’, Hokkaido Kogyo Glj’utsuKenkyusho Hokoku, 1999,73,7 R610 Y. Kosugi, ‘Extraction of Fats and Oils by Supercritical Carbon Dioxide and Component Analysis by NMR’, C‘horinkai Saishin Gijutsu, 1999,3,22 R611 K. Matsuta, K. Sat0 and T. Minamisono, ‘P-NMR on Short-Lived Nuclei and Hyperfine Intractions of Dilute Impurities in Crystals’, Kotai Butsuri, 1999,34, 9 19 R612 T. Minamizono, ‘P-NMR of Implanted Unstable Nucleous and Electronic Structure of Extremely Dilute Impurities in Crystals’, JAERIConJ, 2000,99-013,35 R613 S. Miyajima and T. Nakai, ‘Spectroscopy of Liquid Crystals’, Ekisho, 1999,3,43

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R614 S. Miyajima and T. Nakai, ‘Spectroscopy of Liquid Crystals’, Ekisho, 1999,3, 124 R615 S. Miyajima and T. Nakai, ‘Spectroscopy of Liquid Crystals - NMR Spectroscopy (3)’, Ekisho, 1999,3,205 R616 S. Nakata, ‘Studies on Catalytic Materials by Solid-state NMR. Application to Microporous and Mesoporous Materials and Fluorine Containing Catalysts’, Shokubai, 1999,41, 566 R617 T. Nishi, ‘Recent Progress in Characterization of Multiphase Polymers’, Nippon Gomu Kyokaishi, 1999,72, 535 R618 A. Nishikawa, ‘Antigenic Specificity and Epitopes of Yeast Cell Wall Polysaccharides’, Meiji YakkaDaigaku Kenkyu Kiyo, 1998,28, 11 R619 T. Nishizawa, ‘Basics of Nuclear Magnetic Resonance (NMR) and Characterization of Pitch’, Tanso, 1999, 187, 109 R620 M. Normura and K. Kidena, ‘Characterization of Heavy Oil’, Petrotech (Tokyo), 1999,22,891 R62 1 S. Obika, ‘Syntheses and Properties of Conformationally Restrained Nucleosides and Oligonucleotides Analogues’, Yakugaku Zasshi, 2000, 120, 147 R622 T. Ohno, ‘Isotope Effect in High Temperature Superconductors’, Kotai Butsuri, 2000,35, 185 R623 M. Oikawa, W.-C. Liu, H. Furuta, T. Shintaku, H. Sekljic, N. Fukuda, Y. Fukase, Y. Suda, K. Fukase, S. Kusumoto and T. Kirikae, ‘Synthesis of Labeled Analogues and Functional Study of Lipid A, the Active Principle of Bacterial Endotoxin’, Tennen Yuki Kagobutsu Toronkui Koen Yoshishu, 1999,41, 133 R624 T. Ono, ‘Cu(2) NQRof Y1248 - Oxygen Isotope Effects’, Bussei Kenkyu, 1999,72,456 R625 M. Ono and Y. Wada, ‘Study on Substrate Specificity of Hydrolytic Catalytic Antibody’, YukiGosei Kagaku Kyokaishi, 1999,57, 867 R626 Y. Ono, ‘Design of Catalysts Having Superior Functions. New Development of Environmentally Benign Catalysts. Zeolites. Acidic Properties and Catalysis’, Kikan Kagaku Sosetsu, 1999,41, 35 R627 S. Saito, ‘Analyses of Polymerization Behaviors of Polyolefins by NMR’, Shokubai, 1999,41,586 R628 S. Sato, R. Takahashi and T. Sodesawa, ‘Observation of Formation Process of Ti(OPr)4-Stearic Acid Complex in the Preparation of Porous Titania by Using 13CMAS NMR’, Shokubai, 1999,41,582 R629 T. Shido, ‘Characterization of Zeolites Using MQ-MAS Solid-state NMR’, Shokubai, 1999,41,622 R630 H. Suzuki, T. Takemori and A. Inagaki, ‘Activation of Acyclic and Cyclic Conjugated Dienes in Cooperation with the Three Metal Centers’, YukiGosei Kagaku Kyokaishi, 1999,57,935 R631 Y. Suzuki, M. Ojika, Y. Sakagami, T. Yoshimura, R. Fudo and S. Yamanaka, ‘Structures and Biological Activities of Novel Antimicrobial Compounds, Cystothiazoles, from the Myxobacterium Cyctobacter Fuscus’, Tennen YukiKagobutsu Toronkai Koen Yoshishu, 1999,41, 582

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R632 K.-I. Suzumura, I. Takahashi, H. Matsumoto, K. Nagai, K.-I. Suzuki and N. Nagano, ‘Structural Elucidation of YM-755 18A-D, Novel Cytotoxic Macrolides Isolated from Pseudomonas sp. Q38009’, Tennen YukiKagobutsu Toronkai Koen Yoshishu,1999,41,397 R633 T. Takahashi and T. Erata, ‘Nuclear Magnetic Resonance’, Bussei Sokutei no Shinpo, 1997, 1, 1, ed. S. Kobayashi, Maruzen, Tokyo, Japan R634 K. Takegoshi, ‘Study of Phase Separation Process of Polymer Blend by Solid State NMR ’, Kobunshi Kako, 1999,48, 362 R635 S. Takekuma, ‘Chemistry of Fullerene. Inclusion Complex and Characteristic Properties’, Kikan Kagaku Sosetsu, 1999, 43, 197 R636 Y. Takemura, M. Ju-ichi, C. Ito and H. Furukawa, ‘The Structures of Acridone-Coumarin Dimers from Citrus Plants’, Tennen Yuki Kagobutsu Toronkai Koen Yoshishu, 1998,40,407 R637 A. Tokunaga, K. Ishida and Y. Kitaoka, ‘Pseudogap of Bi2Sr2CaCu2O8+,,from the Standpoint of NMR’, Bussei Kenkyu, 1999,72,444 R638 M. Tsuge, ‘Characterization of Formaldehyde Condensate Resins with Instrumental Analyses’, Nettowaku Porima, 1999, 20,2 16 R639 D. Uraguchi, T. Ooi and K. Maruoka, ‘Chemistry of Chelate-Type Hypervalent Boron and Aluminium. Utilization for Selective Organic Synthesis’, YukiGosei Kagaku Kyokaishi, 2000,58, 14 R640 H. Urata, ‘Effect of Chirality of Ribose on Nucleic Acid Structure and Function’, Yakugaku Zasshi, 1999,119,689 R641 H. Wada, ‘The World’s First Superconducting Magnets beyond 900 MHz’, FSST News, 2000,78,6 R642 K. Watanabe, ‘Pulse NMR’, Toyoda Gosei Giho, 1999,41,44 R643 Y. Yamaguchi and I. Shimada, ‘Novel Approach for Drug Design by Using NMR’, Tanpakushitsu Kakusan Koso, 2000,45,895 R644 K. Yamazaki and M. Suzuki, ‘The Three-Dimensional Structure of TAP and the Binding Mode of TAP to Factor Xa’, Nippon Kessen Shiketsu Gakkaishi, 1999, 10, 177 R645 S. Yamazaki, ‘Recent Developments in Solid-state NMR Related to the Applications to Inorganic Materials’, Shokubai, 1999,41, 595 R646 T. Yasumoto, M. Satake, Y. Onuma, T. Ukena and H. Naoki, ‘Structures and Etiology of Marine Toxins Involved in Fatal Poisoning’, Tennen YukiKagobutsu Toronkai Koen Yoshishu, 1997,39, 193 R647 N. Yoshii and S. Okazaki, ‘Structure and Dynamics of Supercritical Water and Aqueous Solution’, Netsu Bussei, 1999, 13, 84 R648 S. Yoshimura, ‘Drug Discovery from Natural Products’, J. Mass Spectrom. SOC.Jpn., 1999,47, 123 R649 Y. Yoshimura and M. Shirao, ‘Application to the Food Analysis of NMR’, Shokuhin Eiseigaku Zasshi, 1999,4O,J-318 Korean R650 J.-H. Yeo, ‘Utilization of Chemical Shift Tensor on Solid State Nuclear Magnetic Resonance; A Theoretical Approach and Application to Structute Analysis’, Anal. Sci. Technol., 1999, 12, 83A

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Polish R65 1 J. Chruschiel, ‘Progress in the Chemistry of Polymethylhydrosiloxanes’, Polimery ( Warsaw), 1999,44,462 R652 M. Stepien, ‘Molecular Symmetry and Stereochemistry’, Wiad. Chem., 1999,53, 17 Portuguese R653 M. R. M. Palermo de Aguiar, A. L. Gemal and R. Aguiar da Silva San Gil, ‘Characterization of Polymorphism in Pharmaceuticals by Solid State Nuclear Magnetic Resonance’, Quim. Nova, 1999,22, 553

Romanian R654 C . Luca and C. Rusa, ‘Inclusion Compounds of Urea and Perhydrotriphenylene with Macromolecules’, Rev. Chim. (Bucharest), 1998,49, 865 Russian R655 R. A. Cherkasov, N. A. Polezhaeva and V. I. Galkin, ‘Mechanism of Phosphorylation Reactions of Mucochloric Acid and its Derivatives’, Ukr. Khim. Zh. (Russ.Ed.), 1999,65,21 R656 S. G. Sakharov, ‘Isomerism, Dynamic Behavior, and Reactivity of do Transition Metal Complexes with n-Donor Two-Center Ligands’, Zh. Neorg. Khim., 1999,44, 1783 R657 V. D. Skirda and V. I. Volkov, ‘Pulsed-Field-Gradient NMR in Studies of Physicochemical Processes in Molecular Systems’, Zh. Fiz. Khim., 1999,73,362 R658 V. P. Tarasov, ‘Multinuclear Magnetic Resonance in Simple and Complex Halides, Oxides, and Hydrides’, Zh. Neorg. Khim., 1999, 44, 1863 R659 V. S. Urusov and D. Yu. Pushcharovskii, ‘Recent Achievements and New Horizons in Structural Mineralogy and Cristal Chemistry of Minerals’, Vestn. Mosk. Univ., Ser. 4: Geol., 1999,4, 3 Spanish R660 J. F. Lamus, ‘Trends in Enhanced Petroleum Recovery’, Vision Tecnol., 1999,6, 113 R661 S. Ramirez, A. Porta and N. Caffini, ‘Phytochelatins and Related Peptides. Structure, Matabolic Role and Environmental Applications’, Acta Farm. Bonaerense, 1999, 18, 53 R662 J. Tardaguila, J. E. Pardo, R. Gomez, M. Bartamini and A. I. Blanch, ‘Control of Wines Using Nuclear Magnetic Resonance (NMR)’, Aliment., Equipos Tecnol., 1998, 17, 79

2 Theoretical and Physical Aspects of Nuclear Shielding BY CYNTHIA J. JAMESON and ANGEL C. de DlOS

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Theoretical Aspects of Nuclear Shielding

1.1 General Theory. - A convenient method of doing relativistic calculations of shielding makes use of most of the machinery of non-relativistic ab initio calculations and introduces relativity via additional perturbative operators, representing the most important corrections: spin-orbit coupling and the scalar relativistic corrections (the mass-velocity and Darwin corrections). There have been many developments on this front in the past several years, particularly from the groups of Nakatsuji, Malkin and Kaupp, Fukui, and Ziegler. New developments and applications by these groups, with special emphasis on spin-orbit contributions are discussed below. All of the approximate relativistic methods used so far for calculations of shielding involve a formalism that may suffer from singularities in both the variational and perturbational approach. Precautions are taken in such instances, such as the use of a frozen core to avoid the variational instability of the Hamiltonian' and in the perturbation approach one has to go to third or even fourth order to obtain all the contribution^^'^. The time has come for fully relativistic fourcomponent approaches to shielding calculations. Pioneering work by Pyykko in the 1970s4, 5 did not have the benefit of the currently available ab initio machinery, however, ab initio calculations using four-component methods are beginning to emerge. A fully relativistic method, explicitly dealing with the four component Dirac-Coulomb-Breit many body Hamiltonian within the Dirac-Fock approximation, has been formulated in the random phase approximation (therefore neglecting electron correlation) by Visscher et aL6 The DHF (Dirac-Hartree-Fock) wavefunction is obtained and then the effect of additional interaction terms in the Hamiltonian are considered using response theory. This is a relativistic generalization of the RPA (random phase approximation) method. The quadratic response handles shielding calculations. Some of the terms included by Fukui are not taken into account in this four-component formalism, and the formulation also lacks gauge-origin invariance. The common gauge origin is used with the origin placed on the halogen nucleus for the computations involving the HX molecule. The method of including spin-orbit effects is identical to the one used by Vaara el al., as Nuclear Magnetic Resonance, Volume 30

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reported last year.7 As already found by previous approximate approaches, the relativistic effect on the shielding tensors in HX is large for both H and X nuclei. The anisotropy of the H shielding shows a large relativistic effect, coming entirely from the one-electron spin orbit interaction coupled to the FC interaction. The perpendicular component of the experimental spin-rota tion tensor is used to determine the perpendicular component of the paramagnetic part of the shielding in linear molecules, but the paramagnetic contribution to the parallel component of the tensor is neglected. While this vanishes by symmetry for linear molecules in the non-relativistic formalism, the paramagnetic contribution to the parallel component of the shielding for linear molecules is not zero in a fully relativistic formalism.' The identity that relates the spin-rotation tensor to paramagnetic shielding in the perturbed nonrelativistic treatment'"' has in fact not been proven for the fully relativistic case. For the isotropic shielding, the halogen nucleus in HX has very significant correction terms coming from the combination of the mass-velocity external-field and Fermi contact operators, which are larger than the spinorbit corrections.6 This is in agreement with the results obtained previously by others. A correction to the earlier reported' four-component Dirac-Fock finite perturbation theory for shieldings has been made.' The Gordon decomposition first proposed for shielding calculations by Pyper in 198313is used here. The current results for H shielding in the HX molecules are not quantitatively in as good agreement with experiment as the results of Visscher et aZ.,6 although the trends are entirely as expected. Wolff and ZieglerI4 have made use of the variationally more stable relativistic zero order regular approximation (ZORA) in calculations of shielding. Comparisons of the Pauli spin-orbit Hamiltonian and the ZORA Hamiltonian to GIAO-DFT calculations of 207Pband 83Wshielding provide a measure of the relative merits of the two approximate relativistic Hamiltonians and also reveal the relative importance of the scalar and spin-orbit contributions in these systems.I5 These calculations employ a frozen core for the Pb and use the BP86 functional with basis sets of Slater-type orbitals. Results from the ZORA method agree better with experimental chemical shifts than the results from the Pauli scheme for both 207Pband 183Wnuclei. These are the first calculations of Pb shieldings in which spin-orbit coupling has been taken into account. The range of chemical shifts in the tungsten compounds is about 8000 ppm and the ZORA calculations do reproduce the range and also the relative shieldings. The compounds chosen for the Pb calculations only span 1300 ppm. Discrepancies from experimental data should not be taken literally in these cases because intermolecular chemical shifts can be very large in condensed phases, especially for Pb (sometimes hundreds of ppm). The spin orbit term was found to be small in lS3Wbut dominate the trends in 207Pb.'5A similar comparison of the PSO and ZORA methods for 195Ptshows that overall chemical shifts are similar; the ZORA frozen core values are within 1020% of the values of the all-electron ZORA method, indicating that employing the frozen core does not drastically affect the results.16 The calculations find

'

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that the inverse energy gaps and the magnitudes of magnetic coupling matrix elements are both important in explaining the trends. Spin orbit contributions to the overall shielding changes with C1 to I as expected, but an interesting second trend is that trans compounds exhibit greater spin-orbit shielding than the corresponding cis compound, which can be attributed to a secondary effect resulting from ‘trans influence’ leading to different Pt-halide distances for cis and trans isomers. Second and third order spin-orbit contributions have been calculated by Vaara et aZ.17 for the halides HX, CH3X and for the H2Y compounds where Y = 0 to Te, using linear and quadratic response theory. In earlier work, these authors presented a fully analytical approach for the calculation of the oneand two-electron spin-orbit corrections through coupling with the FC and SD interactions using quadratic response theory.’ In this present work, they introduce a linear response formulation for the second order terms arising from the magnetic field dependence of the SO Hamiltonian, together with the quadratic response formulation of the earlier work to calculate the spin-orbit contributions. They find that the third order contributions to the hydrogen shielding are significant and qualitatively similar for the H2Y and the HX systems. For the heavy nuclei, the signs of the various terms are different for the two series, leading to different degrees of cancellation. The second order terms are larger in magnitude than the third order corrections for the heavy nuclei so that the net spin orbit effect on the heavy nuclei is therefore deshielding. Whereas electron correlation decreases the calculated third order spin orbit corrections, the second order terms are not affected. For comparisons with experimental data, especially for Se and Te, may I suggest to these authors that they use the value from experiment that corresponds to [o(H2Se) - o(free Se atom)] or [o(H2Te) - o(free Te atom)] for a better comparison with the corresponding value from their calculations? This would avoid altogether the problem associated with the approximate relativistic correction used for @free Se atom) and o(free Te atom) in formulating the absolute Se and Te shielding scales. l 8 Spin orbit terms for I3C and 19F shielding in the series CF3IF, have been calculated using third order perturbation theory with the SOS-DFPT scheme.’’ The overall SO contributions to shielding in CF3I are large (57.4 ppm) compared to that in CH3I (32.6 ppm). The FC mechanism benefits from the larger carbon 2s character of the I-C bond due to the electronegative F substituents. At the same time the 19F directly bound to iodine have relatively small SO terms since the I-F bonds have relatively little s character. There are recent developments towards finding a better functional for shielding calculations. The exact density functional remains unknown and the development of more accurate functionals remains a very active area of research. The search continues in the desire to provide a better description of the exchange correlation effects in molecules which display large exchange correlation effects on shielding. Current DFT models employed in shielding calculations include those that have been designed from pure mathematics mainly by Perdew and co-workers. These are parameter-free and they use the

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so-called generalized gradient approximation (GGA), i.e., they involve the density and its gradient. They do not include Hartree-Fock exchange (HFX). Examples of this type, that have been used in shielding calculations, are LDA, PW91, PBE. A hybrid functional has been fashioned from these, without any adjustable parameter, by using a fixed amount of HFX (PBEO, see below). Functionals of the other class, for which the pioneering work is due to Becke, can be of the GGA form or hybrid (contain an admixture of HFX) form. They are distinct from the first class in that they contain parameters which have been chosen such that Kohn-Sham calculations with them reproduce a selected set of molecular data. Functionals determined in this way that have been used in shielding calculations include the hybrid B3LYP functional and the B97 functional. A new GGA functional of the latter class, the HCTH (Hamphrecht, Cohen, Tozer and Handy) functional has been introduced.20 Hamphrecht et al. considered the performance of a wide variety of currently used functionals applied to shielding (H, C, N, 0, F) of small molecules. In a comprehensive comparison using hybrid and non-hybrid functionals they provide a fair assessment; the numerical results are directly comparable, since the molecular geometries used are identical, basis sets used are the same. The authors concluded that DFT is not yet competitive with ab initio correlated calculations. The hybrid functional results are about SCF quality, the GGA results are approaching MP2 quality but not as good as MP2. In an examination of the results, it is found that the large discrepancy observed between hybrid and non-hybrid functionals originated from differences in the numerator matrix elements rather than any significant deficiency in the eigenvalue differences.20 Subsequently, Wilson et al. did the same comprehensive comparison of a wide variety of functionals in the computation of shielding in transition metal complexes.21 In previous calculations by others on Fe, Co and V comp o u n d ~ , ~it~ had - ~ ~been found that for first row transition complexes, comparison of the calculated metal chemical shift ranges with experiment indicate that hybrid functionals generally produce similar or superior results to nonhybrid functionals. The latter tend to underestimate the magnitude of the paramagnetic contribution to the shielding. A proper understanding of why the inclusion of exact Hartree-Fock exchange in the hybrid functionals improves the metal shielding is crucial to the future development of improved functionals containing no exact Hartree-Fock exchange. Comparison of B3LYP, LDA, BLYP, BP86, BP91, HCTH, PBE leads to the following conclusion: Hybrid functionals provide a dramatic improvement to the metal shielding results over the non-hybrid functionals. Analysis of the results shows that the poor performance of the non-hybrid functionals originates from their underestimation of the numerator matrix elements rather than any deficiency in their description of the eigenvalue differences in the denominator of the paramagnetic shielding. The relative increase in the magnitude of the denominators following inclusion of a fraction of HFX is less than the corresponding increase in the numerator matrix elements. The net effect is a significant increase in the paramagnetic shielding terms for hybrid func-

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Nuclear Magnetic Resonance

tionals. In this case, introducing the Malkin correction to HCTH worsens the results by making the values even more shielded.21 On the other hand, the ligand shielding are well accounted for by the GGA functionals, including HCTH. Two improvements have been subsequently proposed by these authors. In the first,25they consider making a correction that will force the functional to approach the correct asymptotic behavior. The long-range behavior of all GGA exchange-correlation potentials is incorrect. They derive the expression for the correct asymptotic behavior and join up the correct asymptotic behavior with the GGA potential by empirically choosing the asymptotic form outside the molecular region and the GGA potential inside. This exchange correlation functional is called HCTH-AC (for asymptotic correction). The results are compared with HCTH and also with other GGA functionals for H, C, N, 0 and F shielding in a benchmark set of small molecules. The asymptotic correction improves only the higher lying valence and Rydberg virtual excitations rather than the more important low energy valence ones. The addition of the asymptotic correction has only a small effect in shielding calculations (although it has a dramatic effect in electric dipole polarizability calculations). The difficulty lies in the DFT description of the higher occupied valence orbitals and the lower unoccupied orbitals. The Malkin empirical correction to the eigenvalues does help, but Wilson et al, do not find theoretical justification for it. Even with the asymptotic correction introduced here, present DFT does not offer a significant improvement over regular H F calculations for shielding. In other words, even when the correct asymptotic behavior is imposed, the present exchange correlation functionals which are determined from energy criteria are not optimal for magnetic properties, and these authors doubt that the reasons lie with the neglect of current dependence in the f ~ n c t i o n a l . ~ ~ Their second attempt for improvement26 is a scheme that assumes that the correlation parts of the hybrid functionals are satisfactory but that the nature of the virtual orbitals critically depends upon the exchange part. Accordingly, in the uncoupled expression that arises from GGA theory, they use what they consider to be more optimal orbitals: hybrid Kohn-Sham orbitals and eigenvalues with an adjusted exact-exchange coefficient Cx. The procedure works for both B3LYP and B97 hybrid functionals, with coefficients obtained empirically, Cx = 0.05 for B3LYP and 0.04 for B97. The results for C, N, 0, F, P, S and C1 shielding in a large set of molecules, including the usual pathological test systems, are compared across the board with SCF, B3LYP, B97, HCTH, and also with the best ab initio results available (for some, these are Gauss’ CCSD(T) results,27 and for others CCSD,’* or CCD,29 or MCSCF3*). It appears that the empirically adjusted hybrid functionals (using Cx = 0.05 and 0.04) provide occupied and unoccupied Kohn-Sham orbitals that are appropriate for the calculation of shielding.26 An examination of how well the parameter-free density functionals PBE and PBEO do in shielding calculations (13C, 15N, and 1 7 0 , using the usual set of benchmark molecules), reveals that they are competitive with published MP2 results and better than published B3LYP results, in comparison with experi-

2: Theoretical and Physical Aspects of Nuclear Shielding

51

ment. PBEO is found comparable to B3LYP for 170shielding in O3 molecule, where MP2 fails completely.31732 The ONIOM method of doing molecular calculations subdivides a molecule into several parts or layers, each of which is described at a different level of theory. The innermost layer is described at the highest level of theory. The local nature of nuclear shielding implies that shielding calculations may be efficiently done in this way for very large molecules, with the nucleus of interest in the innermost layer. Tests of this conjecture are offered by Karadanov and Morokuma in a few molecules, the largest being n a ~ h t h a l e n e .These ~ ~ are tough tests since layering fluorobenzene on naphthalene involves disrupting the ring system. In the latter, there is a five-fold saving in CPU time and a sixfold saving in scratch space, compared to conventional approaches that treats the entire molecule with the same high level of theory. Common sense dictates that the innermost layer has to include atoms directly bonded to the nucleus whose shielding is being calculated. Mauri, Pfrommer and Louie (MPL) developed a method to compute the shielding in extended condensed matter systems using periodic boundary conditions, which permits computations on truly infinite systems.34735 A recent example of the use of the MPL method is an examination of the 13C shielding in the surface atoms of diamond for a hydrogen terminated (1 11) surface. The presence of electronic surface states modifies the shielding of surface C atoms from that of the Until recently, only pseudopotentials and plane waves had been used. In this reporting period, the method is extended to an isolated molecule in free space by considering a periodic lattice with a molecule per unit cell and then letting the volume of the unit cell tend to infinity.37 Then, the method is extended to atomic orbital basis sets so that it may be applied to calculations of shielding in molecular systems. Further, a new distributedorigins method is proposed, different gauges for core and valence electrons or individual gauge for core and valence states (IGCV). In this method the authors use IGAIM for the induced first order electronic current for the core and CTOCD (continuous transformation of the origin of the current density at each point r in real space, gauge origin d(r) = r for the valence contribution. Comparing within the DFT LDA scheme the new distributed origins with the standard ones, they find that core-valence transitions are negligible or else nearly independent of the chemical environment around the nucleus in question. Since the convergence properties of the core contributions are poor in the MPL method, they propose using the MPL method with atomic orbital basis sets for the valence contribution and then simply adding to it a rigid core contribution obtained from an atomic calculation. They say their results show that the core contribution to the chemical shift is independent of the chemical environment. This is contrary to what is found by others (for example, refs. 38 and 39). However, in MPL, the core contribution to the first order current is described within the full basis set, as also is the valence contribution. That is, all (core) --+ (valence} + {empty] transitions are included in the former, and all (valence} + {core}+(empty] in the latter, leading to a gauge-invariant separation of core and valence contributions.

52

Nuclear Magnetic Resonance

1.2 Ab initio Calculations. - An excellent comprehensive and insightful review of ring currents by Lazzeretti puts the massive literature on the subject into context!' Recent calculations, especially those that permit the visualization of electron currents generated by a magnetic field at various planes through a molecule, leave little room for conjecture about whether the currents are diamagnetic or paramagnetic, and 7c electron contributions to current densities can be separately viewed. The relationship of the proton chemical shifts and anisotropies of magnetizability to the ideas of ring currents or aromaticity are critically discussed using as basis the highest level ab initio calculations of current densities, including those by the method CTOCD (continuous transformation of the origin of the current density). This latter method amounts to choosing a function d(r) = r instead of a constant, and was proposed originally by Keith and Badere41 Current density maps and shielding tensors for anthracene, phenanthrene and triphenylene have been calculated using the CTOCD method.42 The variant of this method, in which the origin of the current density is shifted toward the nearest nucleus for points close to nuclei, in particular the CTOCD-PZ2 variant, improves the predictions even for modest basis sets. The comparison with experimental shielding tensors is reasonably good, and the isotropic values are even better. The current density maps obtained do yield descriptions of the actual n electron flow induced by an external uniform magnetic field in these systems, and these cannot be deduced on the basis of the very simple ring current model. DFT calculations of shielding tensors of heavy nuclei ranging from IS3Wto 51Vand 31Phave been reported.4349 Effective core potentials and the B3LYP functional were used for PW120M3- and 12 smaller molecules having W as the central atom. The results are disappointingly poor with a correlation slope of about 0.05, that is, calculations are able to recover only about 400 ppm of the experimental chemical shift range of 8440 ppm in the set.43 Buhl reported calculations of Io3Rh and 95M0 shieldings. Unfortunately, rapidly interconverting species in solution do not permit determination of individual Io3Rh shielding for the species,44 while the 95M0 shieldings indicate that electron correlation may be significant, but various DFT methods give different results, with the BPW9 1 functional providing the best overall agreement.45 77Se shielding calculations by Nakanishi in ten small molecules, using the B3LYP functional, leads to reasonable recovery of the chemical shifts range, improving with basis set size.47An investigation of the effects on shielding (to higher shielding) from the small bond angles in the three-membered ring of cyclic trans triphosphirane (P3H3) and comparison with the hetero-analog (CH2)(PH)2used DFT-GIAO B3LYP.49Other 31Pshielding have been calculated without electron correlation, in phosphaalkenes X-P=CY2 using GIA0,50 and in P406S and P407 using IGLO." A review of 3'P shielding calculations has been published by Chesnut and Q ~ i n . ~ * 1 7 0 and 5N shielding tensor components observed in metalloporphyrins have been modeled by DFT-GIAO calculations using the BPW91 functional on smaller analogues with surprisingly good agreement.48DFT calculations of

2: Theoretical and Physical Aspects of Nuclear Shielding

53

9F shielding in halogenated ethanes and pro pane^,^^ in cyclohexyl fluorides,54 and in CF30X, X being F, C1, or Br," led to mixed results. Some experimental chemical shifts are reproduced better by B3LYP, others better by HartreeFock level calculations, using the GIAO method of distributed origins. Although the 19F absolute shielding scale is well-established, these authors compare their calculations with experimental chemical shifts. Furthermore, there are some problems with conformational averaging and optimum molecular geometry in these reported systems. The I9F shielding in the solid alkali metal fluorides have been modeled by calculations in a cluster of nearest neighbors [M6F15+? The results are not very good, with B3LYP and BLYP providing the best slope in the correlation of calculated values with chemical shifts observed. They also showed with the LiF system, that using a larger cluster including next nearest neighbors does lead to different (by about 45 ppm) answers for the 19F shielding. 29Sichemical shifts in a series of 26 simple compounds are found to be more completely recovered by H F than by DFT calculations using the B3LYP f ~ n c t i o n a l A . ~ ~comparison of several hybrid density functionals in their ability to reproduce experimental 13C chemical shifts in a series of 18 small molecules, with mostly CH3 and CH2 sites (and two C=O sites)58leads the author to recommend one of the Adamo-Barone one-parameter hybrid functional^.^^ The set of molecules used is not sufficiently inclusive of the many types of 13Csites (the calculations after all did not do well for the two C=O sites) to judge the various functionals in their ability to reproduce chemical shifts of carbon. Orbital analysis of the results of DFTIGAIM calculations of the 13C shieldings in acetylene, allene and higher cumulenes provides some insight into the shielding contributions in these linear and pseudo-linear systems.60Ruud and co-workers take a DFT look at all the nuclear shieldings in the formaldehyde molecule and compare the results with Hartree-Fock calculations at various basis set sizes. The results are extrapolated to the large basis set limit.61The proton shielding in a set of 80 small molecules having a C-H bond have been subjected to DFT calculations using 15 different procedures (GIAO and IGAIM coupled with various functionals and various geometry-optimization levels).62 The results do not agree with experiment in an absolute numerical sense, but require linear scaling. Of course the authors are using condensed phase experimental data and have no right to compare directly with experiment without making appropriate corrections due to rovibrational averaging and intermolecular effects. Thus, the scaling has only a cosmetic effect. They do find that the GIAO and IGAIM methods converge to the same limit as the basis set size is increased (no surprise there) and that GIAO suffers less degradation in quality with smaller basis sets than does IGAIM. Correlation with experiment is slightly better for B3LYP, but B3P86 or B3PW91 do nearly as DFT-GIAO calculations of "N, 13C, and 'H shielding in a variety of compounds have been reported. Examples are N-containing heterocycle^,^' ortho-substituted fluorobenzenes," estrone, a steroid hormone,65 glucose and related compounds,66mannose and m a n n ~ b i o s eC30H , ~ ~ 12 bowl-shaped hydrocarbons,68 anhydrodeo~ythymidines,~~ l-chl~rosilatrane,~~ chlorinated di-

Nuclear Magnetic Resonance

54

ben~othiophenes,~' and some CH3-0-X compounds.72 DFT-IGLO calculations of I3C shielding in carbodication and ally1 cation^,^' and DFT-GIAO of I'B shieldings of carborane anions74 were used to support the proposed existence of the ions in solution. CHF-GIAO calculations of I5N, I3C, and 'H shieldings were reported in uracil derivative^,^^ nitro-derivatives of p y r i d i n e ~ , ~ ~ derivative^,^^ and fluoropyrazoles. 79 azolopyridines and p y r i d a ~ i n e samidine ,~~ MP2-level calculations of 'B shielding in protonated borane-Lewis base complexes have been carried out .80 DFT-GIAO calculations of the I3C and 7Li shielding, as well as the nucleus independent shielding at the center were reported for the poly-lithiated fullerene, 1.3 Semiempirical Calculations. - An implementation of semiempirical calculations of chemical shifts has been developed in the MNDO framework using GIAOs and analytic derivative theory,82 to provide a general purpose semiempirical method for calculations of 'H, 13C, I5N and I7O chemical shifts in molecules of interest to organic chemists. A reference set of 97 molecules were used to provide parameterization and the method was tested on a larger set of nearly 400 molecules. Three-center terms contribute only a few ppm to the total chemical shifts, but are found to be essential for a qualitatively correct description of 'H chemical shifts. 2

Physical Aspects of Nuclear Shielding

2.1 Anisotropy of the Shielding Tensor. - As the relevance of shielding anisotropy becomes more evident in structure elucidation, the number of measured shielding tensors, in which both magnitude and direction of principal components, continues to increase in this reporting period. Growth in this area has been made possible by several techniques, that do not require single crystals, that have become available for shielding tensor determinations. For example, Dusold et ~ 1 have . employed ~ ~ lineshape fitting of rotational resonance MAS NMR spectra in determining the values and the principal axis system (PAS) of the 13C shielding tensors in the monoammonium salt of the four-13C spin system maleic acid. Their results show that for the two inequivalent I3C sites, the most shielded component lies nearly perpendicular to the molecular plane. The direction, however, of the intermediate component deviates considerably from the molecular framework. For instance, 0 3 2 ~ of C2 lies 23" from the C2-C3 bond. Dusold et have demonstrated that such deviations are significant and are necessary to take into account if the tensor orientation is to be used in the determination of internuclear distances. Assumptions of typical orientation can lead to errors of as much as 0.1 Another technique that allows for the extraction of the shielding tensor axes is based on combined rotation and multiple pulse spectroscopy (CRAMPS).84 Rasmussen et nl.85applied this pulse sequence and determined the magnitude and PAS of the 'H shielding tensor of the hydrogen bonded proton in

A.

2: Theoretical and Physical Aspects of Nuclear Shielding

55

KH2P04. Noteworthy to mention is the direction of the least shielded component, four degrees from the direction of hydrogen bonding. In the following, we use the following definitions used by many solid state NMR spectroscopists, where 0 3 ~2 022 2 01 1, span, i2 = (033 - 01 1) skew, K 3(0i,, - ~ 2 2 ) / ( 0 3 3 - 011) The orientation of the shielding tensor in quadrupolar nuclei continues to gain attention in this reporting period. Medek and Frydman86 have recently proposed a two-dimensional experiment which correlates multiple and single quantum coherences of half-integer quadrupolar sites, a multidimensional extension of the multiple quantum MAS (MQMAS) introduced by Frydman and Hanvood five years ago.87 The 2D spectrum obtained is sensitive to the relative orientation between the electric field gradient (efg) and shielding tensors. Maximum sensitivity is achieved when correlating the highest, - S to +S, multiple quantum transition against the central, - '12 to '/2, transition, and keeping the sample stationary. Representative data have been obtained for 87Rb(S = 3/2) and 59C0(S = 7/2)." Some of the contributions from the Wasylishen group during this year involve less studied nuclei. One of these works illustrates the first evidence for anisotropic shielding of 9Be.88The molecule studied is Be(acac)2, which is tetrahedral when isolated, but in the solid, appears to be distorted. A span of about 3-7 ppm is estimated for the 9Be shielding tensor in this compound. Ab initio calculations at the Hartree-Fock level with a 6-31 1++G(3df,3pd) basis set provide an estimate of about 7-9 ppm. Additional computations on a series of molecules containing Be indicate that paramagnetic contributions are significant and in some cases, the spans can be greater than the total known isotropic shielding range for 9Be, which is about 50 ppm. Another first example of anisotropic shielding has been demonstrated for 63'65Cu in [ C U ( C N ) ~ ] ~A- . span ~ ~ of 42 ppm is observed with the least shielded (and unique) component lying along the C3 axis of the tetracoordinated Cu. The observed span arises from a minor departure from tetrahe$ral symmetry. Along the unique axis the Cu-C bond is slightly longer (2.014 A), compared to the three other bonds (1.992 In addition, the three Cu-C-N bond angles deviate by about six degrees from being linear. 31Pshielding tensors in phosphole tetramer" and various phosphine and phosphite substituted cobaloximes'l have been examined. The phosphole tetramer is shown to be composed of two 31Pisolated spin pairs, thus amenable to the dipolarxhemical shift technique. However, the dipolar tensor turns out to be axially symmetric, precluding unequivocal assignments of shielding tensor orientations. By combining results from ab initio computations and the 2D spin-echo experiment, the ambiguities have been resolved. The spans (1 15 ppm) and skews (0.70) for all four three-coordinated P are the same within experimental error. The most shielded component lies approximately 78" from the P-P bond and resides in the phosphole ring plane. The study on cobaloximes,9' on the other hand, yields the following trends: The 31Pnucleus in phosphines becomes deshielded

A).

56

Nuclear Magnetic Resonance

upon coordination. In contrast, P in phosphates becomes shielded and, in addition, their span increases. Ab initiu calculations have been performed on 27Alshielding tensors in the linear Al(1) compounds, AlH, AlNC, AlCl and AlF,92 which are then compared against recent spin-rotation data. The calculated shielding components that are perpendicular to the bond axis turn out to be correlated to the HOMO-LUMO gap in these linear molecules. Results from other laboratories include the determination of the shielding tensor orientation with respect to the efg tensor of 'lV in BiV04,93 the measurement of the principal values of the 77Seand 13Cshielding tensors from CPMAS spectra in ~elenomethionine,~~ and the uncovering of 99Hgshielding tensor principal values in series of HgZX2 where X = C1, SCN, NCO, CH3C02, and CF3C02.The 77Sesolid state spectrum of L-selenomethionine94 indicates that, similar to L-methionine, there are two inequivalent molecules per unit cell, which give rise to two resolvable tensors. The difference in the 77Seisotropic shielding between the two molecules is 38.6 ppm. In terms of the principal values, these differences can be traced to the least and most shielded components. From the 199Hgstudies:' it becomes apparent that, in contrast to the Hg(I1) compounds, the anisotropy and asymmetry of the 199Hg shielding tensor in Hg(1) compounds are insensitive to the nature of X. The dipolar-chemical shift technique has been applied to determine the orientation of the 5N shielding tensors in polymorphous p ~ l y g l y c i n e . ~ ~ Results indicate that the orientation of the N-H vector with respect to the PAS of the "N shielding tensor differs between the two dominant forms of polyglycine, PGI (p-sheet) and PGII (31-helix). In PGI, the N-H bond lies 16" off the plane defined by the two shielding components, 0 1 1 and 022. This observation can either be explained by a departure of the N-H bond from the amide plane or a departure of the intermediate shielding component from the amide plane. Previous experiments of Lee et aZ.97 and theoretical predictions 2 away from by Walling et aZ.98have indicated that the latter reason, ~ 2 tilting the amide plane, is more likely. Variable temperature measurements of shielding tensors prove to be very useful in elucidating the dynamics of coronene and c o r a n n ~ l e n eAt . ~ ~room temperature, the powder pattern displays a motion-averaged tensor. Lowering the temperature to 100 K freezes the motion and upon comparison with the room temperature spectrum, helps in determining the PAS orientation of the shielding tensor of the bridgehead carbons. Corannulene, a moiety of fullerene, contains a five-fold symmetry axis, about which the molecule rotates at room temperature. After comparing the room temperature and 100 K spectra, the most shielded component of the inner bridgehead carbons is found to lie 13" from the rotation axis, while that of the outer bridgehead carbons lies 26" off the axis. This departure from the rotation axis can be explained by the fact that corannulene is not planar and that 0 3 3 follows the direction of the p orbitals involved in the n bonding. Coronene, on the other hand, indicates a departure of the most shielded component from the rotation axis. In this case, the departure is due to an out-of-plane component of the motion. Sen et al.,"' also employing variable temperature measurements, have investigated bond

2: Theoretical and Physical Aspects of Nuclear Shielding

57

rearrangements in Li3P7.At temperatures lower than - 30" C, the rearrangement is quenched, leading to three subspectra (one for each of the following P sites; apical, equatorial and basal). On the other hand, at higher temperatures, a single average and axially symmetric tensor is observed. By using the PAS obtained from ab initio calculations and assuming a rearrangement similar to the Cope rearrangement, the exchange and motion-average spectra can be successfully simulated. The Oldfield group continues to study porphyrins, addressing the ruffling issue.lo' To this end, various six-coordinated octaethyl and tetraphenylporphyrins have been synthesized. 13C, I5N and 1 7 0 shielding tensors for these porphyrins have been measured in addition to their X-ray structures being determined. DFT calculations reproduce the observed shielding tensors and a set of rules addressing the ruffling issue has been defined. Harper et aZ.lo2have used ab initio calculations in assigning 13Cshielding principal values for the 15 carbon sites in parthenolide. Parthenolide crystallizes in such a manner that two inequivalent molecules exist per unit cell, leading to 29 resolvable frequencies (one pair is degenerate). Using the hybrid density functional method, B3PW91, nine of these pairs can be successfully assigned. Since the computations only involve isolated molecules, it can be deduced that the shift differences between polymorphs can be attributed solely to intramolecular effects. Rich et allo3have compared Hartree-Fock and DFT-B3LYP calculations in predicting the 13C shielding tensors in the two antibiotics, ampicillin and penicillin-V. B3LYP results are closer to experimental numbers. However, in reproducing conformational effects, Hartree-Fock performs equally well. Lastly, Separovic et aZ.lW have illustrated an application of the shielding anisotropy of an indole carbon in determining the orientation of tryptophan residues in gramicidin A.

2.2 Shielding Surfaces and Rovibrational Averaging. - The shielding at a particular nuclear site is intimately related to the distances and angles it makes with its neighboring atoms. Information regarding this dependence can therefore be utilized in structure elucidation. For example, Kikuchi et aZ.105have begun using the known dependence of NMR chemical shifts on secondary structure in qualitatively analysing the structure of four homologous IgGbinding domains of staphylococcal protein A. How a given NMR shielding varies with geometry is described by a mathematical surface, a shielding surface. Most shielding surfaces are normally obtained via ab initio methods as one can freely vary the geometry and then calculate the shielding. For some systems, experiments can be used to derive empirical shielding surfaces. During this reporting period, examples of both can be found in literature. Correlated multiconfigurational self-consistent field (MCSCF) shielding surfaces for acetylene have been constructed by Wigglesworth and coworkers.lo6 Several unexpected trends from this work merit mentioning. For instance, these calculations show that the 13Cshielding of C1 is six times more sensitive to variations in the ClC2H2 angle than it is to changes in the HlClC2 angle. Furthermore, the shielding at C1 is likewise more sensitive to

58

Nuclear Magnetic Resonance

the C2-H2 bond length than to Cl-H1 in the region of small displacements. Moreover, the bond lengths, C 1-H 1 and C2-H2, also have opposing effects on the C1 shielding. Lastly, the dependence of the C1 shielding on C1-HI is nonmonotonic. A maximum value is observed at a compressed value of this bond length, which is expected as the shielding of N in HCN (the united atom) is negative. The calculations compare favorably with experiment. The computed shieldings at 300 K are 117.59 ppm for 13C, to be compared with the experimental value of 116.9 ? 0.9 ppm, and 29.511 ppm for 'H, to be compared with the experimental value of 29.277 ? 0.001 ppm. The sensitivity at C l to the geometry modifications at C2 is not entirely unexpected; this has been suggested by the observed isotope shifts in this molecule. ~ attempted to determine experimentally the depenAsakawa et ~ 1 . " have dence of 13C shielding principal values of amino acids on the secondary structure of peptides and proteins. These experiments are compared to results derived from GIAO calculations. It is observed that the least shielded component of Cp is especially sensitive to secondary structure, an observation reproduced by the GIAO calculations. The C, site similarly shows that its least shielded component is most sensitive to secondary structure. Calculations, however, suggest that all three components are influenced by the dihedral angles. For carbonyl carbons, the intermediate component is shown to be very susceptible to hydrogen bonding. Wei et al. lo* have demonstrated experimentally and theoretically the dependence of the principal components of the I5N shielding in imidazole as a function of the hydrogen bond (donor-acceptor) distance (N-0). In this work, the intermediate component shows the greatest sensitivity to hydrogen bonding. Low temperature NMR has been applied to the study of the dependence of shielding on hydrogen bond di~tance.'~'In this study, the "N shieldings in pyridine I5N and a variety of acids dissolved in a deuterated freon solvent mixture are measured. It is evident in these systems that the "N shielding likewise correlates with N-0 distance. In other systems, geometry factors other than hydrogen bond distances can influence the observed shielding. Valerio et aZ., using an Si(OSiH& model and the SOS-DFT-IGLO method, reproduces well the observed correlation between the shielding of 29Siand the Si-0-Si angle. lo Absolute values closer to experiment can be obtained by employing a larger cluster, R3Si-O-SiR3, where R is OSiH3. These exercises suggest that if one is only interested in a particular geometry dependence of the shielding, a smaller fragment can sometimes suffice. Shimizu et aZ."' have investigated the dependence of the I3C shielding in the biphenyl moiety of dimer liquid crystals on the torsion angle between the phenyl rings. The systems studied in this work are a,wbis[(4,4'-cyanobiphenyl)oxy]alkanes with seven, nine and ten carbon chains. Based on the measured 13C chemical shifts and ab initio calculations, the torsion angle between the phenyl groups in the seven- and nine-carbon-chain molecules is about 40". On the other hand, the nine-carbon-chain alkane exhibits conformers with torsion angles of 30" and 60". 31P shielding continues to be a subject of interest in Chesnut's group. Chesnut and Quin"* have recently examined the angle dependence of 31P

2: Theoretical and Physical Aspects of Nuclear Shielding

59

shielding in phosphines. Experimentally, it is generally accepted that as the bond angle increases, deshielding occurs. There are, however, known exceptions. The dependence of the shielding on bond angle is shown not to be monotonic, an observation already noted years ago.' l 3 Traces on the shielding surface for central atoms in small molecules, showing the dependence on the bond angle, predict a minimum at angles close to the tetrahedral value. The dependence of 31P shielding on bond length has likewise been studied by Dransfeld and Chesnut''4 in H2P-EH, (where E goes through the first two rows of the periodic table). In this study, the shielding derivative of 31Pwith respect to the P-E bond length is found to correlate with the electronegativity of E. 2.3 Isotope Shifts. - There are two reviews published in this time period on this topic. One illustrates the use of isotope effects on chemical shifts in structural studies of intramolecular hydrogen bonded compounds.' Another one surveys progress in the utilization of solvent isotope effects in proteins.'16 Bolvig et al. ' I 7 have tabulated primary tritium and deuterium isotope effects in 55 compounds that have at least one intramolecular hydrogen bond. In this series of compounds, the tritium shift is shown to be approximately 1.4 times bigger than the corresponding deuterium shift. This factor is as it should be; the mass dependence of the isotope shift was predicted by Jameson,'I8 and the ratio of tritium-induced isotope shifts to deuterium-induced isotope shifts, in particular, was given by Jameson and Osten'" in general for tritium to deuterium ratio as {(mT- mH)/mT)/((mD-mH)/mD},or 7.893/5.533 = 1.43 by explicit calculations for the methane molecule. Isotope effects arising from tautomerism can be discriminated from localized isotope shifts by the former's stronger temperature dependence and negative sign. In addition, it has become apparent that the observed localized primary shifts correlate well with the secondary two-bond deuterium-induced isotope shift on 3C, suggesting its potential use, similar to the two-bond isotope shifts, for measuring hydrogen bond strengths. Deuterium isotope effects have been measured for o-hydroxythioamides, 2A-thiazoline and 5-acyl-2-thiobarbituric acids.120 In this study, steric effects present in the amide are found to be responsible for isotope shifts of opposite sign. Another isotope shift study involves the measurement of secondary deuterium isotope effects on I7O shielding in o-hydroxy acyl aromatics and P-diketones.12' Lastly, Heckmann et have observed large (hundreds of ppb) one-bond secondary 13C and two-bond secondary l 8 0 induced isotope effect on 31Pshielding in [(H3C)2CH]3Si-O-C =P. There are several works published in this reporting period that illustrate applications of isotope shifts. Sprangers et al. 123 have found a correlation between the three bond H/D isotope effect on the chemical shift of protein C' sites (3Au(ND)) and the backbone dihedral angle Y , Use of the isotope shifts as additional restraints in the structure determination leads to a significant improvement in the quality of the NMR structures for the a-Spectrin SH3 domain. Hobley et aZ.124have used long-range D-induced secondary isotope effects on I3C to probe hydrogen bond connectivities in spiropyrans and their

'

'

60

Nuclear Magnetic Resonance

merocyanines. A potential tool for assessing solvent accessibility in proteins and nucleic acids is suggested by a recent determination of H2I80 solvent induced isotope shifts in 19FNMR.'25 Downfield shifts of 0.0014-0.0018 ppm have been observed for fluorobenzene, 4-fluorophenylalanine, 3-fluorophenylalanine, 3-fluorotyrosine, 5-fluorotryptophan and 5-fluorouridine. Of course, isotope shifts cannot be divorced from a discussion of shielding surfaces and rovibrational averaging. Such vibrational averaging over the shielding surfaces for acetylene in the work of Wigglesworth et aZ., mentioned in the previous section, yields average shielding values for all isotopomers, thereby providing the isotope shifts that can be directly compared with experimental values. This was an excellent, comprehensive treatment that finally provided the theoretical basis for the curious isotope shifts in the acetylene molecule. From this theoretical work, it is shown that calculations using correlated shielding surfaces and ab initio potential surfaces yield isotope effects that are within 10% of the experimental values.

2.4 Intermolecular Effects on Nuclear Shielding. - Intermolecular effects on shielding can be categorized into two different types, specific and non-specific. Non-specific contributions to the shielding can be simulated by regarding the solvent as a dielectric medium. Mikkelsen et aZ.'26 have extended such treatment to a multiconfigurational Hartree-Fock description of the solute molecule. As in other cases, close-range interactions such as hydrogen bonding are not satisfactorily reproduced by this approach. In this recent paper, the shieldings in H2S and HCN are studied as a function of the dielectric constants exhibited by a wide range of solvents. For specific interactions, it is necessary to incorporate actual solvent molecules in the computations. Ludwig et 127,128 continue to apply the quantum cluster equilibrium (QCE) theory to reproduce chemical shifts in liquids. In these particular studies, the chemical shifts of 'H and 1 7 0 in liquid ethanol are evaluated as a function of temperature. It has become apparent, based on the observed shieldings and other parameters compared against the QCE calculations, that cyclic tetramer and pentamer structures represent the principal components in liquid ethanol. The most popular approach in reproducing NMR shieldings in condensed phases, where specific interactions are present, still involves computations on a cluster of atoms that incorporates a sufficient number of the neighboring atoms contributing to the observed chemical shift. Of course, two questions remain, how big should the cluster be and would the computations be affordable. The procedure is straightforward when the specific interaction exists within a single molecule. In this case, computations on an isolated molecule should suffice. As illustrated by Afonin et u Z . , ' ~ ~ the polar 0-CH3 of anisole induces a dipole on the highly polarizable n: bond, which causes a greater shielding of the cis-ortho-carbon in the aromatic ring. A similar scenario is observed for 'H, 13C and 15N shieldings in molecules containing a vinylic hydrogen close to a basic nitr~gen.'~'The situation becomes more complex when the interaction concerns two separate molecules. Wei et dio8 studied the effect of hydrogen bonding on the "N shielding tensor in

2: Theoretical and Physical Aspects of Nuclear Shielding

61

imidazole. Calculations relating the hydrogen bond distance to the observed principal values are able to reproduce the trends seen in various compounds containing the histidine residue. Networked solids are very challenging systems for shielding calculations. To~sell'~' has demonstrated that it may be sufficient to use small model cluster compounds in predicting 23Nachemical shifts in silicates and aluminosilicates. Anhydrous sodalite, for example, is represented by a Na+ ion enclosed by Si606H12 arranged in D 3 h symmetry. With this small cluster, the 23Nanucleus is calculated to be shielded by 4.5 ppm compared to a Na+ octahedrally coordinated to six water molecules, in agreement with the observed 1.7 k 4 ppm shielding of 23Na in sodalite compared to an aqueous solution of Na+. Another Na+-centered cluster of formula NaSi506HIz3- is used to represent crystalline sodium silicate Na2Si03. This calculation produces a deshielding of 16.1 ppm compared to the hexaaquo Na+ ion, which again compares favorably with experiment, -23 k 3 ppm. The additivity of ligand contributions to the 23Na shielding is examined and proved to be useful in approximating 23Na shielding values in twelve different sites in silicate and aluminosilicate materials. In this additivity approximation, it is necessary to take into account the Na-O distances, in addition to the coordination number. Fiircasiu and H i i n ~ uhave ' ~ ~ studied I7O NMR chemical shifts in hydronium and dihydroxonium ions in their fluoroborates. Using isolated fragments of H30+BF4- and H502+BF4-, the expected deshielding of I7O in the dihydroxonium ion is reproduced. The calculated shielding is found to be sensitive to the relative orientation of the two ions. Better agreement with experiment seems to be achieved when two of the F atoms are hydrogen bonded in H502+BF4- and a pyramidal form of H30+BF4- is assumed. Adding to the ample number of measured solvent-induced chemical shifts are new results for azo dyestuff^,'^^ urea systems,134and thiourea systems.13' Ring-current effects in fullerene continue to be a topic of interest. Endohedral and nucleus-independent chemical shifts are studied for selected lithiated fullerenes,81 fluorinated fullerenes,136 fullerene cation, and arylated fullerunfortunately still use the dispersion model in enes.137 Ylihautala et aZ.'387139 explaining the observed anisotropy of shielding in rare gas atoms in liquid crystals. Vuissoz et aZ.lM have offered an explanation for the observed dependence of 13Cshielding of CO chemisorbed on Pt and Pd electrodes on the electromotive potential. The observed derivatives of the shielding with respect to voltage are 71 ppm/V in Pt and 136 ppmN in Pd. Upon comparison with calculated shielding polarizabilities, these observed values are found to be too large to be attributed to electrostatic effects. Measurements of longitudinal relaxation times provide the clue, as the relaxation times are also observed to change with voltage. Thus, the variations in shielding in these systems are now attributed to changes in the Knight shift, indicating that there is a charge transfer process occurring between the electrode and the adsorbed CO. In addition to the difficulty in incorporating specific interactions, the presence of motion can make the problem in condensed phases intractable.

62

Nuclear Magnetic Resonance

Progress in this area is evident in some of the recently reported results. Pfrommer et al.,14' employing their periodic band theoretical approach, have recalculated the NMR chemical shifts in ice and liquid water. These calculations have combined a DFT approach in calculating the chemical shifts and an a6 initio Car-Parrinello molecular dynamics simulation. The results compare favorably with experiment as they correctly reproduce the observed anisotropy (about 34 ppm) of the 'H shielding, similar to the results obtained by Hinton et ~ 2 l . , employing '~~ (H20)17in GIAO calculations. Using the same theoretical approach, Mauri et al.36 have suggested the use of NMR chemical shifts in probing localized electronic states (surface states) in a crystal. Based on studies on a hydrogen-chemisorbed diamond (1 1 1) surface, the calculated 13C shieldings are found to vary linearly with the density of surface states. Jameson et ~ 1 . have ' ~ ~continued to use Grand Canonical Monte Carlo (GCMC) simulations and ab initio intermolecular shielding surfaces and have successfully reproduced Xe NMR spectra of Xe gas co-adsorbed with CH4 in zeolite NaA. Although the methane molecule moves from cage to cage fast enough that individual Xe,(CH4), peaks are not observed (as had been observed in Xe-Kr mixtures in this zeolite), the position of each Xe, peak indicates the average number of CH4 molecules in the cage. GCMC not only reproduces the average cluster shifts but also the distribution of the have two gases among the cages of zeolite NaA. Lastly, Scheurer et incorporated molecular dynamics in the study of I5N shielding tensors of two glutamine side-chains in the protein ubiquitin. Their results indicate that motion can substantially reduce the effects of hydrogen bonding as various orientations and distances of hydrogen bonds are sampled during the trajectory. 2.5 Absolute Shielding Scales. - Gee et aZ.145have constructed an improved absolute shielding scale for 3 5 / 3 7 ~This l . new scale is based on C1 spin rotation data for HCI, one of the most precisely measured spin-rotation constants for chlorine. 146 To correctly place the absolute shielding obtained from the spinrotation data within the known Cl chemical shift scale, the 3 5 / 3 7 ~ chemical 1 shift of HCl in the gas phase has been carefully measured with respect to the chloride ion in aqueous solution, 28 ? 3 ppm. At equilibrium geometry, without relativistic corrections to the diamagnetic shielding, the absolute shielding of 3 5 / 3 7 ~ in 1 HCl is 962.3 (0.9 ppm. The experimental value that has been used for comparison with a6 initio calculations is 952 ppm. The rovibrationally corrected value at 298 K is 946.4 k 0.9 ppm. Thus, the absolute shieldings for common references are

o(C1-2, neat liq., 298 K) = 632 k 4 ppm. o(CH3C1, neat liq,, 298 K) = 923 k 4 ppm. o(NaC1, aq. infinite dil., 298 K) = 974 k 4 ppm. It should be noted that if D20 is used as a solvent, there is a significant isotope shift of about - 5 ppm for Cl-(aql.

2: Theoretical and Physical Aspects of Nuclear Shielding

3 1 2 3 4 5 6 7

8 9 10 11 12

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

31 32 33 34 35 36 37 38

63

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43 44 45 46 47 48 49

50 51 52 53 54 55 56 57 58 59 60 61

62 63 64 65 66

67 68 69 70 71

72 73

2: Theoretical and Physical Aspects of Nuclear Shielding

74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

101 102 103

65

H. Lee, T. Onak, J. Jaballas, U. Tran, T. U. Truong and H. T. To, Inorg. Chim. Actu, 1999,289, 1 1 . E. Bednarek, J. C. Dobrowolski, K. Dobrosz-Teperek, J. Sitkowski, L. Kozerski, W. Lewandowski and A. P. Mazurek, J. Mol. Struc., 1999,483, 333. P. Cmoch, J. W. Wiench, L. Stefaniak and G. A. Webb, J. Mol. Struc., 1999,510, 165. P. Cmoch, L. Stefaniak, E. Melzer, S. Baloniak and G. A. Webb, Magn. Reson. Chem., 1999,37,493. J. W. Wiench, L. Stefaniak, E. Grech and E. Bednarek, J. Chem. SOC., Perkin Trans. 11, 1999,885. R. M. Claramunt, I. Alkorta and J. Elguero, Heterocycles, 1999,51,355. G. Rasul, G. K. S. Prakash and G. A. Olah, Inorg. Chem., 1999,38,44. M. Buhl, Zeit. Anorg. Allg. Chem., 2000, 626, 332. S. Patchkovskii, W. Thiel, J. Comput. Chem., 1999,20, 1220. S . Dusold, H. Maisel and A. Sebald, J. Magn. Reson., 1999, 141, 78. B. C. Gerstein, C. Chow, R. G. Pembleton and R. C. Wilson, J. Phys. Chem., 1977,81, 565. J. T. Rasmussen, M. Hohwy, H. J. Jakobsen and N. C. Nielsen, Chem. Phys. Lett., 1999,314,239. A. Medek and L. Frydman, J. Mugn. Reson., 1999,138,298. L. Frydman and J. S. Harwood, J. Am. Chem. SOC., 1995,117,5367. D. L. Bryce and R. E. Wasylishen, J. Phys. Chem., 1999,103,7364. S . Kroeker and R. E. Wasylishen, Can. J. Chem., 1999,77, 1962. M. Gee, R. E. Wasylishen, K. Eichele, G. Wu, T. S. Cameron, F. Mathey and F. Laporte, Can. J. Chem., 2000,78, 118. R. W. Schurko, R. E. Wasylishen, S. J. Moore, L. G. Marzilli and J. H. Nelson, Can. J. Chem., 1999,77,1973. M. Gee and R. E. Wasylishen, in ‘Modeling NMR Chemical Shifts. Gaining Insights into Structure and Environment’, J. C. Facelli and A. C. de Dios, eds., Oxford University Press, 1999, p. 259. P. R. Bodart, Y. Dumazy, J. P. Amoureux and C. Fernandez, Magn. Reson. Chem., 1999,37,223. M. J. Potrzebowski, R. Katarzynski and W. Ciesielski, Magn. Reson. Chem., 1999,37, 173. G. A. Bowmaker, R. K. Harris and D. C. Apperley, Inorg. Chem., 1999,38,4956. I. Sack, S. Macholl, F. Wehrmann, J. Albrecht, H. H. Limbach, F. Fillaux, M. H. Baron and G. Buntkowsky, Applied Magn. Reson., 1999,17,413. D. K. Lee, R. J. Wittebort and A, Ramamoorthy, J. Am. Chem. Soc., 1998,120, 8868. A. E. Walling, R. E. Pargas and A. C. de Dios, J. Phys. Chem. A, 1997, 101, 7299. A. M. Orendt, J. C. Facelli, S. Bai, A. Rai, M. Gossett, L. T. Scott, J. BoerioGoates, R. J. Pugmire and D. M. Grant, J. Phys. Chem., 2000,104,149. T. Sen, R. Poupko, U. Fleischer, H. Zimmermann and 2. Luz, J. Am. Chem. SOC., 2000,122,889. R. Salzmann, M. T. McMahon, N. Godbout, L. K. Sanders, M. Wojdelski and E. Oldfield, J. Am. Chem. SOC.,1999,121, 3818. J. K. Harper, G. McGeorge and D. M. Grant, J. Am. Chem. SOC.,1999, 121, 6488. J. E. Rich, M. N. Manalo and A. C. de Dios, J. Phys. Chem. A, 2000,104,5837.

66 104 105 106 107 108 109 110 111

112 113

114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 I30 131 I32 I33 134

Nuclear Magnetic Resonance F. Separovic, J. Ashida, T. Woolf, R. Smith and T. Terao, Chem. Phys. Lett., 1999,303,493. J. Kikuchi, T. Asakurd, K. Hasuda, T. Ito, K. Ohwaku, H. Araki and M. P. Williamson, J. Biochem. Biophys. Meth., 2000,42, 35. R. D. Wigglesworth, W. T. Raynes, S. Kirpekar, J. Oddershede and S. P. A. Sauer, J. Chem. Phys., 2000,112,736. N. Asakawa, M. Takenoiri, D. Sato, M. Sakurai and Y. Inoue, Magn. Reson. Chem., 1999,37,303. Y. Wei, A. C. de Dios and A. E. McDermott, J. Am. Chem. SOC.,1999, 121, 10389. S. N. Smirnov, H. Benedict, N. S. Golubev, G. S. Denisov, M. M. Kreevoy, R. L. Schowen and H. H. Limbach, Can. J. Chem., 1999,77,943. G . Valerio, A. Goursot, R. Vetrivel and D. R. Salahub, Microp. Mesop. Mater., 1999,30, 11 1. R. N. Shimizu, H. Kurosu, I. Ando, A. Abe and H. Furuya, Magn. Reson. Chem., 1999,37,479. D. B. Chesnut and L. D. Quin, Heteroatom Chem., 1999, 10, 566. C. J. Jameson and A. C. de Dios, in ‘Nuclear Magnetic Shielding and Molecular Structure’, ed. J. A. Tossell, Kluwer Academic Publishers, Dordrecht, 1993, p. 95. A. Dransfeld and D. B.Chesnut, Phosphorus, Sulfur Silicon Relat. Elem., 1999, 146, 653. S. Bolvig and P. E. Hansen, Current Org. Chem., 2000,4, 19. P. E. Hansen, Magn. Reson. Chem., 2000,38, 1. S . Bolvig, P. E. Hansen, H. Morimoto, D. Wemmer and P. Williams, Magn. Reson. Chem., 2000,38, 525. C. J. Jameson, J. Chem. Phys., 1977,66,4983. C . J. Jameson and H. J. Osten, J. Chem. Phys., 1984,81,4293. P. E. Hansen, F. Duus, R. Neumann, A. Wesolowska, J. G. Sosnicki and T. S. Jagodzinski, Polish J. Chem., 2000,74,409. S . Bolvig, P. E. Hansen, D. Wemmer and P. Williams, J. Mol. Struct., 1999, 509, 171. G. Heckmann, G . Becker and H. Kraft, Magn. Reson. Chem., 1999,37,667. R . Sprangers, M. J. Bottomley, J. P. Linge, J. Schultz, M. Nilges and M. Sattler, J. Biol. NMR, 2000, 16,47, J. Hobley, V. Malatesta, W. Giroldini and W. Stringo, Phys. Chem. Chem. Phys., 2000,2,53. J. R. P. Arnold and J. Fisher, J. Magn. Reson., 2000,142, 1. K. V. Mikkelsen, K. Ruud and T. Helgaker, J. Comput. Chem., 1999,20, 1281. R. Ludwig, F. Weinhold and T. C. Farrar, Mol. Phys., 1999,97,465. R. Ludwig, F. Weinhold and T. C. Farrar, Mol. Phys., 1999,97,479. A. V. Afonin, M. B. Ferraro, R. H. Contreras and J. C. Facelli, Russ. Chem. Bull., 1999, 48, 1054. A. V. Afonin, A. V. Vashchenko, T. Takagi, A. Kimurd and H. Fujiwara, Can. J. Chem., 1999,77,416. J. A. Tossell, fhys. Chem. Min., 1999, 27, 70. D. Farcasiu and D. Hancu, J. Phys. Chem., 1999,103,754. J. W. Wiench, W. Schilf, L. Stefaniak and G. A. Webb, J, Mol. Struc., 1999,510, 1. M. Witanowski, W. Sicinska, Z. Biedrzycka and G. A. Webb, J. Mol. Struc., 1999,476, 133.

2: Theoretical and Physical Aspects of Nuclear Shielding 135 136 137 138 139 140

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141 142 143 144 145 146

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3 Applications of Nuclear Shielding BY SHlNJl AND0 AND SHIGEKI KUROKI

1

Introduction

The format of this report remains similar to previous years. Chemical and physical influences to nuclear shieldings are considered in the first section. The shieldings of particular nuclear species are described in the following section according to their position in the periodic table. Since there are a huge number of articles on NMR spectroscopy during the period of this review, the coverage of this report is restricted to widely available and common journals, which are published in English, due to space limitation.

2

Various Chemical and Physical Influences on Nuclear Shielding

2.1 Substituent Effects. - 33SNMR chemical shifts have been determined for a series of 10 substituted pheacryl sulfones. Electron-withdrawing and electron-releasing substituents in 4-substituted phenyl-4’-methylpheacyl sulfones, p-MeC6H4COCH2S02C6H4R,cause a ‘reverse’ substituent effect on the 33S NMR resonance. Introduction of a fluorine substituent into an aromatic ring of [2.2]paracycrophane causes distinct shielding and deshielding effects at the anti- and syn-protons respectively, of the adjacent bridge methylene group.2 A recent contradictory claim in the literature was disproved by 2D H,H-NOESY, longrange H,H-COSY and H,C-HMBC experiments. 13CChemical shifts of the azomethine C atom for 4-PyCH:NCbHa (I; Py = pyridyl; X = H, p-Me, p-Br, p-C1, p-OMe, p-NMe2, p-COMe, p-C02Me, mN02, m-CF3) were determined in CDC13 ~ o l u t i o n .Exceptionally ~ good Hammett correlations of I3C NMR chemical shifts of azomethine C atoms with electrophilic substituent constants were obtained. The effect of 29 commonly encountered substituents on the chemical shifts of a,p and C5H5 positions in monosubstituted ferrocenes are tabluated and employed for ‘H NMR assignments in 1,2- and 1, 1’-disubstituted ferrocene

derivative^.^ The ‘H and I3C NMR spectra of some 6-substituted 2-naphthyl Me sulfides were assigned unambiguously using NOE and two-dimensional NMR spectral Nuclear Magnetic Resonance, Volume 30

0The Royal Society of Chemistry, 2001 68

3: Applications of Nuclear Shielding

69

techniques and the influence of substituents on the chemical shifts is disc~ssed.~ A characteristic long distance charge transfer pattern was found for paranitrostyrenes by means of 13CNMR and AM1 molecular orbital calculations, where the olefinic bridge (CH=CH) and the aromatic ring (Ph) carbon centers are perturbed according to the donor-nature of the para-substituent groupse6 An analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C4 center gave a good correlation between the C4 chemical shift and the electronegativity of this vicinal center. 'H 13C, "N, and 29Si NMR spectra of trimethylsilylated para- and metasubstituted benzhydroxamic acids were studied in chloroform solution^.^ Association with chloroform partially compensates the direct substituent effect on the shielding in the case of the Si(0 I ) silicon. The influence of other factors not covered by substituent constants is demonstrated by excellent correlations with the chemical shifts in analogous tert-butyldimethylsilyl derivatives, I3C chemical shifts of the azomethine carbon atom for N-(substituted phenyl)pyridine-4-aldimines, 4-Py-CH=N-C6H4-X, X having a wide range of substituent effects, have been determined in CDC13 solution, and the substituent shielding effects were discussed.' Both trans-syn cyclobutane-type photodimers of 2'-deoxyuridylyl(3'-5')-thymidine were synthesized and examined by 'H, I3C, and 31P NMR.9 Steric crowding of cyclobutane ring substituents is offered as an explanation for the difference in substituent effects between the families of cis-syn and trans-syn pho t odimers. 2.2 Conformation Effects. - The relationships between 13C NMR chemical shifts of aromatic carbons and the inter-ring torsion angles (8) of two phenyl rings in biphenyl and its substituted derivatives are systematically examined. l o The chemical shifts of C1, C2 and C6 carbons (6cl, 6 ~ 2 and , 6C6) of the biphenyl group in the solid state show systematic correlations with 8 when the substituent shielding effects are removed from the solid chemical shifts by subtracting the solution chemical shifts or subtracting the substituent shielding parameters. An increase in 6 ~ 1 , 8 ~or 2 ,6C6 (high frequency displacement of the signals) corresponds to an increase in 8. The isotropic nuclear shielding calculated using ab initio molecular orbital theory with the GIAO-CHF method for the biphenyl agree well with the experimental results. The structure and dynamics for the monoclinic and hexagonal crystals of 1,2O-eicosanediol (HO(CH2)ZoOH) have been investigated mainly by solidstate I3C and 'H NMR. The evaluation of the 13C chemical shift for the CH2 resonance line by considering the y-gauche effect indicates that about 3% gauche conformations are included in the inner CH2 sequences in the hexagonal phase. The two fluorines of cyclopropyldifluoroborane and vinyldifluoroborane show two chemical shifts at low temperatures, due to hindered rotation about the boron-carbon bonds. The rates of rotation were determined at higher temperatures.

''

'

70

Nuclear Magnetic Resonance

Conformational change of N,N'-Bis-(3-aminopropyl)-1,3-~ropanediamine (3P4A) intercalated in a-zirconium phosphate (a-ZrP) is reported. The lineshape analysis of 13C CP/MAS NMR spectra suggests that the conformation of 3P4A assumes the 'straight' form in the higher temperature product while it assumes the 'bent' form in the lower temperature product. According to this model, 31PMAS NMR spectra were consistently assigned.l 3 * 3C CP-MAS solid state NMR spectra of hydrochlorides and perchlorates of buspirone analogues were recorded. l4 As no remarkable differences between carbon chemical shifts of hydrochlorides in solid state and in solution were observed, it was concluded that in solution these compounds adopted the same conformation as in the solid state. The observed 13C NMR chemical shifts of poly(phenylacety1ene) (PPA) in the solid state before and after cis-trans isomerization were investigated on the basis of chemical shift calculations for the cis-transoidal and deflected transtransoidal f o r m ~ .This ' ~ upfield shift of trans-PPA largely was attributed to the increases of the excitation energy from the ground state to the lowest state in the paramagnetic terms of 13C chemical shift on the main-chain carbons with the increase in deflected angle 'I: from 0 to 80".The +SO" deflected conformation of the trans-transoidal chain due to the cis-trans isomerization was confirmed. Three dioximes and three dimethoximes, which contain functional groups in opposite positions of a six-, eight- or ten-membered ring, were synthesized, and their conformational properties and transannular interactions were investigated by spectroscopic (PE, 3C NMR) and theoretical (MMX, AM 1, ab initio HF, and B3LYP) rnethods.l6 'H NMR spectra of (Z)-cocaine and some of its derivatives (a-CPT, P-CPT, nor-0-CIT, cocaine-HC1 and ecgonine-HC1) were analysed, and the spectral parameters obtained were used for conformational analysis in conjunction with theoretical HF/6-3 1G*, MMP2, AM 1 and molecular dynamics calculations. l 7 The dependence of the principal components of the NMR chemical shifts tensors of non-hydrogen nuclei in model dipeptides was studied." The principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. The conformational analysis using the slow exchange 13CNMR spectrum of the eight-membered lactone oxacyclooctane-2-one at - 154.2"C indicated the presence of two conformers with populations of 25.6 and 74.4Y0.l~ The stereochemistry of the preferred conformers of several 3-acyloxy-1,3oxazinanes has been established by NMR. A strong anomeric effect stabilizes the conformation having an equatorial orientation of the lone pair on nitrogen.*' A nitrogen inversion process was found to be the rate-limiting process in the conformational equilibrium.

2.3 Intermolecular and Hydrogen Bonding Effects. - Solid state 'H NMR spectra of glycine (G1y)-containing peptides and polypeptides were measured at 800 MHz and at high-speed MAS of 30 kHz to elucidate the relationship

3: Applications of Nuclear Shielding

71

between the hydrogen-bond length and 'H NMR chemical shift, to add to our previous experimental and theoretical findings that there is a relationship between the hydrogen-bond length and 13C, "N and 170chemical shifts of various kinds of amino acid residues of peptides and polypeptides in the solid state.21 A series of 11 Schiff's bases derived from substituted salicylaldehyde and aliphatic amines was studied in the solid state by "N and I3C CP-MAS NMR. I5N CP-MAS is especially useful for investigation of the tautomerism in the compounds considered, owing to the large difference in the nitrogen chemical shifts of OH and NH tautomers.22In the solid state, three of the compounds examined were shown by "N NMR to exist as OH tautomeric forms, and the remaining eight as the corresponding NH forms. Electronic and spatial structures of some H-bonded complexes ((HCOOH), ( n = 1-4), formic, acrylic acids and their complexes with water, and dimethylsulfoxide) were studied by ab initio methods using MP2/RHF//6-3 1G*, RHF/ 6-31G* basis sets, and also by using density functional theory (DFT) within the B3LYP appr~ximation.~' Analysis of the results for various basis sets and comparison of the calculated and experimentally obtained chemical shifts show that the calculations predict a significant downfield shift for 'H and I3C nuclei for H-bonded systems. The results of calculations of 170magnetic shielding constants strongly depend on the basis set and theoretical approach used. A relationship between intramolecular hydrogen bonding and the cis-trans isomerization of a proline imide bond for proline-containing short peptides were studied by 'H NMR and IR spectroscopy using DMSO-ddCDCl3 mixed solvents.24The percentage of the trans form increases with increasing fraction of CDC13 in the mixed solvents except for compounds without possibility of intramolecular hydrogen bonding. The 'H, 13C,and 15NNMR data reported for azo-compounds show that in DMSO solutions all of them exist in the azo form only and do not participate in the azo-hydrazoimine e q ~ i l i b r i u mThe . ~ ~ NMR data for compounds 1 and 2 show the presence of a weak hydrogen bond for the non-protonated forms, between N10 and the 2-NHCH3 proton. Infrared and NMR measurements were performed for various small peptide amides in chloroform solutions.26The IR results have shown that one of the two NH of the C-terminal NH2 groups takes part in an intramolecular hydrogen bond while the other NH is almost free from the hydrogen bond. NMR chemical shifts of the two NH protons of the NH2 groups provided support the IR results. Hydrogen bond lengths on enzymes derived with high precision (< k 0.05 from the proton chemical shifts (6) were compared with those estimated from the fractionation factors (+) of the proton derived from protein X-ray crystal10graphy.~~ Hydrogen bond distances derived from proton chemical shifts were obtained from a correlation of 59 0-H -.. 0 hydrogen bond lengths, measured by small molecule high-resolution X-ray crystallography, with chemical shifts determined by solid-state NMR in the same crystals. The

A)

72

Nuclear Magnetic Resonance

increased precision in measurements of hydrogen bond lengths by NMR has provided insight into the contributions of short, strong hydrogen bonds to catalysis for several enzymatic reactions. Using 'H NMR spectroscopy, the hydroxyl proton chemical shift 6 of methanol solved in carbon tetrachloride at methanol mole fractions ranging from 0.01 to 0.20 was measured at 20°C and 40°C under pressures of up to 2000 bar.28 On the basis of a simple monomer-tetramer equilibrium for methanol molecules, it was possible to separate the effects of hydrogen-bond strength and extent on the 'H chemical shift. Analysing the pressure and temperature dependence of the chemical shift of tetramer methanol clusters, a pressure-induced strengthening and a temperature-induced weakening of hydrogen bonds in clusters of methanol molecules is found. Fractions of chelate hydrogen-bonded enol tautomers, corresponding to two potential minima in the intramolecular hydrogen bond, are estimated from I3C NMR spectra of solid and dissolved compounds with 1,3-dicarbonyl fragm e n t ~For . ~ ~most of them such tautomerism has not been investigated so far. Two crystalline substances were probed at various temperatures, and specific temperature dependences of chemical shifts were observed, reflecting the growth of the main tautomer fraction on cooling. 13C NMR chemical shift tensor components are reported for a 13C-labeled Gly amide carbonyl-carbon of glycylglycine (Gly'Gly2), GlyGly -HN03 and GlyGly-HC1-H20.30It was found that the 13C chemical shift tensor components (011, 0 2 2 , and 033)for the Gly' amide carbonylcarbon in GlyGly and GlyGly.HN03 with a N-H O=C type of hydrogen bond depend on the hydrogen-bond length. In addition, the magnitude of the deviation of the directions of the 0 2 2 components from the C=O bond axis depends on the hydrogen-bond angle. The experimental result for GlyGly-HCl-H20with a 0H O=C type of hydrogen bond was also discussed. 'H, I3C, 14N, "N, I7O, and 19F NMR data are presented for 2,8-bis(trifluoromethyl)-4-quinolino1(I) and its complexes with 1,8-bis(dimethylarnino)naphtalene (11) in 2: 1 and 1:1 molar ratio^.^' GIAO-CHF MO calculations for [ 1HI and[4H] forms of free I and its anion are also performed. The [4H] form of I predominates in MeCN solution. In both complexes with 11, I forms a semianion containing an intermoleculer hydrogen bond. The proton chemical shift of supercritical water was analysed by computer simulations with emphasis on its relationship to the number of hydrogen bonds per water molecule and the dipole moment of a water molecule.32The chemical shift was shown to be proportional to the number of hydrogen bonds. Ab initio calculations at the MBPT(2)/(qzp,qz2p) level of theory were carried out using the gauge-invariant A 0 method to evaluate 'H NMR chemical shifts for the H-bonded proton in two series of complexes, the first containing Cl-HN and Cl-H-Cl H-bonds, and the second 0-H-0, N-H-0, and N-H-N Hbonds.33 A 'H NMR study of a series of recently synthesized N-alkylpivaloylamides in CCl4 solution and in the presence of 0-electron donors (Me2CO and THF)

'

3: Applications of Nuclear Shielding

73

has been carried Chemical shifts for free NH and NH=O H-bonded protons were obtained. Equilibrium constants for 1:1 amide-acetoneketrahydrofuran H-bonded complexes at 25 "C were determined. Mixtures of MeOH with 2 strong proton donors, CHC13 and halothane (CF3CHClBr), were studied.35The behavior of these systems is governed by aggregate-formation through H-bonding, where MeOH self-association and complexation with the proton donors compete. In order to obtain information about these aggregate-forming equilibria, 'H NMR chemical shifts of the CHC13 or halothane proton and of the OH proton of MeOH were measured as a function of concentration and temperature. 2.4 Solvent Effects. - IR and multinuclear NMR studies of trimethyl phosphate in 24 different solvents were undertaken to investigate the solutesolvent interactions and to correlate solvent properties, such as acceptor number and dielectric constant, with the IR band shift and the I3C and 3'P NMR chemical shifts.36Furthermore the cross-correlation between the IR and NMR shifts in different solvents was studied. There is a correlation between IR and NMR shifts: the infrared, 13C and 31Pshifts can be predicted for most solvents used in this study, if one of them is known. A recently developed method to calculate singlet and triplet gauge-originindependent magnetic properties, such as the magnetizability, the nuclear shieldings, and the indirect spin-spin coupling constants, for solvated molecules were described and examined.37

Isotope Effects. - Polycyclic bis(diorgan0tin) derivatives were synthesized oxalic acid with diorgafrom bis(2-hydroxy-3,5-di-tert-butyl-phenylanilido) notin dichlorides, R2SnC12, and the isotope-induced chemical shifts 'A 12' 13C(119Sn) for five-coordinate tin nuclei at natural abundance of 13C were determined.38A positive sign of 'A 12'13C ('I9Sn) was found for R = Me, Bu, and Ph. Deuterium isotope effects on 1 7 0 chemical shifts are studied in a series of intramolecularly hydrogen-bonded o-hydroxy acyl aromatics and dike tone^.^^ Geometries and chemical shifts are calculated using DFT ab initio methods. Relationships between hydrogen bond strength represented as either Ro-0, RO-H or RO-H, and the experimental parameters, 'A I7O(OD) and 6 1 7 0 are demonstrated for non-tautomeric compounds. These plots do also provide a method for establishing tautomerism in new compounds. 'A 170(OD) as well as 170are found to be dominated by hydrogen-bonding in terms of RO-H or RO-H distances and not influenced very much by the Ro-0 distance. The proton transfer equilibrium in a series of Schiff bases derived from 5nitrosalicylaldehyde and aliphatic amines was investigated by means of variable temperature NMR spectra and the deuterium isotope effect on I3C NMR chemical shifts.40 13C shieldings, 'A 13C('80/l60), of carbon monoxide (99.7% 13C,0.9% 170, and I 1.8% "0 enriched) in a variety of solvents and of the Fe-C-0 unit of several carbonmonoxy hemoprotein models with varying polar and steric 2.5

74

Nuclear Magnetic Resonance

effects of the distal organic superstructure and constraints of the proximal side are reported?' The disecondary phosphines PhHP(CH&PHPh, n = 2, 3, were prepared in high yields (SOYO)after short reaction times (6h) and converted into the mesoand racemo-diastereomers of [ (PhHP(CH&PHPh) W(CO)4], n = 2, 3.42 12" 3C isotope effects on 3'P shieldings and I 2Jp-plcouplings were calculated from the I3C NMR spectra of the complexes. Kinetics and I3Ckinetic isotope effects in the decarboxylation of phenylpropiolic acid in HCOOH/H20, 1:l v:v, solution have been examined between 80 and 140°C 43

3

Shielding of Particular Nuclear Species

During the period of this review, the NMR spectra of most elements have received at least some chemical, biological, or physical invenstigation. Due to the space limitations, structure determination and related studies of natural products or macromolecules will be excluded, and the review articles are given for the most popular nuclei ('H, 13C, 14*15N,and I9F). 3.1 Group 1 ('H, *H, 3H, 697Li,23Na,*'Rb, 133Cs).- 3.1.1Hydrogen ( ' H ) ( I = 1/2). - A review with 40 references was given on the 'H chemical shifts of H20 and organic molecules adsorbed on to different types of carbon surfaces (exfoliated graphite, graphite oxide, intercalated graphite, graphitized and nongraphitized activated carbons, C black, carbosils). Values of chemical shift differ in accordance with the structure of the active sites.44A review with 100 references was given on the 'H MAS NMR of zeolites.45The dynamic nature of the protons in zeolites such as H-ZSM-5, metallosilicates, molecule sieves and heteropoly compounds was examined by temperature dependence of 'H MAS NMR in the range of 298 to 473 K. The temperature dependent lineshape of 'H MAS NMR spectra of acidic protons in zeolites showed the first and unambiguous evidence for proton migration in zeolite structures. A review with 40 references was given on the topic of low-barrier hydrogen bonds (LB-HBs) and the question of how they are involved in enzyme function.46 The authors report 'H NMR chemical shift data and surveys of structural preferences for well-studied 0-HO systems, and also for less studied, but biological important N-HO systems, in particular the imidazole and imidazolium functionality. A review was given on the advances in highresolution solid state 'H and I3C NMR spectroscopy applied to study of conversions of hydrocarbons and alcohols on zeolite catalysts of an acidic nature.47 The potential of NMR spectroscopy in studies of mechanisms of chemical reactions and analysis of compounds formed in situ is considered. A review with 30 references was given on the application of NMR in studying polymer colloidal systems.48 Standard solid state NMR measurements in telechelic fluorocarbon polymer systems and in polymer films made of latex dispersions were presented. Phase behavior of polymer and surfactant in latex

3: Applications of Nuclear Shielding

75

films is analysed by ‘H-NMR. A review was given on advances in structural and conformational analysis of fluorinated carbohydrates by characteristic ‘H, 13C, and I9F NMR chemical shifts and coupling constants.49 A review was given on some recent progress on characterizing the catalytic activity and selectivity of a Bronsted-acid site in a zeolite using ‘H NMR.” 3.1.2 Deuterium (’H) ( I = 1). A review was given on 2H NMR studies of

polyelectrolytes interacting with deutero-labelled surfactants in different surfactant self-assembled architectures, including micelles, monolayers, and bilayer membrane^.^' 3.1.3 Lithium (677Li) ( I = 1,213). Different extra-framework lithium and sodium cation sites in faujasite-type zeolites were discriminated by a new 6Li and 23Na MAS NMR method termed SUPAS MAS NMR.52The long-range and short-range motion of lithium ions into an electrochemically intercalated Li3,La2/3 - .Ti03(LLTO) sintered pellet was studied by a.c. impedance spectroscopy and 7Li solid state NMR.536Li and 7Li solid state MAS NMR were used to investigate the local coordination environment of lithium in a series of . ~ NMR ~ chemical shifts xLi20( 1 -x)P2O5 glasses, where 0.0% x ~ 0 . 5 5 6Li were studied on adsorbed lithium ions in strongly acidic cation exchange resins with different degrees of c r ~ s s l i n k i n gSolid-state .~~ 7Li NMR revealed different Ni/Co distributions in an inverse spinel structure of LiNi,Co(l- ,]VO4 cathode materials prepared by either a high-temperature solid-state reaction method or a low-temperature solution co-precipitation method.56 Two nitrogen ceramic phases, the oxynitride LiSiON and the nitride LiSi2N3, were studied by 6Li and 7Li NMR.57 7Li, ‘H, and I3C NMR analysis of di-tert-butyl 2-lithio-2,4,4’trimethylglutarate and of short-length living poly( tert-butyl methacrylate) chains is reported.58 The growth twin-domain structure of a LiCsS04 single crystal grown by slow evaporation was investigated using 7Li and 133CsNMR at room temperature.” 3.1.4 Sodium (23Na) ( I = 3/2). Na clusters containing 10 to 9.0 a dimeric diethyldioxastannane, which is reflected by the presence of the 2JSn-o-117Sn coupling of 156 Hz. Between pH 5.0 and 9.0 no evidence of any interaction between the diethyltin moiety and the nucleotides has been found.

7

Three-Bond Hydrogen-Hydrogen Couplings

The vicinal proton-proton couplings remain the most often exploited parameter in elucidation of the conformation and configuration of organic, and in particular, bioorganic molecules. The 3 J H H couplings measured in proteins deserve special attention and most of coupling data measured for biomolecules belong to this type of couplings. The attention is due to the abundance of local information they carry which is important to the structure elucidation of well-structured compounds. In unstructured or partially structured compounds these couplings are the sole source of information. The peptides and proteins for which vicinal protonproton couplings have been used in solution structure elucidation are listed in Table 1 and some examples are discussed in detail.

3 3 5 6 8 10 13 14 17 21 26 27 35 37 37 38 38 42 48

~

peFtides and proteins for which the solution structure has been calculated with 3 J H H [p -HGlu]3, cyclo-0-tripeptide HTH, helix-turn-helix peptide mimic ~(DPAAXA); X = Ala or Aib C(D PFASFF) DF-c(CFDWKXaaC)Xbb-NH2, 10 sandostatin analogs cc-lactalbumin (1 11-120) p23wt-m, cytoplasmic domain of p23 GS14K3L4, analogue of GS14, a peptide based on gramicidin S uperin 3.6, an antibiotic peptide from the Australian toadlet, Uperoleia mjobergii dAgcap24 3&m, N-capping peptide of hepatitis delta antigen (24-38) MVIIC, o-conotoxin dAg24 50, N -terminal leucine-repeat region of hepatitis delta antigen gurmarin, a sweet-taste-suppressing polypeptide from the Asclepiad vine Gymnema sylvestre GGN4, gaegurin 4, antimicrobial peptide from Rana rugosa, oxidized and reduced BmKTX, a toxin from the Chinese scorpion Buthus Martensi ct2p8, x-helical hairpin of ~ 8 ~ (~5 4~8 ) ‘ ’ PP3, membrane binding C-terminal residues of the bovine milk component DLP-1, a defensin-like peptide from platypus venome CbnB2, carnobacteriocin from the lactic acid bacterium Carnobacterium pisicola LV 17B

~

10 13 36 53+73 187

J

a

peptides and proteins for which the secondary structure has been determined with 3 GCN4brM 1, a bicyclic peptide neurotensin Pol (1200-1235) [N] ZR7,Z-repeat 7 of titin, complexed with EF34, C-terminal portion of a-actinin [C/N] HasA

Name

Table 5.1 Peptides andproteinsfor which 3JHHcouplings have been applied as a structural parameter

13 20 30 25 19 22 28 29 31 24 15

C

3 6 8 5 14-19 8 10

10 73 24 31 128

b

Cont’d

209 210 21 1 212 213 214 21 5 216 217 218 219 220 22 1 222 223 224 225 226 227

205 53 206 207 208

Ref.

I-?

E

B

$

k

Ah-AMP1 , antimicrobial Protein 1 from Aesculus hippcastanum aPI 1, a single repeat from NaProPI, precursor protein of proteinase inhibitors AMCI- 1, chymotrypsirdcathepsin G inhibitor- 1 from Apis melliferu HIV-1 Nef protein (2-57) HIV-1 Nef protein (2-57) with a myristolyated N-terminus [NJ MTA, [Cd7]-metallothionein-Afrom the sea urchin Strongylocentrotus purpuratus [C/N] HP67, F-actin-binding headpiece of chicken villin (10-76) [Nl p8MTcp’, human, C12A mutant [N] CopZ, copper chaperone from Enterococcus hirae, up0 [C/N] MURDBD, DNA-binding domain of Mu-repressor (13-8 1) [C/N] PsaE, photosystem I accessory protein E from cyanobacterium Nostoc sp. strain PCC 8009 [NJ BTK SH3, human, complexed with a proline rich peptide from p120cb’ 1-309, human chemokine of the CC subclass [Nl calbindin Dgk, P43M mutant, monolanthanide-substituted, refinement with PCS [C/N] HPV-16 E2-C, human papillomavirus E2 DNA binding domain, denatured, residual structure [C/N]TFIIEPc, the central core domain of TFIIEP (66-146) [C/N] KH3, C-terminal module of heterogeneous nuclear ribonucleoprtoein K (380463) [C/N] HuC BRD2 (124-208), N-terminal domain of Hu antigen C [C/N] HuC BRDl (36-123), N-terminal domain of Hu antigen C [NJ ‘F12F1, N-terminal F1 module pair from mature human fibronectin (17-109) [N] cytochrome b5, oxidized [C/N] PDZ2, second PDZ domain of human phosphatase hPTPlE (1361-1456) [Nl D4Pc, cytokine-binding domain of the P-chain (338438) [Nl 6F1‘F2, gelatin-binding domain of mature human fibronectin (274-374) BacTrx, the thioredoxin from Bacillus acidoculdarius, oxidized [GIN] L30, the yeast ribosomal protein complexed with its regulatory RNA [N] cytochrome b562from E. coli, containing iron 111, R98C mutant [N] cytochrome b562from E. coli, oxidized [C/N] carp provalbumin, F102W mutant

Name

Table 5.1 cont’d

50 54 56 56 56 65 67 68 68 69 71 58+14 74 75 80 81 84 85

88 93 94 96 101 101 105 105 106 106 108

a

C)

31 44 42 80 61 66

C)

81

C)

43 73 24 50 39 65 38 56 88 88 14 18 81 42 53 99 135 45 33

h

228 229 230 23 1 23 1 232 233 234 235 236 237 238 239 240 24 1 242 243 244 244 245 246 247 248 249 250 25 1 252 253 254

ReJ

C

b

a

the number of amino acid residues the total number of vicinal backbone and side chain proton-proton couplings measured number not reported

[C/N] NusB, a transcription antiterminator protein from E. coli [N] SOD, co per zinc superoxidase dismutase, monomeric, reduced, mutant [CIN] NtrC gted,DNA binding domain of NtrC from Salmonella typhimurium (380-489), 3 x A mutant [Nl BPV-1 E2 DBD (3 10-410), E2 DNA-binding domain from bovine papillomavirus, homodimer [D/C/N] MAP30, RIP anti-HIV anti-tumor plant protein

139 153 90 x 2 101 x 2 263 C)

43 44x2 79 x 2 ca.280

255 256 257 258 259

' 2.

%

$

154

Nuclear Magnetic Resonance

Stocker and van Gunsteren260have calculated 3 J H N H u couplings in hen egg white lysozyme using GROMOS96 and GROMOS87+ force field. The MD calculations have been carried out for 2 ns and the converged values have been compared with the experimental ones. The comparison illustrates substantial improvement in the latest version of GROMOS force field. Barone and co-workers261have investigated conformational behaviour of palanine zwitterion in aqueous solution using post-Hartree-Fock and DFT calculations. Remarkable agreement has been observed between the vibrationally averaged 3 J H H couplings and those measured experimentally. The effect of motional averaging of vicinal couplings has been analysed by Mark and coworkers.262The analysis is based on 50 ns simulations of a model heptapeptide. Stote et ul.263have applied MD in the analysis of multiple conformations of the important cell adhesion peptide RGDW and its analogue D-RGDW. The calculated values of vicinal 3 J H H couplings averaged over populations of all the resulting structures have been compared with the experimental ones and quite satisfactory agreement has been found. Scalar 3 J H H couplings have been calculated by Hermans and c o - w o r k e r ~ ~ ~ ~ for individual Ala, Asn, Asp, Gly and Val residues from their conformational distributions in known protein structures. The authors have also carried out calculations of molecular dynamics distributions for these residues. The resulting couplings obtained in both ways have been compared and their validity for description of random-coil state of proteins has been discussed. The secondary structure of five peptides with the sequences corresponding to helical fragments of phage 434 Cro protein has been investigated by Padmanabhan et uZ.265with the aid of 3 J H H couplings. From the conformational changes observed for the peptides in aqueous, 40%(by volume)TFE/water and 7M urea solutions the authors have drawn some conclusions about the possible early events in the folding of the entire protein. In search of the possible contribution of p-structures to protein folding an analysis with the use of 3JHH couplings has been carried out by Searle and co-workers266 for a p-hairpin peptide in water. Identification of the potential initiation sites of T4 lysozyme folding has been traced by Najbar et aZ.267The secondary structures of ten peptides corresponding to the regions with patterns typical of secondary structure in lysozyme have been examined with 3 J H H couplings. ~ ' J H c couplings have been The DFT calculations of 3JHH, 2 J Hand performed by Czernek et al.268 for relatively large molecules, anhydrodeoxythymidines. Especially good agreement between the DFT-predicted and experimental values has been observed for the 3 J H H couplings. Vicinal 3 J H H couplings are usually used for mono- and oligo-nucleotides to establish conformational equilibrium in their sugar part. Thus, Chattopadhyaya and c o - w o r k e r ~have ~ ~ ~determined the group electronegativity of CF3 in 3'- 0-CF3-thymidine using analysis of temperature-dependent 3 J H H couplings as a direct experimental tool. They have also applied these couplings to study the influence of changes in the electronic character of guanine on the sugar conformation in guanosine 3',5'-bipho~phate.~~' Polak and P l a ~ e c ~ ~ ' have studied metal-nucleotide interaction with the aid of that coupling.

5: Applications of Spin-Spin Couplings

155

Table 5.2 Nucleosides, nucleotides and oligonucleotidesfor which JHHhas been used as a structural parameter Name

Ref:

a novel pyrazolo[1,5-~]pyrimidineC-nucleoside 4’-thionucleosides,a series of ten antiherpeticC5-substituted 2’-deoxyuridines,a series of 28 of monofunctional adducts trans- and cis-[Pt(5’-GMP-N7)(PyAc-N,0)(NH3)] adduct d(TCGT)-N7(3)-Pt(PyAc- 0,N)(NH3) 3’-S-P03-linkedribo- and deoxyribodinucleosidemonophosphates a pyrimidine-rich loop hairpin in a cruciform promoter for N4 vRNAP in a 1 1 -mer DNA duplex the (+)-trans-anti-benzo[g]chrysene-dA-dT d(GAAAACGTTTTC)*and d(GAAAAMe5CGTTTTC)2 [C/N] SL13, a 13-nucleotideRNA containing AUA triloop d( 5‘-CGCTAGCG-3’)2,TOT01 1 Et, complexed with the bis-intercalatingdye

272 273 274 275 275 276 277 278 279 280 28 1

Several other examples of nucleotides whose structures have been established using vicinal proton-proton couplings are listed in Table 2. More and more attention is being paid to analysis of couplings involving hydroxyl protons. Exhaustive investigations of intra- and inter-molecular hydrogen bonds of alcohols in DMSO have been presented by Bernet and Vasella.282Almost sixty compounds have been used and the analysis in great part is evidenced with the help of 3JH,0Hcouplings. Couplings of this type have been analysed in galabioside and its S-linked 4-thiodisaccharide analogue by Sandstrom and c o - w ~ r k e r s .On ~~~ the basis of the coupling values the authors have been able to draw conclusions on hydrogen bonds formed in the investigated compounds. 3 J H H couplings have been also used for the conformational analysis of other sugars; some examples are listed in Table 3. The spectral characteristics which also included JHH couplings has been reported by Cuccia el al.293for an isomeric, alternating heterocyclic pyridinepyridazine molecular strand which has encoded helical self-organization and self-assembly into helical fibres. 3 J couplings ~ ~ have been applied in the conformational analysis of novel diterpenes, excoecarins M and N from resinous and also reported for two novel isomeric triterpenoid saponins, phelasin A and phelasin B, isolated from Anthocephalus cadamba’ and for four triterpenes of the ursane, artane, lupane and friedelane groups isolated from three Brazilian plants.295The 3 J ~ and 4 J couplings ~ ~ have been applied by Cameron et al.296to elucidate the stereochemistry of new sesquiterpene and brominated metabolites, i. e. 6hydroxyfurodysinin-0-methyl lactone, 2-(2,4’-dibromophenoxy)-4,6-dibromoanisole and dehydroherbadysidolide, isolated from the tropical marine Dysidea sponge. An extensive use of vicinal H-H couplings has been made by Brukwicki et al.297 in their studies on the conformational equilibrium in quinolizidinepiperidine alkaloids. In particular, they have shown, using the J H 7 H 13 coupling, that the fraction of the conformer with the boat ring C in the conformational

~

Nuclear Magnetic Resonance

156

Table 5.3

Carbohydratesfor which couplings have been used as a structural parameter in conformational analysis

Name

unnatural dihexoses linked by a -CHZ- bridge in the positions (3 +3) a-D-arabinofuranosyloligosaccharides,portions of polysaccharides from Mycobacterium tuberculosis xyloglucan oligomer XXXG, X = a-D-Xylp(1 +6)-P-~-Glcp and G = P - D - G ~ c ~ glucotriose, terminal unit from oligosaccharide of glycoprotein presursor [C] a rigid tetrasaccharideepitope in the capsular polysaccharide of Vibrio cholerae 0139 octasaccharide fragment of the 0-specific polysaccharide from Shigella dysenteriae type 1 0-antigen lipopolysaccharide from Yersinia enterocolitica serotype 0:28 0-antigen polysaccharide from the enteroinvasive E. coli 0173 0-antigenic polysaccharide from enteroinvasive E. coli 0136 [C]flexible polysaccharide of Streptococcus mitis 522 a

Re$ 3

JHH

284

3JH H

285

3

JHH

286

JHC

287

3

1 3 JHC, JHP

288

3 JHC

289

'JHC

290

type of couplings measured

equilibrium in N-methylangustifoline amounts to 34 %. The J H 7 H 17 coupling measured for (+)-2-thionosparteinium perchlorate has been used by Borowiak et al.298to show that ring C of this compound has a boat conformation. The J H H couplings have been determined for neoadifolin, a new indole alkaloid from the heartwood of Adina cordgolia.299 The conformation of taxol, the anticancer drug isolated from the bark of Taxus brevllfolia has been the subject of the studies performed by Snyder and c o - w o r k e r ~The . ~ ~vicinal proton-proton couplings have been applied to solve the solution structure of the anticancer agent, etop~ide.~" A comparison of the calculated and experimental 3&H values combined with molecular mechanics calculations and NOE results has allowed Barrero et aL302 to conclude that a DU conformer is preferred in solution for two melampolides previously isolated from Mikania Minima. Proton-proton couplings have been reported for some novel flavanone and for brefeldin-A, a 13the first isoflavanone-benzofuranone biflav~noids,~'~ membered ring fungal m e t a b ~ l i t eand ~ ~applied in the structural analysis of ellagic acid derivative^.^" Molecular modeling studies on the stereochemistry of 2-aryl-3-(2',4'-dinitropheny1thio)cyclohexenes has been performed by Devanathan et ~ 1 . ; ~J 'H~H couplings have been reported for these compounds. An analysis of J H H couplings has allowed the identification of the substituent patterns on 1,3,5 ...polyoxygenated-2,4,6...polymethylated alkane chains.307

5: Applications of Spin-Spin Couplings

157

Vicinal H-H couplings have been used by Lewin et al.308in their studies on the conformation of two isomeric 3a- and 3P-aminotropanes, applied by Klemm et aZ.309to corroborate the trans geometry of 2,3-dichloro-5-ethyl-2,3dihydrothieno[2,3-b]pyridineand by Baron et ~ 1 . to ~ elucidate ' ~ the conformation of l -(2'-methylimidazolyI)-2-hydroxyindanein solution. Further examples of application of 3JHH couplings involve conformational analysis of 4-methyl- 1,3,2-diok~aborinane,~~ the identification of vicinally substituted cyclohexane isomers3* and the elucidation of the conformational preference of some tetrahydropyrroto[ 1,2-4[1,3,4]oxadiazinederivativesB313 The 3JHH couplings have been measured by T r ~ j i l l o - F e r r a r afor ~ ~a~series of arylmaleamic acids, arylmaleimides, arylsuccinamic acids and arylsuccinimides, by De Rosa et aL315for a series of variously N-substituted pyrroles, by N o w a k ~ w s k a for ~ ' ~some N-bromoalkyl-(E)-4-azachalconebromides and by Huwyler et al. for 3,3'-bi(tricyclo[3.2.2.02*4]nona-6,8-dienylidene, the promising precursor of triafulvalene. The stereochemistry of a large series of octahydroisoquinoline derivatives has been elucidated by Ghiviriga et aL318by the use of vicinal proton-proton couplings. It is worth mentioning that a very characteristic pattern of 3JHH couplings has been observed for the epoxide derivatives of these compounds allowing the assignment of the epoxide relative stereochemistry by means of the proton spectrum only. ' ~ a series of H-H couplings have been measured by Kolehmainen et ~ 2 1 . ~for chlorinated dibenzothiophenes and by Di Maio et aZ.320for cis- and trans-10substituted decal-2-ones. The solution conformation of a new unsymmetrical metalla crown ether, cisMo(CO)~{2-(2,2'-02Cl~ H ~ ) P O ( C H ~ C H ~ O ) ~ - ~ - C ~ O H , ~ - ~ ' - O P ( ~ , ~ ' - O ~ has been studied by H a r i h a r a ~ a r m a by ~ ~the l use of the vicinal H-H couplings. The results obtained indicate that both ethylene groups in this compound are gauche. 3JHH couplings have been measured by Klaui et aZ.322for two unusual mixed-ligand indium complexes, [LoM,InTp][InC14] and [LoMeInTp][PF6], where L o M ~ -= [ T $ ( C ~ H ~ ) C O ( P ( O ) R ~ Tp} ~ ] -= ~ ~[HB(pz)3]-. ~ Ab initio Hartree-Fock SCF and multiconfigurational SCF calculations have been carried out by Guilleme et al.323to study the dihedral angle dependence of the vicinal proton-proton coupling in an ethane molecule.

8

Three-Bond Couplings between Hydrogen and Heteronuclei

Riiterjans and c o - ~ o r k e r have s ~ ~ presented ~ the joint calibration of Karplus coefficients of six types of vicinal couplings and evaluation of the corresponding backbone torsion angles in protein. This self-consistent analysis represents the state of the art in the field. Montelione and c o - w o r k e r ~have ~ ~ ~ developed HYPER, a hierarchical algorithm for automatic determination of protein dihedral-angle constraints based on a set of NMR-derived constraints including scalar-coupling ones.

number of residues total number of vicinal couplings measured (homonuclear 3JHHcouplings are also included if measured) types of heteronuclear couplings measured measured only number not reported

71 81 113 117 102+18 126 137 141

>I65 193 225

85 79 225 ca. 90 69

3Jcc 326 327 328 329 Jcc, JCN 330 Jcc, 3JCN 3 ‘Jcc, 3Jcc, JCN 33 1 332 3 3Jcc 3 333 Jcc, JHN

Ref:

a

type of vicinal heteronuclear couplings measured; 3

J homonuclear ~ ~ couplings have been also measured in most cases.

purine nucleoside analogues with 2,3-epoxypropyl or 3-amino-2-hydroxypropyl moiety 5’-O-(guanosine-2’-0-phosphonomet hy1)cytidine trans-syn cyclobutane photodimers o f dUpdT d(CCCCGGGG), an A-type double helix having B-type puckering of the deoxyribose ring the P5 helix of a group I intron ribozyme complexed with Co(NH3):+ a nonselfcomplementary undecamer DNA duplex AAA [GIN] the BS2 operator DNA sequence [C/N] the BS2 operator DNA sequence complexed with the Antennapedia homeodomain

Name

3 3 JHP, JCP

a

71 334 335 336 337 338 339 339

Ref:

Table 5.5 Nucleosides, nucleotides and oligonucleotides for which heteronuclear vicinal couplings have been used as a structural parameter

a

anti-HIV hemokine vMIP-I1 kringle 2, K2 module of human plasminogen, complexed with AMCHA DnaB (24-136), N-terminal domain of E. coli helicase BIR2, a second BIR domain of XIAP, inhibitor of apoptosis 0.5P Fv, complexed with the V3111Bpeptide P1053 N-TIMP-1, N-terminal domain of human tissue inhibitor of metaloproteinases- 1 ARID, the Dead ringer protein (262-398) from Drosophila melanogaster CBFP heterodimerization domain

3 3 JHC, JHN, 3 JHN 3’JHC, 33JHN

Table 5.4 Peptides and proteins for which heteronuclear couplings have been used as a structural parameter in 3 0 structure calculations

6

s2

00

wl

L

159

5: Applications of Spin-Spin Couplings

The resulting very rapid calculations allow one to restrict substantially the conformational space of the investigated proteins. Further examples of three-bond heteronuclear couplings between hydrogen and heteronuclei used in structure determination of labelled sugars, proteins and nucleotides are listed in Tables 3,4and 5, respectively. The averaging of structurally dependent couplings in cyclic peptides is a difficult problem in conformational analysis. In order to overcome it Baysal and M e i r o ~ i t c hhave ~ ' ~ introduced ~~~ a method based on extensive search of conformational space for the definition of the structure and population of most stable microstates. Structurally relevant 3 J H C and 3 J H H couplings are calculated as averages over the microstate contributions weighted by the corresponding population. Although this method is very much field-dependent, it yields couplings which agree quite reasonably with the experimental data. For the first time, Batta and K o ~ k r have ~ ~ ' measured heteronuclear couplings 2 J and ~ 3 J~H C involving hydroxyl protons and skeleton carbons. The experiments have been carried out for very high concentrations of 1.7 M of trehalose and sucrose in water. Bendiak342has measured 3 J H C and 4JHC heteronuclear couplings between the carbon of the carbonyl group and the ring protons. These experiments have been performed for 3C-labelled carbony1 groups in peracetylated saccharides. The method has been proposed by the author in lieu of permethylation. 3&C and 3 J H H couplings have been used by Martin and P i t ~ e to r ~solve ~~ the solution conformation of short-chain phosphatidylcholine. 3JHC, 'JHCand 35HH couplings have been helpful in structure elucidation of glycerol and sugar systems in glycoglycerolipids isolated by Lang and c o - w ~ r k e r sfrom ~~ a sponge-associated Microbacterium species. A comparative investigation of the consistent valence and extensible systematic force fields has been performed by Martins et aZ.345using the conformation of erythromycin A in benzene as a model compound. The ~ ~ structures were validated against 3 J H H and 3 J couplings. An extensive use of 3 J H C and 3JHH couplings has been made by GharbiBenarous et aZ.346in conformational analysis of josamycin, a 16-membered macrolide free and bound to bacterial ribosomes. A set of *JHCand 3JHC couplings has been reported by Fraziio Teixeira et al.347for a new flavan, identified as 4',7-dihydroxy-8-prenylflavan,which was isolated from the ethanolic extract of the stem bark of Brosimum acutfolium. The junction of the two alkaloid moieties in two known bis-indole alkaloids isolated from Tabernaemontana Zaeta has been established by Medeiros et al.348by the use of two- and three-bond H-C couplings. 3 J H C couplings have been of some help in elucidation of the structures of a v ~ of ~~ series of 3- and 5-aminopyrazoles synthesized by Emelina and P e t r ~ and derivatives of 5-arylmethylene-2,4-imidazolidinediones and 5-arylmethylene-2thioxo-4-imidazolidinonesstudied by Fresneau et al.350 The concerted use of 3 J H C and 3JHNcouplings has been applied by Groger et in order to distinguish between two diastereoisomers of allylic derivative of aspartic acid,

'

Nuclear Magnetic Resonance

160

A very rich set of spin-spin couplings has been reported by McMahon et al.352for a series of the siloxyaluminum products obtained by cleavage of cyclodimethylsiloxanes by dialkylaluminum hydrides. In the Si NMR spectra of some of those compounds such as, for example, (?-Bu)2Al(pOSiMe2H)(pOSiMe2-0SiMe2H)Al(t-B~)~, a very small 3JHSi coupling of 1.0 Hz only has been observed. 3 J H p and 3 J H H couplings for structural Raetz and c o - ~ o r k e r have s ~ ~ used ~ characterization of six modified lipids containing 4-amino-4-deoxy-~-arabinose and/or phosphoethanolamine substituents. 3 J H H and 3 ~ H coupling p values typical of trans and gauche rotamers (3J(HH)g = 4.2 HZ; 3 J ( ~=~13.9 p HZ, 3 J ( ~ p ) = r 33.0 HZ) have been = 2.3 HZ, applied by Jedrzejczyk et al.354in order to elucidate the population distribution of the rotamers in the phosphonic analogues of aspartic acid. A set of 1,3,2-0xazaphosphorino [4,3-a] isoquinolines, a new ring system, has been synthesized by Martinek et al.,355and 3 J H p couplings measured for these compounds have been applied as their conformational probe. 3 J H p couplings across the P-Ru-S-H fragment, which follow the Karplus relationship, have been observed by Ma et in the low temperature NMR spectra of the cis-RuX2(P-N)(PPh3)(SH2) (X = Cl, Br, P-N = [o-(N,Ndimethylamino)phenyl]diphenylphosphine). A JHy coupling of ca. 5 Hz has been observed by Eppinger et al.87in the H and Y NMR spectra of Y[N(SiHMe&I3. The small value of this coupling strongly suggests that the coupling takes place across a three-bond rather than across a one-bond path. Several cis and trans-Pt(Ypy)2X2 complexes where Ypy is a methyl derivative of pyridine and X = Cl or I, have been studied by Tessier and R ~ c h o n . ~ ' ~ The 3JHpt and 3Jcpt couplings consistently larger for cis than trans isomers have been observed. 3JHpb couplings of ca. 20 Hz have been determined by Claudio et aZ.,358for two Pb(1I) complexes, [Pb(EDTA)I2- and [Pb(EDTA-N4)I2'.

9

Three-Bond Couplings not Involving Hydrogen

The possibility to determine torsion angle restraints from 3 J and ~ ~ 3JCHN couplings in proteins has been investigated by Hennig et al.359 Vicinal carbon-carbon and proton-carbon couplings across the glycosidic linkage in disaccharides have been calculated by Cloran et al.360using DFToptimized geometries, the same hybrid functional and a specially designed basis set as a function of the backbone torsion angles. The resulting torsional dependencies were compared with the experimental Karplus curves for these coupling pathways recently reported by Bose et ~ 1 . ~ ~ ' Malloy and c o - ~ o r k e r shave ~ ~ ~ used 233Jcccouplings for isotopomer analysis of glutamate. This analytical method appears to be a useful tool in tracing citric acid cycle intermediates. 3 Jcp, 3 J H p and 3 J H H Couplings measured by Maestre and for

5: Applications of Spin-Spin Couplings

161

diribosylribitol phosphate have been used in determination of the conformations of the ribose residues and the phosphate linkage in this compound. The structure of the nucleotide part of the natural organometallic corrinoidB12 cofactor, methylcob(III)alamin, has been solved by Krautler and cow o r k e r ~with ~ ~ the help of vicinal 3 J ~ couplings p supported with 3JHC and 3 ~ H couplings. p Carbon-phosphorus couplings across one, two and three bonds have been measured by Gancarz et aZ.365for a series of variously substituted phosphop In particular, the nates, special attention having been paid to 3 J ~ couplings. analysis of concentration dependent spectra of diethyl 2-hydroxypropylphosphonate has led the authors to the conclusion that the compound exists in solution as a mixture of various conformers whose relative populations depend on the total concentration of hydroxyphosphonate. "JcP(n = 1-3) couplings have been reported by Akacha et aZ.366for a series p have been of new P-phosphonylated hydrazones. 3 J H p and 3 J ~ couplings determined by Ferreira et aZ.367 for some dioxaphosphinopyridines and pyridoazaphosphinines. A set of the C-Sn couplings including the vicinal and geminal ones has been determined by LyEka et aZ.368for bis(tributylstanny1) L-( +)-ascorbate and bis(tributylstanny1) 0-5,6-isopropylidene-~-(+)-ascorbate. A vicinal C-T1 coupling of 220 Hz for the methyl group and a geminal C-TI coupling of 205 Hz for the carbonyl group of the acetyl fragment have been observed by Tung et aZ.369in the spectrum of acetato-N-tosylimido-mesotetraphenylporphyrinatothalium(111). A vicinal F-F coupling has been used by Colmenares et aZ.370to follow the cis-trans isomerization process of 11,12-difluororhodopsin and related oddnumbered fluororhodopsins. A small 3JFF coupling of 7.5 Hz only has been observed for the cis isomer and a large one of 107.5 Hz for the trans compound. An analysis of the 31P NMR spectra carried out by Borkenhagen et aZ.371 for the products of the reaction of the trimethylsilylphosphinimidoderivative of bornanylene(dimethy1phosphino)methyl imine with diorgano-chlorophosphines has yielded a set of nJppcouplings (n = 1-3), which has been used as evidence that these compounds are of the structure RR2P=N-P'+'R",PR"2. 3Jpp couplings have been found by Bakalarz et aZ.372to be of particular use in their studies on elucidation of the structure of diethyl(2-tert-butylphenylphosphinoyl-3-N,N-diethylaminopropyl)phosphonate and diphenyl (3-N,Ndiethylamino-2-tert-butylphenylphosphinoylpropyl)diphenylphosphineoxide. 1 JP107Ag and 3 J p 1 0 7 ~ gcouplings of 636 Hz and 37 Hz, respectively have been determined by Sz3yk and G ~ l i n s kfrom i ~ ~ the ~ low temperature spectrum of [Ag2(C4F9C00)2dppm]complex (dppm = bis(dipheny1phosphino)methane) providing evidence that the compound and some of its other analogues exist in the form of dimeric trigonal Ag(1) complexes with bridging carboxylates and dppm in solution. The corresponding l&Ag and 3 J ~couplings ~ g are 715 Hz and 42 Hz, respectively.

Nuclear Magnetic Resonance

162

10

Couplings over More than Three Bonds

Long range 4JHH and 5 J H H couplings have been measured for mono-sulfated galactosylceramidesby Iida-Tanaka and I s h i z ~ k a . ~ ~ ~ An analysis of the H NMR spectra of fullerene C-60 cycloadducts with 1-(4nitrophenyl)-3-phenylnitrililideperformed by Chertkov et uZ.375has yielded proton-proton couplings across four and more bonds. The proton-proton couplings across four and three bonds have been for 27 brominated diphenyl ethers. reported and analysed by Hu et The temperature dependence of a six-bond proton-proton coupling, 6 J C H O g H , has been applied by Lounila et aZ.377in their studies on the formyl rotational energetics and the conformational analysis of 2,6-dichloro- and 2,6dinitro-benzaldehydes. A set of n J H C couplings (n = 1-4) has been reported by Eifler-Lima et for salazinic acid isolated from some species of Parmotrema. The 2D selective and double-selective J-HMBC methods have been applied by Yamaguchi and co-workers to measure the natural abundance H-N longrange couplings in harmane,42 i. e. 3-methyl-4-carboline, the basic framework of P-carboline alkaloids and in some substituted pyridines.379 A conformational analysis by the use of the NMR parameters such as 4JHF, 1 J c F and 2 J c has ~ been performed by Abraham et for 3-fluorobutan-2one and 3,3-difluorobutan-2-one. N o solvent dependence has been observed for these parameters in the case of 3,3-difluorobutan-2-one; this provides strong evidence that this compound exists in solution in the form of one conformer only, which is in agreement with the ab initio calculations. A study on indirect fluorine coupling anisotropies in p-difluorobenzene has been performed by Vaara et A full set of the ab initio and experimental couplings which also includes those across four and five bonds between the C, F and F, F atoms, respectively has been reported for this molecule. An impressive set of J coupling data has been collected by Brey and for the oxetanes containing fluorine on or near the ring. This included not only those across one and two C-F bonds but also a large number of n J ~ ~ couplings, n = 2-6. Some trends and rules governing these parameters have been discussed by the authors. It has been shown by Stern and W e ~ t m o r e ~that * ~ tris(perfluoropropanoy1 and perfluorobutanoy1)benzoylmethanate complexes of rhodium(II1) exist as nearly statistical 1:3 mixtures of their respective fac and mer octahedral isomers, whereas the analogous perfluoroalkanoylthiobenzoylmethanatecomplexes have afac-octahedral structure. Very small 3JFF couplings of 0-3 Hz only and rather large couplings across four bonds (of 9 Hz) have been observed for all the compounds studied. An eight-bond F-F coupling of 3.4 Hz has been observed by Mahon et aZ.384 in the F NMR spectrum of R ~ ( d p p e ) ( C 0 ) ~ ( O S 0 2 C F and ~ ) ~marginally larger (3.8 Hz) in the spectrum of Ru(dppe)(CO)(H20)(OSO~CF3)~.It has been suggested by the authors that both through-bond and through-space coupling mechanisms should be taken into account in this case.

163

5: Applications of Spin-Spin Couplings

11

Couplings across the Hydrogen Bond and through Space

Direct observation of couplings through the hydrogen bond has provided a new possibility for structural studies of many classes of compounds and several new methods have been designed specifically for measurement of this important type of coupling. Meissner and S ~ r e n s e n ~have * ~ presented 3D S3 HNCO pulse sequences for measurements of and 3h.!cpN couplings in proteins. The same authors386 have reported 3D hHNCO TROESY pulse sequence that allowed them to measure 3hJHNCa couplings for the first time. Cordier et ul.387have measured the 2hJHNCTcouplings in perdeuterated protein with the help of 2D 1HN-’3C’HMQC experiments. Also for perdeuterated protein Wang et ~ 1 . have ~ ~ ’published 3D 3hJCFN HNCO pulse scheme that allows measurement of the 3 h J C r N couplings. Dingley et ~ 1 . ~have ~ ’ used the J N c H(N)CO experiment to measure 3hJC,N couplings present in DNA. Pervushin et al. 390 have proposed [”N, ‘HI-ZQ-TROESY sequence that facilcouplings in large biomolecules. itates measurements of Ih.lHN and 2hJNN Hennig and Williarn~on~~’ have introduced 2hJ(N,N)COSY pulse scheme that permits measurement of 2hJ” couplings in the absence of the observable intervening proton. A standard 2D COSY experiment has been modified by Fierman et al.392to detect small 2hJ couplings of the order of 0.3 Hz between hydroxyl groups in 1,3- and 1,4-diols. The possible use of scalar couplings across hydrogen bonds as a novel conformational restraint in the calculation of structures of multimolecular assemblies has been briefly discussed in a review by Wider and W u t h r i ~ h . ~ ~ ~ Some other new data are presented in Table 6. Availability of new data facilitates deeper understanding of the nature of the hydrogen bond. Observations of scalar couplings occurring through hydrogen bonds have been briefly reviewed by G e m m e ~ k e rThe . ~ ~author ~ has suggested that the so called ‘through-space J couplings’ reported earlier in the literature are rather real scalar couplings transmitted by the valence electrons. Exponential correlation between 3hJCrN and hydrogen bond length taken from the Xray data has been proposed by Bax and c o - ~ o r k e r s Dingley . ~ ~ ~ et u~~~~ have observed the 3 h J c ~ Ndependence on the H..-O=C valence angle in hydrogen bonds of the N-H..-O=C type in proteins. Dingley et u E . ~ ’ ~ have found that the IhJHN and 2hdNN couplings in a DNA triplex can be nicely correlated with the isotropic shift of the imino proton. A quantitative investigation of the trans-hydrogen 2 h J N C coupling involved in polypeptide N-H-..O=C hydrogen bonds and the scalar 2hJ” couplings across N-H. ..N hydrogen bonds of Watson-Crick and Hoogsteen base pairs has been performed by Scheuer and B r i i s ~ h w e i l e rusing ~ ~ ~ DFT calculations. The calculations provided a theoretical explanation of these couplings that corroborates their quantum-chemical origin. DFT calculations of the scalar couplings IhJHNand 2hJ” of the N-H...N hydrogen bonds in the anion [C=N--.L...N=C]-, L = H, D and of the cyclic hydrogen-bonded formamidine dimer (HCNNH& have been performed by Benedict et al.399The existence of the scalar couplings has been 2hJHN~t

hydrogen bond type, symbols of nuclei involved in bold b type of couplings measured number of residues (or nucleotides) versus number of couplings measured

a

a

Compoundsfor which scalar couplings were measured through the hydrogen bond

CIN] an immunoglobulin binding domain of streptococcal protein G C/N] chymotrypsin inhibitor 2 (1 9-83) C/N] chymotrypsin inhibitor 2 (19-83) D/C/NI ubiquitin D U N ] MAP30, RIP anti-HIV anti-tumor plant protein Nl apomyoglobin, H24-H 1 19 side chain-side chain interaction C/N] Hoogsten-Watson-Crick DNA triplex C/N] Hoogsten-Watson-Crick DNA triplex C/N] d(G4T4G4),Oxy- 1.5 DNA quadruplex C/N]d(G4T4G4), Oxy-1.5 DNAquadruplex N] HIV-2 TAR RNA complexed with argininamide C/N] DNA duplex, complexed with [NI Antennapedia homeodomein C/N] DNA duplex, complexed with EN] Antennapedia homeodomein

Name

Table 5.6

3h JCN 2h JHC 3h JHC 2h JHC 3h JCN 2h JNN 2h JNN Ih JHN 2h JNN 3h JCN 2h JNN 2h JNN Ih JNN

b

816 30/4 28+69/12 28+69/12

815

56/38 65/16 691 6 76/8 263/65 U1 32/14 32/8

c)

19 385 385 387 388 394 395 395 389 389 39 1 390 390

Ref

5: Applications of Spin-Spin Couplings

165

invoked by the authors as strong evidence of the covalent character of the hydrogen bond. Two papers devoted to the calculations of spin-spin couplings across the hydrogen bond have been published by Del Bene et al.4007401 In one paper400 the Fermi contact contributions have been calculated for hydrogen-bonded 00, N-N and N-0 in ten model compounds, in the otherM1the authors have shown that relationships exist between the X-Y distance in an X-H-Y hydrogen bond, the anharmonic proton-stretching frequency, the H NMR chemical shift, and the X-Y spin-spin coupling. The EOM-CC (equation-of-motion coupled-cluster) results for J and *J in [FHFI- and its higher homologues [F(HF),]-, n = l a , have been reported by Perera and Bartlett.402 The long-range couplings between the N-methyl protons and quaternary carbons bound to antimony have been observed by Tokunaga et ~ 1 in the . J-~ HMBC 2D NMR spectra of 2-(N,N-dimethylaminomethyl)phenylbis(4methylpheny1)stibine and its dichloride showing direct evidence of the existence of intramolecular Sb---N nonbonded interaction (Figure 8); 4 J ~ ~ ~ =3 , 4 JNCH3,C16 = 2.3 Hz for the parent compound and 4 J N ~ H 3 , ~=I 4.6 Hz, 4 JNCH3,$16 < 1.O Hz for the dichloride have been observed.

CH,

CH3

Figure 8

Applications of F NMR to probe long-range interactions in rigid organic molecules, cubanes and dynamics of proteins and other biological systems have been presented by Gakh et aLM4 Correlations between long-range couplings involving fluorine and other chemical and physicochemical properties of the compounds have been obtained, and examples of unusual couplings which cannot be explained in terms of through-bond and through-space mechanisms have been discussed by the authors. 6JHFand 'JCF through-space couplings have been observed by JaimeFigueroa et ~ 1 . between ~ ' ~ the fluorine and the methyl on the acetyl group ('H and I3C) in a series of 4,5-substituted 1-acetyl-8-fluoronaphthalenes. The magnitude of these couplings depends on the nature of the substituent at C4, the internuclear distance and the solvent. A C-F coupling across formally five-bonds of 11 Hz has been observed by

~

~ 9

166

Nuclear Magnetic Resonance

Chen et ~ 1 . for 4 ~some ~ derivatives of 3,5-diaryl-4,4-fluoro-4-bora-3a,4a-diazas-indacene and interpreted in terms of a through-space interaction. This coupling has been used to draw conclusions regarding the structures of these molecules. Mallory and c o - w ~ r k e r have s ~ ~ ~continued their interesting studies on the through-space fluorine-fluorine couplings. The J F F couplings have been measured by them for a set of eighteen compounds related structurally to 1,8difluoronaphtalene. An analysis of these data led them to the conclusion that an exponential relationship exists between J Fand ~ dFF, and a linear relationship between J F F and the extent of the overlap interaction between the in-plane fluorine 2p lone-pair orbitals. Through-space transmitted F-F couplings (formally across seven bonds) of ca. 6 to 9 Hz have been observed by Aizikovich et aL408in a series of N-substituted fluoroquinolones. Investigation of the F-F spin-spin couplings in 1-(x-fluorophenyl)-8-(yfluoropheny1)naphthalenes (x, y = 2, 3, 4) which obey a through-space mechanism has been performed by Ernst and Sakhaiie409 The solution behaviour of the complexes ( N B U & [ M ~ ( ~ - L L ) ~ (M R ~=] Pd, Ni, Pt, LL = pyralozate (pz), 3,5-dimethylpyrazolate (dmpz), 3-methylpyrazolate (mpz), indazolate (indz), R = C6F5; and M = Pd, LL = pz, dmpz, mpz, indz, R = 2,4,6-C6F$2 has been re-investigated by Espinet et aZ.410by the use of NMR spectroscopy. They observed the presence of the mixture of two diastereoisomers (head-to-head and head-to-tail) for the asymmetrically substituted azolates (mpz and indz) and a strong through-space coupling between the endo ortho fluorine nuclei of different MR2 fragments in the F NMR spectra of those diastereoisomers whose boatlike structures place these atoms at short distances. It should be added that the incorrect interpretation of the H and F NMR spectra and the unjustified proposals of equilibria and dynamic behaviour of these complexes has been earlier offered by Lopez et aZ.41

12 1

2 3 4 5

6 7 8 9 10 11

References K . Kamienska-Trela and J. Wojcik, Specialist Periodical Report: Nuclear Magnetic Resonance, 2000,29, 165 (Royal Society of Chemistry). R.K. Harris, J. Kowalewski and S. Cabral de Menezes, Pure & Appl. Chem., 1997,69,2489. R.H. Contreras, J.E. Peralta, C.G. Giribet, M.C. Ruiz de Azua and J.C. Facelli, Ann. Rep. NMR Spectrosc., 2000,41, 55. H. Fukui, Progress NMR Spectrosc., 1999,35, 267. L. Visscher, T. Enevoldsen, T. Saue, H.J.A. Jensen and J. Oddershede, J. Comput. Chem., 1999,20, 1262. J. Vaara, K. Ruud and 0. Vahtras, J. Comput. Chem., 1999,20, 1314. K.V. Mikkelsen, K . Kuud and T. Helgaker, J. Comput. Chem., 1999,20,1281. J. Autschbach and T. Ziegler, J. Chem. Phys., 2000, 113, 936. P. Pyykko, Theor. Chem. Acc., 2000,103,214. L. Ernst, Progress NMR Spectrosc., 2000, 37,47. H . Giinther, J. 5ruz. Chem. SOC.,1999, 10,241.

5: Applications of Spin-Spin Couplings

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

167

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J. Vaara, J. Kaski and J. Jokisaari, J. Phys. Chem., 1999,103,5675. W .S. Brey and M.L. Brey, J. Fluorine Chem., 2000, 102,219. G.A. Stern and J.B. Westmore, Can. J. Chem., 1999,77, 1734. M.F. Mahon, M.K. Whittlesey and P.T. Wood, Organometaliics, 1999, 18,4068. A. Meissner and O.W. Ssrensen, J. Magn. Reson., 2000,143, 387. A. Meissner and O.W. Ssrensen, J. Magn. Reson., 2000,143,431 . F. Cordier, M. Rogowski, S. Grzesiek and A. Bax, J. Magn. Reson., 1999, 140, 510. Y.-X. Wang, J. Jacob, F. Cordier, P. Wingfield, S.J. Stahl, S. Lee-Huang, D. Torchia, S. Grzesiek and A. Bax, J. Biomol. NMR, 1999,14, 18 1 . A.J. Dingley, J.E. Masse, J. Feigon and S. Grzesiek, J. Biomol. NMR, 2000, 16, 279. K. Pervushin, C. Fernandez, R. Riek, A. Ono, M. Kainosho and K. Wiithrich, J. Biomol. NMR, 2000, 16, 39. M. Hennig and J.R. Williamson, Nucl. Acid. Res., 2000, 28, 1585. M. Fierman, A. Nelson, S.I. Khan, M. Barfield and D.J. O’Leary, Org. Lett., 2000,2,2077. G. Wider and K. Wiithrich, Curr. Opinion Struct. Biol., 1999,9, 594. M. Hennig and B.H. Geierstanger, J. Am. Chem. SOC.,1999, 121, 5 123. A.J. Dingley, J.E. Masse, R.D. Peterson, M. Barfield, J. Feigon and S. Grzesiek, J. Am. Chem. Soc., 1999,121,6019. G. Gemmecker, Angew. Chem. Int. Ed., 2000,39, 1224. G. Cornilescu, B.E. Ramirez, M.K. Frank, G.M. Clore, A.M. Gronenborn and A. Bax, J. Am. Chem. SOC.,1999,121,6275. C. Scheurer and R. Briischweiler, J. Am. Chem. Sac., 1999,121,8661. H. Benedict, I.G. Shenderovich, O.L. Malkina, V.G. Malkin, G.S. Denisov, N.S. Solubev and H.-H. Limbach, J. Am. Chem. Soc., 2000,122, 1979. J.E. Del Bene, S.A. Perera and R.J. Bartlett, J. Am. Chem. SOC.,2000, 122, 3560. J.E. Del Bene and M.J.T. Jordan, J. Am. Chem. Soc., 2000,122,4794. S.A. Perera and R.J. Bartlett, J. Am. Chem. SOC.,2000, 122, 1231 T. Tokunaga, H. Seki, S. Yasuike, M. Ikoma, J. Kurita and K. Yamaguchi, Tetrahedron Lett., 2000,41, 103 1 . Y.G. Gakh, A.A. Gakh and A.M. Gronenborn, Magn. Reson. Chem., 2000, 38, 551. S. Jaime-Figueroa, L.J. Kurz, Y. Liu and R. Cruz, Spectrochim. Acta A, 2000,56, 1167. J . Chen, J. Reibenspies, A. Derecskei-Kovacs and K. Burgess, Chem. Commun., 1999,2501. F.B. Mallory, C. W. Mallory, K.E. Butler, M.B. Lewis, A.Q. Xia, E.D. Luzik Jr., L.E. Fredenburgh, M.M. Ramanjulu, Q.N. Van, M.M. Francl, D.A. Freed, C.C. Wray, C. Hann, M. Nerz-Stormes, P.J. Carroll and L.E. Chirlian, J. Am. Chem. Soc., 2000, 122,4108. A.Ya. Aizikovich, M.V. Nikonov, M.I. Kodess, V.Yu. Korotayev, V.N. Charushin and O.N. Chupakhin, Tetrahedron, 2000,56, 1923. L. Ernst and P. Sakhaii, Magn. Reson. Chem., 2000,38, 559. P. Espinet, A.M. Gallego, J.M. Martinez-Ilarduya and E. Pastor, Inorg. Chem., 2000, 39, 975. G. Lopez, J. Ruiz, G. Garcia, C. Vicente, J. Casabo, E. Molins and C. Miravitlles, Inorg. Chem., 1991, 30, 2605.

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6 Nuclear Spin Relaxation in Liquids and Gases BY R. LUDWIG

1

Introduction

The aim of this report is to cover the progress of work in the field of magnetic relaxation and self-diffusion in liquids and gases over a period of twelve months from June 1999 to May 2000, and is a continuation of the report given last year.' As in previous periods, this review is limited to work on comparatively simple liquids and solutions of physico-chemical and chemical interest, as publications in the field of macromolecules and biological chemistry are covered elsewhere in this volume. Of course, such a distinction is sometimes problematic, as innovative work dealing with solutions of complex molecules may be of interest for research in the field covered here. Thus, at the risk of duplication, some interesting studies dealing with more complex systems are mentioned briefly. At the beginning of this chapter it is convenient to quote some authoritative reviews in the subject area. More specialized reviews will be discussed in the corresponding subsections. Also, some important general trends are briefly highlighted here. Details will be discussed later in this chapter. Methods of measuring simple spin-lattice TI relaxation times are summarized by Kingsley.* This review includes optimization, data analysis, elimination of errors, signal intensity formulas, chemical exchange, and spatially localized measurements. Bertini et aL3 reviewed the solution structure of paramagnetic metalloproteins. In paramagnetic systems the broadening of the NMR lines is due to the fast nuclear relaxation rates, which are induced by the coupling of the nucleus with unpaired electrons. The authors developed an NMR method which allows the structure determination of solved paramagnetic proteins. The water exchange reaction between coordination shells around metal ions in aqueous solution is a fundamental reaction in understanding the reactivity of these ions in chemical and biological systems. Helm and Meerbach4 summarized NMR methods and theoretical techniques to study water exchange on metal ions. Recent advances in the NMR analysis of bio-molecules in the liquid and solid state are reviewed by Siegal et aL5 The high power of combining liquid chromatography with NMR spectroscopy is discussed by Albert.6 The author presented current developments ~~

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employing the on-line coupling of capillary as well as supercritical fluid separation. Gounarides et aL7 reviewed applications of pulse field gradient diffusion NMR to mixture analysis and ligand-receptor interactions. This method is unique for analysing complex mixtures and for detecting intermolecular interactions. Kushner et aL8 reviewed solvent isotope effects by using light and heavy water. An article by Macdonald' reported deuterium NMR studies of polyelectrolytes interacting with deutero-labeled surfactants. Keusters et al. l o examined the parallels between magnetic resonance and optical spectroscopy, with the goal of determining to what extent the benefits of 2D NMR might be extended into the optical regime. Radiation damping is a phenomenon due to the interaction between the magnetization and the resonant circuit. Radiation damping effects were almost ignored in NMR for about thirty years. Since 1980, NMR has found wide applications in biological systems with the use of high magnetic fields. Radiation damping effects, which are strongly related to the strength of the magnetic field, have become a hot topic, and publications on radiation damping are increasing quickly. Miller et al." reviewed the interplay among recovery time, signal and noise in series- and parallel-tuned circuits. The authors discussed how the quality factor of the probe affects the signal-to-noise ratio, the probe recovery time and radiation damping effects. The capabilities and limitations of toroid cavity detectors for high-resolution NMR spectroscopy and rotating frame imaging are reviewed by Momot et a1.12 Radiation damping effects produce unexpectedly small effects because of self-cancellation of magnetization and short induction decay times. Radiation damping artifacts using water signal suppression techniques are reported by Price.13 Studies of slow molecular motions in supercooled and glassy systems using NMR techniques has become more popular and the number of papers increased steadily during the past few years. Meanwhile there are some authorative reviews in this field. Buznik l4 summarized nuclear physical methods to study glassy systems. In particular, the relation between the spectral parameters and microscopic structural characteristics of glass-forming systems is shown. Recent developments of phosphate glasses for a variety of technological applications have been reviewed by Brown. Spectroscopic and diffraction studies of simple phosphate glasses are described. Ngai et investigated the break down of the Stokes-Einstein-Debye relation in glassforming liquids. The authors concluded that the shear viscosity has a significantly stronger temperature dependence than either the self-diffusion coefficient, or the translational diffusion coefficient of tracer molecules of comparable size. The non-exponential relaxation in super-cooled liquids and glasses has been reviewed by R.V. Chamberlin. l 7 This review mainly deals with the basic question; is the observed response homogeneous where all regions of the sample exhibit intrinsic non-exponential behavior or is it a result of a homogeneous distribution of relaxation times? Physical models of diffusion for polymer solutions and gels are summarized by Masaro and Zhu.'*

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The use of hyperpolarized noble gases in preclinical and clinical applications has increased drastically. Non-radioactive noble gases with a nuclear spin 1/2 (3He, '29Xe) can be hyperpolarized by optical pumping. Polarization is transferred from circularly polarized laser light to the noble-gas atoms via alkali-metal vapors (spin exchange) or metastable atoms (metastability exchange). Hyperpolarization results in a non-equilibrium polarization five orders of magnitude higher than the Boltzmann equilibrium, compensating for the several thousand times lower density of noble gases as compared with liquid state hydrogen concentrations in tissue and allows for short imaging times. Hyperpolarization can be stored sufficiently long to allow for transport and application. Song et a l l 9 reviewed the use of laser-polarized xenon and presented selected experiments including biological systems and polarization transfer to molecules in solution and on surfaces. Spin polarization-induced nuclear Overhauser effect (SPINOE) is a recent experimental finding that accomplishes the transfer of spin polarization of laser-polarized xenon and helium to other nuclei. Song2' showed how cross-relaxation rates between xenon and other molecules can be measured to investigate molecular interaction and to probe the chemical and structural environment of the binding sites. Further progress in this field of using polarized noble gases will be described in detail in Chapter 4. Relaxation and self-diffusion techniques in solution are widely used to study hydrogen-bonded systems. The nuclear quadrupole coupling constant (NQCC) presents a sensitive probe for the strength of hydrogen bonding. Unfortunately in the liquid phase this property can not be measured in a direct way. Using a new NMR method, the first reliable results for the DQCC in liquid heavy ammonia were obtained as a function of temperature by Hardy and Zeidler.21y22The value of 231 kHz amounts to only 80% of the gas phase value. These results support the hypothesis that hydrogen bonds play an important role in liquid ammonia, similar to the situation in water. 2

General, Physical and Experimental Aspects of Nuclear Spin Relaxation

1 General Aspects. - Long range dipolar couplings were successfully interpreted in terms of a classical average dipolar field, generated by average nuclear spin magnetization of the sample, and acting on the spins as an additional component of the magnetic field in the usual Bloch-Redfield equation of motion for the spins in each molecule. This classical formalism23324 ignores the quantum correlations between spins on different molecules. Recently, analogies were observed between the well understood experimental manifestations of intramolecular N-quantum coherences and those of the long range dipolar couplings. Some authors already proposed to replace the classical formalism by a direct discussion of the behavior of the density operator for the whole spin system, A remarkable method by Warren et al.25-31 carefully takes into account the small long range dipolar couplings. Unfortunately, the used rigid lattice formalism precludes any discussion of relaxation and molecular diffusion. For these

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problems the classical formalism seems appropriate. It was soon found out that both formalisms are equivalent, leading to the same predictions in agreement with experimental results. In the classical approach Jeener23 uses a density operator for all spins of the sample with dipolar couplings, being careful not to drop the long range interactions. The approach of Warren et a1.28showed that the correlation between spins on different molecules does not contribute to any observable effects. In contrast to this equivalence of both approaches, Kimich and Ardelean32concluded in their article on effects of the dipolar field in pulsed liquid NMR, that the two formalisms do not describe the same physical phenomena and that experiments may help to clarify the issue. Using a rather trivial extension of the classical formalism, Jeener33 now comes to the conclusion that both formalisms are exactly equivalent in all respects. On a simple example he showed that the two approaches lead to the same density operator for the whole spin system, with the same equations of motion for the matrix elements. The effect of translational diffusion of Hahn spin-echo pulse sequences in the presence of field gradient pulses is well known. The so-called pulsed gradient spin echo (PGSE) has been reviewed in several This technique and its applications are usually discussed in the chapter ‘SelfDiffusion of Liquids’ of this report. PGSE has become a standard method for the determination of diffusion coefficients. Recently, Ardelean and a new technique, which considers signals arising only K i m m i ~ hpresented ~~ after nonlinear evolution effects. This method may be referred to as pulsed gradient non-linear echo (PGNE) experiment. This sort of coherence refocusing is related to the class of multiple echoes that was first discovered already two decades The authors examined peculiar features of the echo attenuation by diffusion that can be expected after nonlinear evolution. A general formalism is presented comprising both the ordinary PGSE and the unconventional PGNE scenarios. In particular it is shown theoretically as well as experimentally that nonlinear evolution after the gradient pulses contributes to attenuation even if diffusive displacements in the proper gradient pulse intervals are negligible. 2.2 Experimental Aspects. - The magnetic field dependence of the nuclear spin-lattice relaxation rate provides a detailed report of the spectral density functions that characterize the intra- and inter-molecular fluctuations that drive magnetic relaxation. Wagner et addressed the difficult sensitivity and resolution problems associated with low magnetic field strength by using two magnets in close proximity and shielded from each other. The sample is stored in the high magnetic field, pneumatically driven to the variable satellite field, then returned to the high field for detection at high solution. A magnetic shield effectively decouples the two magnets so that varying the satellite field strength has minimal effect on the field strength and shim of the high field magnet. The disadvantage of the sample-shuttle magnet-pair system is the restriction imposed on the relaxation times by the finite shuttle times. This rate maximum is about 20 s - ’ for most practical reasons. The authors demon-

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strated that the sensitivity gains over switched-current magnet systems permit characterization of solute inter- and intra-molecular dynamics over the time scale range from tens of microseconds to less than a picosecond. This range permits investigations of number of crucial chemical dynamics, while high sensitivity permits examination of a variety of solute spins. It is well known that accurate determination of integral intensities of broad lines is difficult when spin relaxation during the applied pulses cannot be neglected and/or when ringing of the tank circuit interferes with the signal. Hedin and F ~ r presented o ~ ~ an extension of the analytical solution of the generalized Bloch equations,47 which is then used to evaluate the signal intensity obtained in a composite pulse experiment designed to cancel ringing effects. Boulat4*applied the method of the first derivative of the matrix exponential to analyse and compare several experimental schemes that were proposed to avoid the effects of multi-step magnetization transfer, referred to as spin diffusion, in the measurement of the cross-relaxation rate constant between a pair of spins. It has been recognized for some time that the quality of high-resolution NMR spectra is degraded by temperature gradients which results from pure temperature stabilization of the sample. Good temperature control is a crucial factor in the design of NMR probes and variable temperature units. Loening and Keeler49 introduced two methods which make it possible to measure, quantitatively and in a straightforward manner the temperature profile and the rate of convective flow in liquid samples. Damyanovich et aZ?’ designed a low-temperature, high-resolution NMR probe head which eliminates the problem of electrical discharge commonly experienced during radio frequency pulse cycling in a helium environment. The sample coil and all high-voltage elements could effectively be isolated in vacuum, while at the same time permitting good thermal contact between the sample and cryonic gas. The authors checked their probe design in a temperature range between 4.6 and 77 K using pulses less than 50 ms. Peshkovsky and MacDermott” reported NMR spectra of deuterated nitrobenzene recorded in the presence of strong electric fields using a home-built probe and electric amplifier. A two-dimensional detection method was used in which fields are applied only during the variable delay associated with the indirect dimension. The authors observed a strong dependence of the order parameter on concentration: neat nitrobenzene ordered significantly more than nitrobenzene diluted in a nonpolar solvent. The degrees of order for the diluted nitrobenzene were rationalized in terms of simple electrostatic theories.

2.3 Relaxation in Coupled Spin Systems. - In liquid-state NMR, spin relaxation due to cross-correlation of two anisotropic spin interactions can provide useful information about molecular structure and dynamics. These effects are manifest as differential line width or line intensities in the NMR spectra. Recently, new experiments were developed for the accurate measurement of numerous cross-correlated relaxation rates in scalar coupled multi-spin

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systems. The recently introduced concept of transverse relaxation optimized spectroscopy (TROSY) is also based on cross-correlated relaxation. B r ~ t s c h e routlined ~~ the basic concepts and experimental techniques necessary for understanding and exploiting cross-correlated relaxation effects. Strongly coupled spin systems provide many curious and interesting effects in NMR spectra, one of which is the presence of unexpected lines. A physical reason is given for the presence of these combination lines. Bain53analysed the X part of the spectrum of an ABX spin system as an example. Kaikkonen and K o ~ a l e w s k calculated i~~ a relaxation matrix for a multipolar AX spin system under the on-resonance spin-locking condition. They demonstrated that the spin-lock leads to many relaxation pathways being blocked, resulting in a considerably simplified relaxation network. Cross-correlated relaxation rates y are commonly obtained from constant time experiments by measuring the effect of the desired cross-correlated relaxation on an appropriate coherence during the constant time T. These measurements are effected by systematic errors, which derive from undesired cross-correlated relaxation effects taking place before and after the constant ~~ the time period T. In their paper, Carlomagno and G r i e ~ i n g e rdiscussed sources and the size of these errors in an example of two pulse sequences and proposed experimental approaches to avoid them. investigated unitary bounds and controllability of quantum Unitidt et evolution in NMR spectroscopy. Some features are exemplified in relation to heteronuclear coherence transfer by planar and isotopic mixing in liquid state NMR. Luy and G l a ~ e presented r~~ analytical polarization and coherence transfer function for three dipolar coupled spins '/2 under energy matched conditions. Cornilescu et al.58established a correlation between 3JNcand hydrogen bond length in proteins. Cordier et al.59 demonstrated that J connectivity between amide protons and hydrogen-bond-accepting carbonyl carbons can be observed in a perdeuterated protein. The authors pointed out, that for technical reasons the 3JHccouplings are more difficult to measure than the 3 J N c . Kontaxis et a1.60 evaluated cross-correlation effects and measured one-bond couplings in proteins with short transverse relaxation times. Several methods for the selective determination of spin-lattice and spin-spin relaxation rates of backbone protons in labeled proteins are presented by Millet et a1.61Permi et a1.62presented intensity modulated HSQC- and HMQC-type experiments to measure 3JHNH a in proteins. Richter et a P 3 introduced a new experiment for the determination of sugar conformation in large oligonucleotides from analysis of dipole-dipole cross correlated relaxation. Wang et al.64 presented a method to measure 15N-'H d i p ~ l a r / ' ~ N CSA longitudinal cross-correlation rates in protonated proteins. The coupled relaxation dynamics is described by an iterative fitting procedure to the solution of differential equations. Guenneau et al? measured longitudinal and rotating frame relaxation times through fully J-decoupled homonuclear spectra. The authors could show that these spectra involving Lorentzian Lines can be readily obtained by straight-

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forward processing of the 2D data arising from a conventional spin-echo sequence used in a so-called J-resolved experiment. If NMR systems are to be used as practical quantum computers, the number of coupled spins will need to be so large that it is not feasible to rely on purely heteronuclear spin systems. The implementation of a quantum logic gate imposes certain constraints on the motion of those spins not directly involved in that gate, the so-called (spectator' spins; they must be returned to their initial states at the end of the sequence. Linden et a1.66tested their ideas for a NMR quantum logic gate on the six coupled protons of inosine. Developing also showed how to sequences for NMR quantum computers, Linden et manipulate nuclear spins while freezing the motion of coupled neighbors. In another paper these authors proposed new pulse sequences that exchange information between two weakly coupled nuclear spins.68 Cross-relaxation dynamics between laser-polarized xenon and a surface species is studied by MacNamara et al.(j9by using a simple three-spin model. Wrackmeyer and Bernatowicz7' observed the first indirect nuclear 1'9Sn-73Ge spin-spin coupling, Spin-spin coupling and chemical shifts in macromolecules were discussed by Case.71 2.4 Dipolar Couplings and Distance Information. - Since the early days of NMR, longitudinal and transverse relaxation rates have been used for structural and dynamics investigations of molecule^.^^-^^ Among the different relaxation mechanisms present in solution, the dipolar interaction is the dominant one for protons and therefore has been most frequently used for structure d e t e r m i n a t i ~ n .Such ~ ~ studies rely on the dependence of the crossrelaxation rate on the interproton distance after assuming that the molecule is rigid and spherical. This assumption no longer holds when molecules have a high degree of flexibility or when their shape is anisotropic. Indeed, the relaxation rate depend not only on the structure, for example on the internuclear distance, but also on the reorientational dynamics on the spins.77i78 Three possible strategies are chosen to separate these structural and dynamic contributions. First, the correlation times are studied as a function of or solvent viscosity.81Second, the relaxation rates are measured by varying the Larmor frequency.82Third, experiments can be carried out in which the spins are placed in conditions where they relax differently from when they are only subjected to laboratory-frame longitudinal relaxation. TJT2 or selective Tlhon-selective Tl83 experiments were the first experiments of this kind. Later on, some authors tried to measure the ratio of the longitudinal to transverse cross-relaxation rates as a rapid method to obtain a local estimation of the rotational correlation time by studying dipolar cross-relaxation with NOESYg4 and ROESY85786 sequences.87788 Unfortunately, the transverse measurements are strongly influenced by coherence transfer between the spins through scalar coupling. Thus, some authors started to study spin dynamic processes in liquids using off-resonance r.f. irradiation.89-92Desvaux and B e r t h a ~ ldescribed t~~ this approach, which gives access

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to information about motions occurring on the micro- to milli-second time scale and on the nanosecond time scale. Vincent and B ~ d e n h a u s e ndemonstrated ~~ that on off-resonance rotatingframe Overhauser spectroscopy (OFF-ROESY), spin-diffusion effects can be suppressed in selected spectral regions by double-selective inversion of the magnetization in the middle of the cross-relaxation interval. The resulting experiments were called 'quenching of undesirable indirect external trouble in off-resonance rotating frame Overhauser spectroscopy' (QUIET-OFFROESY). This method yields direct information about cross-relaxation rates between selected pairs of spins, as if the two-spin subsystems were isolated from their surroundings. The authors could show that the precision of the determination of internuclear distances can be significantly improved if spin diffusion is quenched. Chiarparin et al.95gave a comprehensive description of the effects on twospin coherences of cross-correlation between fluctuations of two different relaxation mechanisms in NMR. Dipole-dipole interactions between four nuclei and chemical shift anisotropy of two of these nuclei were considered. The same authors96 presented a method to determine both cp and n: backbone angles in proteins from measurements of cross-correlated relaxation effects. Using dipolar couplings and molecular fragment replacement, Delaglio et aZ.97 determined protein structures. Measurements of the nuclear Overhauser enhancement (NOE) is one of the mainstays of structural studies of molecules in NOES arise when longitudinal magnetization is transferred between nuclear spins as a result of their mutual dipolar relaxation. Since the magnitude of dipolar relaxation between a pair of internuclear distances is inversely proportional to the sixth power of their internuclear distance, the NOE decreases rapidly with increasing internuclear distance and is generally observed up to 4.5-5.0 When several nuclei are in close proximity, magnetization can be transferred both directly from one spin to another and indirectly by way of one or more additional nuclei. This effect, which may distort the size of the measured NOE, is known as spin diffusion. Harris et al.98described a robust and accurate method for measuring nuclear Overhauser enhancements that are largely free of spin diffusion effects. The authors considered the effects of using imperfect radio frequency pulses. Eykyn et al.99presented one-dimensional NOE experiments applicable to labeled macromolecules which allow the manipulation of specific spin diffusion pathways and thus unambiguously identify clandestine spins through which the direct NOE is mediated. Clore et a1.Im derived an expression for the dipolar coupling R-factor which provides a quantitative and readily interpretable measure of the agreement between observed and calculated dipolar couplings. A new method for thc simultaneous measurement of magnitude and sign of D-l(C-H) and D-l(H-H) dipolar couplings in methylene groups has been introduced by Carlomagno et al."' A new pulse sequence allows to measure both dipolar couplings at the same time, using spin state selective transfers. Carlomagno et al. lo2 proposed an experiment to measure dipolar-dipolar

A.

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cross-correlated relaxation rates to study the nature of the motion within protein backbone. Peti and Griesingerlo3presented measurements to determine magnitude and sign of H,H-dipolar couplings in proteins. Theory, experimental aspects, and the use in structure calculation of cross-correlated relaxation rates measured on zero- and double-quantum coherences in liquid state NMR is presented by Reif et Residual dipolar couplings for pairs of proximate magnetic nuclei in macromolecules can easily be measured using high-resolution NMR methods when the molecules are dissolved in dilute liquid crystal media. Al-Hashimi et al.lo5 showed that these couplings can be used to constrain the relative orientation of molecular fragments. Koenig et al. lo6 measured dipolar couplings in a transducin peptide fragment weakly bound to photo-activated rhodopsin. Tjandra et al. lo7 presented a direct refinement against protonproton dipolar couplings in NMR structure determination of macromolecules. Permi et al.'" presented a new method for measurement of one-bond 13C-13C a scalar and dipolar couplings from a two-dimensional 5N-1H correlation spectrum. Pulse sequences for measurement of one-bond 5N-1H coupling constants in the protein backbone were introduced by Lerche et al. lo9 Recognition of protein folds via dipolar coupling is shown by Annila et al.' lo The solvent dependence of rotational energetics and formly-proton longrange spin-spin coupling behavior of benzaldehydes using dipolar couplings and temperature dependence of long-range couplings was studied by Lounila et al.

'

''

Exchange Spectroscopy. - Chemical exchange during NMR experiments is commonly understood to be any reorganization of the system that causes a change in any parameter of the spin system under examination. Such parameters include chemical shifts, scalar and dipolar coupling interactions, and spin relaxation rates. Most treatments of chemical exchange phenomena include relaxation in an approximate way, usually through an empirical approach. 2*1l 3 Authors who treat relaxation in a rigorous way usually employ the development of Redfield,74but in an ad hoc manner, usually by inclusion of the only most important relaxation matrix elements for a specific system under study. Cuperlovic et al.li4 developed a general computer program that is flexible enough to treat arbitrary spin systems in a variety of exchanging situations while retaining all features of the relaxation matrix. The authors introduced their theory for describing chemical exchange with simultaneous treatment of relaxation phenomena in the manner of Redfield. The dipole-dipole, chemical shift anisotropy, and random fluctuating field relaxation mechanisms are included. 15-' l7 The authors pointed out that their treatment is general and could be expanded to include other mechanisms. Molecular tumbling is treated either as rotational diffusion' lg91l 9 or by a model-free approach. Schurr et a1.l2l examined the effects of site exchange due to slow conformational changes in rapidly rotating molecules in solution in detail. Significant gaps in the currently available theory were filled by the authors. A continuous 2.5

'*'

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Nuclear Magnetic Resonance

Gaussian exchange model is formulated in which a nucleus undergoes continuous one-dimensional motion in a harmonic potential well that is located in a linear chemical shift gradient. NMR measurements of laser-polarized gaseous '29Xe produced by spinexchange optical pumping with a narrow-linewidth laser at a high magnetic field of 4.7 T is reported by Sun et al.122 Luo et al.123 reported experimental results of frequency-selective laser optical pumping and spin exchange of Cs with '29Xe and I3lXe in a high magnetic field of 11.71 T. Their results showed that hyperpolarized '29Xe and I3'Xe NMR signal exhibit alternating phases when the laser frequency for l ~ ~ the relevance of pumping cesium atoms is changed. Bauerle et ~ 1 . discussed the multiple spin exchange Hamiltonian to describe thermodynamical properties of solid 3He film absorbed on graphite. The evolution from a pure Heisenberg behavior to a novel spin liquid state is explained. Stith et aZ.'25 investigated the transfer of polarization from I2'Xe to solute protons in aqueous solutions to determine the feasiblility of using hyperpolarized xenon to enhance 'H sensitivity in aqueous systems near room temperature. For superfluid states of 3He with spatially uniform texture inside a porous medium a model of rapid exchange between liquid atoms and solid atoms frozen on to the substrate surfaces is used by Hook and Kaplinsky126to derive the NMR frequencies. Chemical exchange amongst five conformers of a tenmembered ring compound containing two amide bonds and a disulfide was ' ~ ~ intermolecular nuclear studied by Bain et al.127 Liepinsh et ~ 1 . measured Overhauser effects between the protons of various small solvent or gas molecules and a non-specific lipid transfer protein from wheat. The NOEs between water and the protein were weak or could not be distinguished from exchange-relayed NOEs. The structure of N-aminopyrazole in the solid state and in solution was determined by Jimenez et aZ.'29 using NMR spectroscopy and computational methods. In the liquid state hydrogen bond exchange rendering the two amino groups equivalents was found to be fast in the NMR time scale. Spin exchange in dry and swollen natural rubbers has been investigated by Grinberg et al.I3' The spin exchange occurs between the protons of CH and CH3 (andor CH2) groups and was estimated to be about 0.04 s. 2.6 Radiation Damping. - With the trend of designing supersensitive probes and very-high-field magnets, the phenomenon of radiation damping is increasingly being perceived as starting to invade the realm of normal nuclear '~~ the scope and magnetic resonance. 131-134 Szantay and D e m e t e ~ - explored limitations of the various special tools available to diagnose the presence of radiation damping. It is shown that the recovery rate of the M,-magnetization is the most sensitive parameter to indicate weak damping. This effect can be measured by using suitable gradient techniques. A wide variety of amplitude- or phase-modulated pulses, which are designed for selective excitation, inversion or refocusing, are severely perturbed by radiation damping in the presence of large macroscopic magnetization. Chen

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et al.136 presented a general method to compensate for radiation damping during application of selective r.f. pulses. The authors pointed out that their method has general applicability for any amplitude- or frequency-modulated r,f. pulse. This is demonstrated experimentally for inversion pulses. Rourke and A ~ g u s t i n e showed '~~ that nonlinear evolution of the LandauLifshitz type can be exactly linearized including the radiation-damped spin system and the super-radiant system in the semi-classical regime, in the presence of time-varying driving fields. The resultant linear system is simply that of a spin '/2 particle with radiation damping rate, or superradiant characteristic time. Mao and Chen138studied the relation between the delay time and the tipping angle for superradiance. This relation includes a radiation damping time constant. The strong '29Xe NMR signal of laser-polarized samples, which will be discussed in detail in Chapter 4, may cause also radiation damping in a highresolution spectrometer. This is true even if the sample is a low-pressure gas or dissolved xenon. This non-linear effect prevents proper inversion of the xenon polarization, which is required to detect the cross-relaxation between xenon and proton in SPINOE experiments. Berthault et ~ 1 . presented l ~ ~ a simple way to properly invert intense nuclear magnetization by combining a field gradient pulse and a frequency-sweepadiabatic pulse. Loria et al.'40 proposed two modifications to sensitivity-enhanced gradientselected TROSY-based triple-resonance NMR experiments that reduce the overall duration of the pulse and minimize radiation damping effects on waterflipback solvent suppression.

2.7 Quadrupolar Interactions. - Quadrupolar coupling constants are sensitive probes of hydrogen bonding. Their accurate knowledge is a major prerequisite when trying to extract dynamical information from relaxation data of quadrupolar nuclei. If the relaxing mcleus is covalently bound, the NQCC is determined by the electric field gradient (e.f.g.) arising from the intramolecular charge distribution at the nuclear site. Liquid phase values for NQCC can be obtained only in an indirect way. The standard procedure is to first determine correlation times for molecular reorientation from dipoledipole relaxation rates. For example, the molecular reorientation time obtained from dipolar 13C-1H relaxation data is used to determine the deuteron NQCC in 13C-2Hfrom quadrupolar relaxation. In this particular case the procedure is correct, because the principal axis of the e.f.g. tensor coincides with the direction of the dipole-dipole interaction. The method also works for 'H-I7O and 2H quadrupolar interaction in the OH ' discussed the group of w a t e ~ - ' ~and ~ *a' l~~~o h o l s . ' Farrar ~ ~ ~ ' ~et ~ 1 . ' ~ recently use of these 170-enriched samples of alcohols to measure the correlation time of the OH internuclear vector. It works well when the hydroxyl proton exchanges rapidly, and it fails when slow exchange effects are not considered. Details are very difficult to interpret without the help of theoretical calculations. As noted in previous reports,' there are now theoretical methods to predict the e.f.g. in the liquid state. The first method is based on Molecular

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Dynamics Simulations (MD). From the bulk configurations, molecular clusters are chosen and used in supermolecular ab initio calculations. This technique to determine 2H and 1 7 0 has been successfully applied by Huber et aZ.1469147 NQCC in liquid water. The same procedure has been used by Merkling et a1.148to calculate the DQCC for the hydroxy-D in methanoYCC14 mixtures at various compositions. The computed DQCC-values are about 2-20% higher than the experimental ones but show a similar decrease with increasing methanol mole fraction. This kind of procedure was also applied to calculate the NMR spin-lattice relaxation time of 21N in liquid and supercritical states.'49 In a second method, molecular clusters were calculated by pure ab initio methods. A Quantum Cluster Equilibrium (QCE) M ~ d e l , ' ~ ' ?which ' ~ ~ is fully based on quantum thermodynamics treatment, generates temperature- and pressure-dependent molecular cluster populations. Changing cluster distributions lead to temperature-dependent H-bond-sensitive properties such as NQCC, chemical shifts and vibrational frequencies, which are calculated for each of the clusters. With this procedure the temperature dependence of 2H and I7O NQCC for ethanol were calculated by pure theoretical methods.'52 It could be shown that NQCC's show different temperature behavior depending on the degree of hydrogen bonding. The main advantage of this method is that larger molecular clusters show strong cooperative effects, which cannot be considered by pairwise-additive potentials. Only the large cooperative enhancement leads to values for NQCC, which are typically known for the liquid phase. Deuteron quadrupole echo spectra have been used by deLangen et a1.'53to determine the character and rate of local reorientations of the molecular chains in polyethylene. At high pressure (up to 3300 bar) polyethylene showed a hexagonal phase between the orthorhombic phase and the liquid phase. Quadrupolar 21Naand 7Li relaxation times were measured by Delville et to determine ion diffusion in aqueous dispersions of clays. The spectral densities of both 3/2-spin nuclei exhibit a transition between two dynamical regimes characterized by a plateau and a power-law decrease.

2.8 Intermolecular Dipolar Interaction in Diamagnetic and Paramagnetic Solution. - Bertini and L ~ c h i n a t presented '~~ new applications of paramagnetic NMR in chemical biology. The methodological accessibility to solution structure and dynamics investigations of paramagnetic metallobiomolecules has afforded the ability to tackle the redox pairs of electron transfer proteins of which at least one is paramagnetic, to study the orientation effects of high magnetic fields on paramagnetic biomolecules, and finally to study the role of metal-based cofactors in protein folding and stability. It is well established that the Solomon-Bloembergen-Morgen (SBM) theory invokes so many restrictions that its quantitative use is very limited. Many of the problems arise from the presence of zero-field splitting (z.f.s.) in addition to the Zeeman interaction. This is particularly so for non-rigid complexes of low symmetry, where both static and transient e.f.s. are present. The former is modulated by the overall reorientation of the complex; the latter is due to

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vibrational and rotational motions of the ligands.' 56i1 57 Meanwhile there exists an extensive literature on theories beyond the SBM level, although the various theoretical approaches have been quite different. 58-162 Recently, Strandberg and W e ~ t l u n d 'presented ~~ a paramagnetic proton nuclear spin relaxation theory of low-symmetry complexes for electron spin quantum number S = 5/2. The new theory bases on a generalization of the modified Solomon-Bloembergen-Morgan (MSBM) equations and includes the effects of static zero-field splittings (z.f.s.), hyperfine coupling, and angular dependence. The model gives the difference from MSBM theory in terms of a correction term delta, which is given in closes analytical form. The authors implemented their theory into a computer program, which calculates solvent water proton TI NMRD profiles using both MSBM and the new model. Miller et al. reported an experimental test of one of the central predictions of the theory of paramagnetic enhancement of the NMR relaxation rates (NMR-PRE) in solution for spin S > 112. For S greater than or equal to 1, zero-field splitting (z.f.s.) interactions are present that, when larger than the electronic Zeeman interaction, act to align the spatial quantization of the electron spin motion along the molecule-fixed principal axis system of the z.f.s. tensor. When the z.f.s. is comparable or greater than the Zeeman energy, the NMR-PRE has been predicted theoretically to be a function of the angular variables that specify the orientation of the electron-nuclear interspin vector in the molecular coordinate frame. The measured axial/equatorial TI ratio for S = 1 of about 2.2 was larger as expected theoretically, but in agreement with the results of spin dynamics simulations carried out by the method of Abernathy and Sharp.'65 obtained a more reliable absolute shielding scale for chlorine by Gee et combining NMR methods and molecular dynamics simulations. The experimental paramagnetic contribution the chlorine shielding tensor was available from accurate 35C1/37CIspin-rotation data for HCl in the gas phase. Variable temperature and variable pressure 7O nuclear relaxation rates were measured for the Eu(T1) aqua ion by Caravan et aZ.'67 The authors measured the highest water exchange rate ever occurring at Eu-(aq)2+. The consequences for MRI contrast agents are discussed. A similar technique is used by Toth et al.168 to demonstrate an improved water exchange on potential contrast agents. New transition metal complexes were proposed as contrast agent for magnetic resonance imaging. Anelli et aZ.169 synthesized sulfoamide-functionalized gadolinium DTPA complexes, Paramagnetic metalloproteins were investigated by Volkman et al. 70 Telser et aZ.17*measured the exchange couplings of [Fe3S4]' clusters in four proteins by electron spin-lattice relaxation. Kobayashi and Tachibana17* observed the transbilayer distribution of a chlorpromazine using a paramagnetic shift agent. Biekofsky et al.173presented NMR approaches for monitoring domain orientations in calcium-binding proteins in solution using partial replacement of Ca2+by paramagnetic Tb3+. Paramagnetic NMR spectroscopy has been used by Fukui et al.'74to study

'

'

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Nuclear Magnetic Resonance

the electronic structure of a stable phenalenyl radical. Odintsov et uZ.'~' measured the particle size effect on transverse NMR relaxation in aqueous char suspension. Goudemond et al.176 studied electron-nucleus interactions in highly doped rare earth phosphate glasses measuring paramagnetic relaxation times. Boughriet et u Z . ' ~ ~ reported spectroscopic studies of vanadyl-calcitel ~ ~ 61Ni, 59C0 and 63Cu in water-oxygen systems. Mishina et ~ 1 . measured order to clarify the electronic states of their compounds. Spin-lattice relaxation and broad-band spectra were measured by Zorin et al.17' in aqueous solutions of copper(I1) and nickel(I1). Measured hyperfine tensor components are compared to calculated results by Mattar and Stephens."'

2.9 Slow Motions in Glasses. - Studies of slow molecular motions in supercooled and glassy systems using NMR techniques has become more popular and the number of papers increased steadily during the past few years. In particular the homogeneous versus heterogeneous scenario for the dynamics of glass-forming polymers was discussed intensively.l 8 * In the heterogeneous scenario the non-exponentiality of the a-relaxation is attributed to a superposition of relaxation rates, whereas in the homogeneous scenario the nonexponentiality is intrinsic in nature. Charnberlir~'~~ reviewed experiments and theory of the non-exponential relaxation in liquids, glasses, polymers and crystals. He mainly focused on the two types of nonexponential behavior und their universal significance. Structural relaxation in colloidal supercooled fluids and colloidal glasses has been investigated by Weeks et al.'84The cluster size distribution, structure and dynamics could be derived. The dynamics of fast and slow moving clusters were referred to both relaxation processes. Viot et ~ 1 . used l ~ experimental ~ NMR data to develop a heterogeneous picture of arelaxation for fragile supercooled liquids. Wendt and Richert l g 6 studied the heterogeneous relaxation pattern in supercooled liquids near the glass transition. In the chosen temperature range the average structural relaxation time varies more than four orders of magnitude. Their analysis allowed detecting fluctuations in terms of the resulting apparent homogeneity within the long time tail of the decay. Proton, deuteron, and carbon NMR methods were applied by Qi et al.i87 to study various isotopic species of supercooled and glassy propylene carbonate. The authors investigated the molecular diffusion via static field gradient diffusometry and yielded evidence for a particularly pronounced enhancement of the translational over the reorientational dynamics. Four time stimulated echoes confirmed that the non-exponential relaxation in propylene carbonate is dynamically heterogeneous in nature. The dynamical heterogeneity in a-relaxation and P-relaxation of glass forming liquids was also studied by Bohmer et al.188 Partial deuteration of ortho-terphenyl, propylene carbonate and toluene provided additional information on intramolecular motion. Isotope effects of the dynamics of a supercooled van der Waals liquid were studied by Jorglg9 using likewise deuteron magnetic resonance. The length scale of dynamic heterogeneity is called the characteristic length.

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Hempel et al. 190 studied this characteristic length of dynamic glass transition near the glass transition temperature Tgfor a wide assortment of glass-forming substances. The lifetime of spatially heterogeneous dynamic domains in polystyrene melts was investigated by Wang and Ediger. l g l In most glass-forming materials external perturbations are relaxed in a nonexponential fashion. It is shown by BOhmerlg2 that the degree of nonexponentiality is phenomenologically correlated with the departure from simple thermally activated behavior as measured by the fragility index. While phenomenological correlations have proven very useful for rationalizing the properties of many glass formers, they have provided little direct insight into the nature of the non-exponential relaxation itself. In this respect the author discussed some recent reduced 4D deuteron magnetic resonance experiments on a supercooled liquid. Wang and Ediger' 93 measured anomalous translational diffusion and reported a new constraint for models of molecular motion near the glass transition temperature. Blochowicz et aZ.194analysed the spin-lattice time T1 of simple organic glass formers by introducing a spectral density obtained from broadband dielectric susceptibility data. A quantitative comparison of dielectric and NMR spectroscopy was carried out in full detail for glycerol, yielding almost identical l ~ ~ spectral densities at the Larmor frequency in both cases. Doss et ~ 1 . studied the glass-former salol using carbon and deuteron NMR. The measurements of the spin-lattice relaxation times provided clear evidence for an anisotropic reorientation of the molecules in the supercooled liquid. Close to the glass transition a bimodal longitudinal magnetization recovery is observed which indicated the presence of relatively slow intramolecular degrees of freedom. Vogel et a1.'96investigated systems benzene in polystyrene as model for binary glass formers by applying multidimensional 2H NMR experiments. The authors demonstrated that the molecular larger component of these systems essentially behaves as a neat glass former, whereas the small benzene molecules show a completely different scenario. Ngai'97 investigated a correlation between the logarithm of the most probable secondary p-relaxation time at the glass temperature and the Kohlrausch-Williams-Watts exponent of the primary a-relaxation correlation function for a whole host of chemically different glass-forming materials such as amorphous homopolymers, small molecule van der Waals liquids, plastic crystals, and inorganic compounds. The author concluded the similarity between the P-relaxation and primitive relaxation. A modified KohlrauschWilliams-Watts function was also used by Wachner and Jeffrey'98 to evaluate molecular reorientation in sugadwater glasses. The reorientation is dominated by small angle jumps. Brougham et al. '99 performed field-cycling NMR relaxometry of proton tunnelling in a partially disordered system of hydrogen bonds, which is analogous to glassy systems. The proficiency of this method and the advantages over conventional narrow band spin-lattice relaxation measurements is discussed. T I , TI, and 7'2 relaxation times for D20 in silica sol-gels are used by

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Wonorahardjo et aZ.200to monitor porosity and surface interactions within the silica framework as a function of aging. Site specific rotational mobility of anhydrous glucose near the glass transition has been studied by 2D echo decay 13C NMR.201The same method was used by Tracht et aZ.202to study the segmental dynamics in supercooled polyvinyl acetate, yielding detailed information about geometry and time scale of the motion close to the glass transition. Paris et measured lithium mobility in glassy systems. The ionic conduction mechanism in glasses was studied by Akai204using 7Li NMR and electrical conductivity. The dynamics of lithium ions in lithium bismuthate glasses have been investigated by Pan.205 A general basis for transport phenomena in ionic glasses is reported by Baranovskii and Cordes.206 A general theoretical approach for the mixed alkali effect in glasses is discussed by M a a ~ . ~The ' ~ approach bases on the idea that the covalent host network creates different structural energy landscapes for different types of mobile ions. Green et aL208also discussed dynamics of mixed alkali glasses and liquids. The effect of paramagnetic impurity in the structure of sodium disilicate glass is reported by Mortuza et Ilan and Loring2I0could explain relaxation in a supercooled polymer melt within the dynamically disordered Rouse model. 2.10 Models for Molecular Dynamics. - Some years ago, a model-free approach to the interpretation of NMR relaxation in macromolecules was suggested by Lipari and Szabo.1209211 For both isotropic and anisotropic overall motion, it was shown that the unique information about fast internal motions contained in relaxation experiments can be completely specified by two model-independent quantities: a generalized order parameter, S, which is a measure of the spatial restriction of the motion, and an effective correlation time, t,, which is a measure of the rates of motion. This so-called Lipari-Szabo model-free approach along with Bayesian statistical methods was used by Andrec et aZ.*12 to describe the overall tumbling of macromolecules in solution. The authors could demonstrate that this procedure is able to estimate the isotropic rotational correlation times, as well as the rotational diffusion coefficients for axially symmetric anisotropic tumbling. Daragan and Mayo2I3 used the model free approach to analyse NMR relaxation data for biomolecules undergoing anisotropic rotational diffusion. A simple protocol for using this anisotropic motional model is provided, along with an example on a small a-helical peptide. The LipariSzabo treatment was also applied to relaxation data of sugars214and calixarene215in solution. Diezemann and Nelson2I6analysed the non-exponential relaxation in supercooled salol near the calorimetric glass transition temperature in terms of a free-energy model for the primary relaxation in glass-forming liquids. Diezemann and S i l l e ~ c ugeneralized ~~~ the frequently used model of isotropic rotational Brownian motion via small but finite angular steps to the case in which molecular reorientations by different angles take place around arbitrary axis. A simple approximation allowed giving analytical expressions for experi-

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mentally relevant correlation functions, including spin relaxation rates. The authors compared their results with earlier treatments of rotational Brownian motion. A combination of I3C relaxation measurements and molecular dynamics simulations of benzene in neat liquid and in solution with polystyrene was used by Witt et aL2’* to determine the molecular dynamics of these systems. The behavior of neat benzene could be explained in the framework of a simple rotational diffusion model. The anisotropy changed from 1.9 to 1.3 between 280 and 360 K. The data permitted the description of diffusion in this fluid by the rough hard sphere (RHS) model. Wakai et aL2” measured the tumbling and spinning diffusions of acetonitrile in water and organic solvents by spin-lattice relaxation times for 2H and I4N. The observed large anisotropy of the spinning to tumbling modes represents the anisotropy of the solvation shell and is explained by neither the free rotor model nor the hydrodynamic continuum model. Self-diffusion coefficients for monomethylamine and trimethylamine were measured by Chen et aZ.220in the temperature range between the melting pressure curve and 423 K at pressures up to 200 MPa.

3

Selected Applications of Nuclear Spin Relaxation

3.1 Pure Liquids. - The temperature dependence of hydrogen bonding in neat alcohols was studied by Huelsekopf and Ludwig.221A combination of theoretical methods and NMR measurements clearly lead to the result that stronger hydrogen bonded alcohols such as ethanol and benzyl alcohol exist of larger cyclic structures whereas 2,2-dimethyl-3-ethyl-3-pentanol is built upon monomers and dimmers only. The behavior of the I3C spin-relaxation time as a function of temperature is reported for eight quinoline family compounds by Gauthier et aZ.222 The relaxation time evolution could be interpreted using a stretched exponential correlation function. Sliwinska-Bartkowiak et aZ.223reported experimental measurements and molecular simulations of the melting and freezing behavior of simple fluids in porous media. The experimental studies are for carbon tetrachloride and nitrobenzene in controlled pore glass and vycor. Data from dielectric relaxation measurements were compared with those obtained by NMR relaxation time experiments. 3.2 Non-Electrolyte Solutions. - The study of hydrophobic interaction in aqueous solutions is still popular and of great importance in biomolecular systems. The tert-butanol molecule is known to fit perfectly into clathrate-like water structures. Thus Mayele et ~ 1 investigated . ~ ~the hydrophobic ~ selfassociation of this alcohol in binary aqueous mixtures by NMR methods. The occurrence of two structurally different regions is discussed in the light of literature results from other methods. In a consecutive paper the authors225

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showed that adding urea to these binary mixtures lead to an increase of the translational mobility of the solute tert-butanol in a certain mixture range, whereas it always decreases the mobility of the bulk water. Harris and Newitt226 measured the intra-diffusion coefficient of deuterated water in aqueous solutions of tert-butanol as a function of pressure, temperature and composition. The data are consistent with the concept that water is more structured at small alcohol concentration. The pressure effect on hydrophobic hydration was reported by Wakai et al.227The deuteron relaxation times were measured for deuterated benzene in dilute solutions of water and methanol at pressures up to 300 MPa. In methanol the correlation time increased monotonically with pressure, whereas in water the rotational mobility was not affected by pressure up to 100 MPa. This result implies that strong hydration shells due to hydrophobic effect resist compression up to this pressure. Shimizu et al.228studied water dynamics in aqueous denaturant solutions by measuring 1 7 0 spin-lattice relaxation times. The results indicate that the effect of hydrophobic hydration of denaturants becomes important for the denaturation of a protein with increasing hydrophobicity of the denaturants. Olechnowicz et aZ.229measured 'H relaxation times in highly concentrated water protein solutions. The structure of gellan in dilute aqueous solution was investigated by performing a series of selective one-dimensional NMR experiment^.^^' Takamuku et al.231determined the structure and dynamics of 1,4-dioxane-water binary solutions by X-ray diffraction, mass spectroscopy and NMR relaxation. The 2H spin-lattice relaxation rates showed a minimum for the rotation of water molecules at a mole fraction of about 0.3 for dioxane. Multiple field 13C NMR relaxation times of calix[4]arene in solution with chloroform and dimethylsulfoxide were measured as a function of field strength and temperature. Shultes et aL2* virtually found identical relaxation time for both, the aliphatic and the aromatic carbons at all three magnetic fields. Ito et al.232 performed relaxation time and NOE experiments to determine dynamical properties of calixarene analogs containing thiophene ring. Proton relaxation measurements of water associated with high methoxy and low methoxy pectins were performed by Kerr and Llor and M ~ n o investigated z ~ ~ ~ tautomeric equilibria of pyridoxine in water by 13Cand "N nuclear magnetic resonance. Mao et aZ.235studied the cohesive interaction among polymer chains in a polyacrylamidelwater solution by 2H relaxation time measurements. The same measurements were performed by Tornblom et aZ.236to investigate the surfactant aggregation behavior in aqueous decylammonium chloride solutions. Yamazaki et measured 3C relaxation times of polyacrylic acid in aqueous solutions and studied the effects of charge density on local chain dynamics. Solution dynamics of perfluorobenzene, benzene and perdeuteriobenzene in carbon dioxide was studied as a function of temperature and pressure by Y ~ n k e r measuring *~~ the I9F, 'H and 2H relaxation times.

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3.3 Electrolyte Solutions. - The ionic mobility in liquid and gel electrolytes plays an important role for the development of advanced batteries. Thus measurements of self-diffusion coefficients and relaxation times of ions in such systems become more popular. Ward et al.239applied pulse field gradient spinecho techniques to measure the self-diffusion coefficients of 7Li, ''F and protons in polymer gel electrolytes. The results for the gel electrolytes are compared with those for the corresponding liquid electrolytes and are found to be similar. However, the authors detected small but significant differences between both systems. The picture that emerges is of a two-phase material with regions of liquid electrolyte existing within a polymer gel rich matrix. 'H, 7Li and 19F pulsed-gradient spin-echo NMR diffusion measurements of gel electrolyte systems were performed by Hayamizu et ~ 1 . ~ The ~ ' solventdependent behavior of the measured lithium and anions were clearly shown. Gel electrolytes in different kind of organic solvents were measured by Aihara et al.241to study the structure of the gels and the diffusion mechanism of the ions. The self-aggregation of sodium n-hexyl sulfate in aqueous electrolyte solution was investigated by Ruso et al.242NMR self-diffusion coefficients of non-aqueous electrolyte solutions for rechargeable lithium batteries were measured by Capiglia et Reiche et a1.244determined the electrochemical properties of gel electrolyte films. Stallworth et d.245 studied the electrical conductivity of liquid and hybrid electrolytes by NMR techniques. Substantial differences were found for both systems. In particular, the electrical conductivity is of the hybrid electrolytes is lower than expected. NMR measurements of self-diffusion in polymer gel electrolytes are reported by Williamson et .1.246,247

Ruso et al.248studied the self-association of propranolol hydrochloride in aqueous electrolyte solution. NMR techniques were used by Lu et aZ.249to control the electrochemical production of highly effective polyaluminium chloride. Vapor-liquid equilibrium measurements in acetone/methanoY NaSCN system at different salt molarities were performed by Iliuta and T h y r i ~ n ~to~ ' emphasize the anomalous crossover effect between salting-in and salting-out on the acetone, reported previously at different salt mole fractions. The selfassociation of amphiphilic penicillins in aqueous electrolyte solutions was reported by Taboada et ~ 1 . ~ ~ ' 3.4 Transition Metal Complexes. - The study of ligand exchange upon metal ions has become one of the major topics in this subject area. To get a better understanding of exchange processes, exchange times were measured extensively as a function of temperature, pressure and various magnetic field strength. Merbach and co-workers' 6 13252 have investigated extensively aquacomplexes of Eu3+and Gd3+to study the solvent exchange process depending on the nature of the ligand. Relaxation properties of lanthanide complexes with DTPA and EDTA were investigated by Wei et aZ.253The authors pointed out that the presented

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complexes have favorable properties such as high water solubility and chemical stability required of clinical useful contrast agent. Orre11254reviewed the dynamic NMR spectroscopy in inorganic and organometallic chemistry. This overview of the last five years research included the field of transition metal complexes. Ruthenium hydrides were studied by Castellanos et al.255 using 'H relaxation. NMR paramagnetic relaxation measurements and Raman spectroscopy were applied to obtain structural information on transition metal cation binding to adenine mononucleotides and their analogues. Lang et al.256discussed the reliability of this methodological approach. From 13C paramagnetic relaxation rates Kubota and Tsu~ h i y concluded a ~ ~ ~ preferential solvation of some divalent transition metal ions in watedmethanol mixtures. Yilmaz et ~ 1 . ~studied ~ ' the influence of transition metal ions Mn2+, Fe3+ and Cr3+ on TI relaxation times of serum, blood and red cells. The obtained relaxation times were compared to those of pure water. In situ NMR spectroscopy using parahydrogen-induced is a powerful tool to study transition metal-catalysed hydrogenations because the signals are enhanced by several orders of magnitude. Ulrich and B a r g ~ demonstrated n ~ ~ ~ that this so-called parahydrogen-induced polarization (PGIP) technique could be used also in the presence of paramagnetic shift reagents. Bakhmutov et aZ.260measured TI relaxation times and quadrupole coupling constants in transition metal complexes. Lisowski26' used 'H NMR to study new heterodinuclear macrocyclic complexes. The hydrogen bonding interaction of rhenium hydrides with two different proton donors was studied in solution by Messmer et a1.262From relaxation time measurements of the hydrides, hydrogen bond lengths were obtained.

3.5 Molten Salts. - Structural and dynamic approaches of molten salts were studied by Bessada et al.263using a combination of NMR experiments and Molecular Dynamics simulations. A new family of molten salts is reported by MacFarlane et al.264Some of the pyrrolidinium imides are molten at room temperature, while the smaller and more symmetrical members have melting points around 100 "C. The solubility and the polymerization of polyacrylonitrile in low-temperature molten salts have been investigated by Hettrich et ~ 1 . It~ was ~ ' found that the polymer dissolves in eutectic mixtures of molten sodium thiocyanate and potassium thiocyanate. Lefebvre and Conway266reported studies on nucleation and growth of aluminum phase electrodeposited from solutions of AlC13 and LiAlH4 in tetrahydrofuran onto smooth substrates. Novel high salt content polymer electrolytes were studied by F o r ~ y t h Relaxation .~~~ time measurements suggest that the glass transition temperature decreases with increasing salt concentration. Thomas et u Z . * ~ ~characterized novel ferrocenylimidazolium salts by NMR techniques. Vanadium oxide complexes in roomtemperature chloroaluminate molten salts have been studied by Bell er aZ.269 Schreiter et ai.270 prepared molten salts from chloride salts. Holbrey and

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S e d d ~ n ~studied ~' the phase behavior of molten salts by NMR and IR spectroscopy.

4

Nuclear Spin Relaxation in Gases

Laser-polarized noble gases such as 3He or '29Xe exhibit a nuclear spin polarization exceeding the equilibrium nuclear spin polarization by orders of magnitude, resulting in a corresponding sensitivity gain in NMR experiments. B r ~ n n e reviewed r ~ ~ ~ recent developments in the application of laser-polarized '29Xe, especially with respect to materials research. The improvements of spin exchange optical pumping allows the production of laser-polarized '29Xe under continuous flow. The author discussed applications of this new technique for the enhancement of MRI and NMR spectroscopy. In another review article B r ~ n n e summarized r~~~ the benefits of using laser-polarized noble gases with respect to surface and biological magnetic resonance. Song et a l l 9 reported some applications of optical pumping methods noble. Selected experiments devised to exploit this enhancement sensitivity are reviewed, including xenon NMR/MRI studies of biological systems and polarization transfer to molecules in solution and on surfaces. The application of spin polarization-induced nuclear Overhauser effects to spin-polarized helium and xenon is discussed by the same author.20 G o ~ d s o introduced n ~ ~ ~ the general topic of biomedical application of laserpolarized noble gases. Special attention is given to a description of the injection techniques and their potential advantages and limitations. An outlook for biomedical applications of laser-polarized noble gases is given, particularly in light of recent low-held magnetic resonance experiments. Albert et al.275measured TI relaxation times of '29Xe in blood and discussed the role of oxygenation, which was found to markedly increase the TI and to shift the RBC resonance to a higher frequency. Magnetic resonance angiography with hyperpolarized '29Xe dissolved in a lipid emulsion was investigated by Moller et al.276 The sensitivity and resolution in 3D NMR microscopy with hyperpolarized noble gases was studied by Moller et aZ.277 The first 3He NMR experiments using low temperature prepolarization were presented by Kober et al.278The obtained NMR signal was 100 times larger than the thermal equilibrium signal. Peled et aZ.279 introduced a single-shot pulsed gradient stimulated echo sequence to address the challenges of diffusion measurements of laser polarized 3He or '29Xe gas. Laser polarization enhances the NMR sensitivity of these noble gases dramatically, but creates an unstable, nonthermal polarization that is not readily renewable. The authors presented a new method, which permits parallel acquisition of the several measurements required to determine a diffusive attenuation curve. As a demonstration, the diffusion coefficient of a sample of laser-polarized I2'Xe gas is measured via this method. Stith et al.125 have investigated the transfer of polarization from 129Xeto solute protons in aqueous solutions to determine the feasibility of using hyperpolarized xenon to

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Nuclear Magnetic Resonance

enhance 'H sensitivity in aqueous systems. In spite of the very large '29Xe polarizations, the 'H signal enhancements gains will be less than 10% and often substantially smaller. The transport of gases in porous materials is a crucial component of many important processes in science and technology. Kaiser et a1.280presented a new technique, which allows the visualization of gas flow and diffusion in porous media obtained by NMR pulsed-field gradient experiments. Bonardet et a1.281reviewed the use of physisorbed 129Xeas a probe to investigate microporous and mesoporous solids. Reeves et a1.282studied the mesoscopic behavior of 3He quasiparticles in a dilute 3He/4He solution in vycor glass. Using both laser-polarized and thermally polarized xenon gas, Mair et al.283probed porous media with gas diffusion NMR. Mair et al.284 demonstrated NMR imaging of the flow and diffusion of '29Xe gas undergoing convection above the evaporating laser-polarized liquid xenon. Continuous flow NMR with hyperpolarized xenon for the characterization of materials and processes is reported by Moudrakovski et aZ.285Experimental data are compared with results from Monte Carlo simulations. The impact of multi-NMR spectroscopy on the development of noble-gas chemistry was summarized by Gerken and Schrobilgen.286 The authors discussed the role of NMR spectroscopy in the structural studies of noble-gas species regarding a variety of NMR sensitive nuclei. An understanding of the fluoride ion donor-acceptor behavior of noble-gas fluorides and oxide fluorides belong to the numerous important advances of noble-gas chemistry. Taha et studied the Berry pseudorotation of SF4 by gas-phase NMR. Experimental and calculated kinetic parameters were compared. The imaging obstructed ventilation with NMR using inert fluorinated gases such as SF6 is reported by Kuethe et a1.288The molecular structure of N-methylsilylaziridine in the gas phase has been determined by Mitzel et aZ.289using NMR and IR spectroscopy. The application of NMR techniques to characterize gas reservoirs is reported.290 If properly used, NMR tools confirm the interpretation by conventional tools and improve on permeability and fluid estimation. 5

Self-Diffusion in Liquids

5.1 Experimental and Theoretical Aspects. - Diffusion measurements using the non-linear stimulated echo are presented by Ardelean and K i m m ~ h . The ~~' non-linear stimulated echo is generated by a sequence of three radio frequency pulses in high magnetic fields in the presence of pulsed or steady field gradients. The latter can be used without knowledge of the relaxation times. Remarkably, the attenuation of the non-linear stimulated echo by diffusion is substantially stronger than in the case of the ordinary stimulated echo. In a consecutive paper4 both authors showed that the NMR diffusometry based on nonlinear echo signals permits one to measure small diffusion coefficients with moderate field gradients. The nonlinear echo

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experiments demonstrated that the coherence pathway dominating by far is of a purely single-quantum nature. Strong effects of modulated demagnetizing fields on the attenuation of Hahn echoes in liquids are shown theoretically as well as experimentally by Ardelean and K i m m i ~ h The . ~ ~ authors ~ concluded that the contribution of intermolecular multiple-quantum coherences transfer echoes to the multiple echoes is negligible. The demonstrated effects in this study are of major impact fro NMR spectroscopy with high-field instruments as they nowadays are in ubiquitous use. A two-dimensional NMR nutation spectroscopy scheme consisting of a single radio frequency (r.f.) pulse and a free-evolution period is suggested by Kimmich et al.293 It allows a multiple spin echo generation by gradients of the radio frequency amplitude. Ardelean et al.294suggested a two-pulse NMR nutation spectroscopy scheme that leads to a new type of spin echoes. Based on these echoes, a method for diffusion measurements is proposed that simultaneously provides the spin-lattice relaxation time and the self-diffusion coefficient. The methods of measurements of spatially resolved diffusion coefficients using radio frequency field gradient295.296 produce 1D profiles whose amplitude is not only a function of the local self-diffusion coefficient but also is modulated by cosine functions of spatial coordinates. Due to this modulation diffusion-weighted images cannot be obtained unless cumbersome data processing is used. Valtier et al.297 now present a new sequence, which avoids this modulation and yields straightforward manner true self-diffusion maps. Taubenreuther et al.298investigated nonlinear spin dynamics in a bulk fluid 3He sample at zero pressure in pulsed NMR experiments. The typical exponential decay of the first echo amplitude was only found for small effective diffusion coefficients. Non-linear effects from the dipolar-demagnetizing field in 3He at very high magnetic field were studied by Owers-Bradley et al.299The authors reported the first observation of long-lived free induction decays and provided a theoretical interpretation. Dvinskikh and Furo300 introduced a robust approach for combining multiple-pulse homonuclear decoupling and ROSE NMR for accurately measuring molecular diffusion coefficients in systems with nonvanishing static homonuclear dipolar couplings. Zavada et ~ 1 . ~ "introduced a propagator representation of anomalous diffusion and applied it to NMR correlation and spectral density functions in context with the orientational structure factor formalism. 5.2 Selected Examples. - Tornblom et ~ 1 . ~studied '~ rotational diffusion in dilute to concentrated micellar decylammonium chloride solutions by field dependent NMR relaxation. The water self-diffusion in a non-ionic surfactant/ water/oil system was measured by Coppola et al.303The same authors304 studied self-diffusion in aqueous solution and lyotropic mesophases of am'~ diffusion in dry and philic block copolymers. Grinberg et ~ 1 . ~observed swollen natural rubber. Diffusional behavior of n-alkanes in the rotor phase

204

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was observed by Yamakawa et al.306Courivaud et ~ 1 . ~investigated ' ~ the enhanced n-hexane diffusion in porous media. The phase behavior and molecular dynamics of tert-butyl cyanide confined within silica pores were studied by Aksnes et al.308 measuring self-diffusion and relaxation times. The internal dynamics and order parameters in surfactant aggregates were determined by Schonhoff et al.309Menzel et aL3'' presented NMR imaging and one- and two-dimensional self-diffusion propagator measurements of the liquids phase in salt water ice. For all samples highly anisotropic self-diffusion was observed. Translational diffusion coefficients of pentasaccharides in solution were reported by Rundlof et d3' Deuteron spectroscopy and deuteron fieldcycling NMR relaxometry of the hydration water of lipid bilayers was reported by Seitter et al.312Chen et ~ 1 . measured ~ ' ~ the temperature and density dependence of self-diffusion in the lower N-methylsubstituted amides. The data were analysed with the current models for fluid dynamics. Aramaki et al.314investigated the effect of water-soluble alcohols on the surfactant aggregation and its structure by pulsed field gradient N M R self-diffusion measurements and other experimental methods. Self-diffusion was also measured for benzenes in carbon dioxide as a function of pressure and temperature by Y ~ n k e r . * ~ ~ Fleischer et ~ 1 . ~investigated ' ~ the self-diffusion of polybutadiene star solutions in toluene over a broad concentration range from dilute to the ordering region with pulsed field gradient NMR. Pulsed field gradient NMR has been successfully applied to a study the intraparticle diffusion and exchange kinetics in chromatographic media by Tallarek et ~ 1 . ~ ' ~ Akimoto et ~ 1 . studied ~ ' ~ non-linear spin dynamics of dilute 3He/4Heat very high magnetic field over temperature ratio. Spin diffusion coefficients of 3He at Pulsed NMR a frequency of 920 kHz have been measured by I~hikawa.~'* investigations on normal-fluid 3He in restricted geometries are reported by Taubenreuther et aL319

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21 1

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266 267 268 269 270 27 1 272 273 274 275 276 277 278 279 280 28 1 282 283 284 285 286 287 288 289 290 29 1 292 293 294 295 296 297

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298 299 300 30 1 302 303 304 305 306 307 308 309 310 31 1 312

313 314 31 5 316 317 318

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7 Solid State NMR Spectroscopy BY A.E. ALlEV AND R.V. LAW

1

Introduction

Solid state NMR spectroscopy has now become an enormous field of research: in excess of 1500 publications used this technique in one way or another in the past year. This review takes highlights from all areas of research which we have for clarity’s sake divided into subject and further sub-divided by nucleus type. 2

Reviews

Review papers covering theoretical aspects of SSNMR include numerical simulation techniques of SSNMR experiments I and application of Lie algebra to N M R . ~ The condition for an echo formation has been reviewed in the context of half-integer quadrupole system. The common features of the classical echo phenomena and the coherence transfer echo have been highlighted. The article also explains how the measurement of a series of coherence transfer echoes achieves the HR feature of the MQ MAS experiment. SSB manipulation techniques in MAS NMR have been reviewed by Anzutkin .4 Various methodological aspects of half-integer quadrupolar nuclei have been extensively reviewed. Recent advances in experimental developments have been reviewed by Smith and van Eck.’ The basic theoretical aspects underlying HR MQ MAS NMR experiment, as well practical aspects involved in the optimisation of its sensitivity, have been presented.6 Also discussed has been the occurrence of unusual SSB patterns along the MQ domain; additional references to ongoing progress in the area that has appeared in the recent literature have also been presented. The advantages and drawbacks of various techniques that utilise MQ MAS NMR for studying molecular sieves and related materials have been r e ~ i e w e d .The ~ techniques reviewed include zfiltered MQ MAS with rotor synchronisation, MQ MAS with CP and MQ MAS with REDOR. Examples of applications include studies of various types of aluminophosphate. 59C0NMR of tetrahedric clusters in solution and in the solid state at high fields has been reviewed.* Some recent progress in solid state Nuclear Magnetic Resonance, Volume 30

>:g The Royal Society of Chemistry, 200 1 214

7: Solid State N M R Spectroscopy

21 5

47349Ti NMR of inorganics and gels has been reviewed.' Some complementary data from 170and I3CNMR and powder XRD has also been included. The application of SSNMR techniques for the structural determination of polysaccharide lo and wood and other lignocellulosic materials has been reviewed. Published spectroscopic observations pertaining to the crystal structure of native celluloses have been reviewed with emphasis placed on observations from I3C SSNMR.12 Considerable discussion has been devoted to the allomorphic composition of cellulose crystallites in higher plants. Published methods of NMR lineshape analysis for the higher plant celluloses have been reviewed and critiqued, both from the point of view of lineshape theory and from the point of view of self-consistency of inferences that are based on lineshape analyses for different carbons. Review articles on SSNMR applications to biological materials have included MAS studies of oriented membrane proteins. l 3 Advantages, practical aspects, and general perspective of MAS on oriented proteins have been presented. Characterisation of polymorphism in pharmaceuticals by SSNMR has also been reviewed.l4 An overview has been given on recent NMR structural studies of phosphorus chalcogenide glasses and melts.15 It has been shown that MAS in combination with DQ NMR provides detailed quantitative information on the network structural features present in ionically conducting thiophosphate glasses, spectroscopic and diffraction studies of simple phosphate glasses,l 6 comparative studies of amorphous phosphates by HPLC, NMR and XPS,17 and applications of SSNMR in microstructure studies of glasses.18 DQ NMR measurements exploiting the dipole coupling between the phosphorus atoms as well as a novel 2D exchange experiment using the scalar coupling have been described for measuring the connectivities of phosphate tetrahedra in glasses. It has been shown that bonding scenarios of up to four connected tetrahedra and the relative orientations of CS tensors can be determined by 2D NMR. This determination provides an additional verification of connectivities and can enable access to bonding angle data in future. Reviews covering applications of SSNMR techniques to materials of catalytic importance have included.20721722723724*25926 Finally, developments in SSNMR and analytical TEM for advanced understanding of the nature of the nearly amorphous calcium silicate hydrate phases in hardened cement pastes have been reviewed by R i ~ h a r d s o n . ~ ~

'

''

3

Theory

The CP dynamics between 'H and "F in a Viton-type fluoroelastomer have been studied using ''F MAS and 'H-+19F CP MAS NMR at 188.29 M H z . ~ ~ The phenomenological theory of spin thermodynamics based on the spin temperature hypothesis was employed to describe the polarisation transfer between the 'H and "F spin baths. Simultaneous fitting of the evolution of magnetisation in the standard CP and a modified CP(T0RQUE) experiment,

216

NucIear Magnetic Resonance

using the exact solutions of the equations for the spin thermodynamics, gave unique sets of the parameters THF, T1,('H) and TlP(l9F) for five separate peaks in the 19F spectra. The values of Tl,('H) and T1,(19F) obtained are consistent with those independently measured by spin-locking experiments. Theoretical description and experimental observation of residual dipolar couplings between quadrupolar nuclei in HR SSNMR have been reported.29 Nonsecular dipolar couplings between pin-'/^ nuclei that are in close proximity to quadrupolar spins have been extensively documented in SSNMR, particularly when involving directly bonded S = 13C, I = 14N spin pairs. The most notable characteristic of residual dipolar couplings is that they cannot be entirely averaged away by conventional MAS. Nonsecular dipolar couplings can also be expected to arise when both I and S are quadrupolar nuclei. Theoretical and numerical analysis have been presented for homo- or heteronuclear quadrupolar spin pairs in the high field limit. Variable field MQ MAS NMR results have also been presented for a variety of compounds possessing 11 B-14N, "B-' 'B and 55Mn-55Mn spin pairs, that validate these theoretical predictions and illustrate the valuable information that can be extracted from analysing these residual dipolar couplings. The research potential as well as resolution limitations that according to theoretical calculations these effects will impart on MQ MAS spectra recorded at low or moderate magnetic fields have thus been evidenced. The 29Si and 27Al NMR chemical shifts of the two crystallographic sites of the zeolite mazzite have been evaluated from the Nh4R shielding tensors calculated using the SOS-DFPT method.30 The calculations were carried out on one-site and two-site models, including from one to three coordination shells around each site. The effects of the cluster size, basis set extension, and choice of the exchange and correlation functional have been analysed. The effect of geometrical and electronic factors on the NMR chemical shifts have been analysed, showing their dependence on the cluster size and also on the shape of the zeolitic system. The SOS-DFPT has been used to calculate 29Si shielding constants and chemical shifts in zeolites.31The calculations were carried out on one-site (1T) Si(OSiH3)4and two-site (2T) R3SiOSiR3 (R = OSiH3) models, including three coordination shells around each site. The 29SiNMR chemical shifts have been shown to be very sensitive to the local geometry. A linear correlation between chemical shifts and average SiOSi angles has been established, taking into account two different zeolites, i.e. mazzite and zeolite-beta. The use of 1T models allows the assignment of the experimental spectra, whereas that of 2T models, containing eventually four-membered rings, improves considerably the calculation of the absolute 29Sichemical shifts, including those of silicon sites in aluminated zeolites. To assist in the assignment and interpretation of 23Na NMR spectra in silicate and aluminosilicate minerals and glasses the 23NaNMR shieldings and the EFGs at the Na for a number of Na-containing species have been ~ a l c u l a t e d .Included ~~ are Na(H20),+, n = 1, 2, 4, 5, 6 and 8, and Na+ complexes with SiH30H, SiH30Na and O(SiH3)2shieldings and EFGs for Na-

7: Solid State N M R Spectroscopy

217

centred clusters extracted from crystalline Na2Si03 and anhydrous sodalite have also been evaluated. Based on 6-31G* SCF optimised geometries and the GIAO method with a 6-31G* basis set a calculated increase in shielding with coordination number (CN) for the Na(OH2)n+,n = 4, 6, 8 series was found. This agrees with experimental trends. Calculated changes in the Na shielding as water is replaced by bridging or nonbridging silicate 0 atoms were also consistent with experimental observations. Theoretical studies, based on calculation of deshielding contributions for individual 0-containing ligands and experimental values for the Na-0 distances, were further extended to 12 different Na sites in silicate and aluminosilicate minerals which have recently been studied experimentally. Experimental and theoretical evidences of enhanced effects of CSA on the DQ dipolar SSB patterns have been presented.33The DQ MAS NMR response to a five-pulse sequence of dipolar coupled spin-'/2 pairs in a powder sample of barium chlorate monohydrate has been investigated for different spinning frequencies and different durations of excitation and reconversion periods. It has been shown that the basic understanding of the effect of chemical shift interaction on MQ SSB patterns will be useful in the quantitative analysis of MQ spectra for structural purposes and may be exploited for measurements of 'H CSA. Analytical expressions for MQ signal generation of quadrupole nuclei have been derived.34 Combined with numerical simulation of the double rotor motion, a strategy has been suggested for partial SSB suppression in MQ NMR spectra. Synchronisation of MQ excitation and selective flip pulses with outer rotor motion increases outer rotor speed effectively two times. This has also been demonstrated experimentally by TQ-SQ correlation spectra of 23Na and 67Rb. The mathematical foundation of the determination of protein structure from orientational constraints has been described.35 The tools used are vector algebra, gram matrices, and determinants. The report describes methods applied to the determination of protein structure by SSNMR. Examples have been given relating to the structure of the peptide gramicidin A. The phenomenological theory of spin thermodynamics based on the spin temperature hypothesis has been employed to describe the CP dynamics between two abundant nuclei, 19F and 'H, when the number of fluorine atoms is not substantially less than the number of hydrogens.36The influence of T1,s of both nuclei and the relative magnitude (heat capacity) of the two spin baths have been incorporated explicitly into the analysis in order to derive values for the parameters involved in the CP dynamics. Numerical calculations have been performed to clarify the difference in the evolution of magnetisation in variable contact time CP experiments between the 'H -+ I3C and 'H -+ I9F cases. A new type of CP-drain experiment has been developed for observing the residual 'H magnetisation after 'H + I9F CP. Direct polarisation I9F MAS, 'H -+ I9F CP, and 'H -+ 19F CP-drain MAS NMR spectra have been measured for a fluorinated polyimide, 6FDNODA. The CP dynamics between 'H and 19F for the polyimide have been analysed on the basis of the spin thermodynamics

218

Nuclear Magnetic Resonance

theory. The constant for polarisation transfer has been determined by the analysis using the effective CP parameters, which were directly obtained from the CP and CP-drain experiments, together with independently measured values of T I(, H) and TIp(19F). CP from a spin I = '/2 nucleus (e.g. 'H) to a spin S = 3/2 nucleus (e.g. 23Na) or a spin S = 512 nucleus (e.g. 27Alor "0) in static powder samples has been investigated. The results of conventional SQ, TQ and five-quantum CP experiments have been presented and discussed. Based on a generalisation of an existing theory of CP to quadrupolar nuclei, computer simulations have been used to model the intensity and lineshape variations observed in CP NMR spectra as a function of the rf field strengths of the two simultaneous spin-locking pulses. 37 These intensity and lineshape variations can also be understood in terms of the spin S = 3/2 or 5/2 nutation rates determined from experimental quadrupolar nutation spectra. The results of this study are intended as a preliminary step towards understanding single and MQ CP to quadrupolar nuclei under MAS conditions and the application of these techniques to the MQ MAS NMR experiment. The spin-locking mechanism of the spin I = 312 nuclei under MAS has been theoretically and experimentally investigated, and the criterion of adiabatic passage around zero-crossings of the quadrupole splitting was inferred from the time-dependent Schrodinger equation.38 It has been shown that SOQ interaction and off-resonance play important roles in the spin-locking of the quadrupolar nuclei, and they were responsible for the great loss of the spinlocking signals. The spin-locking might be achieved by minimising the effect of the SOQ interaction by using a rf offset. A formalised many-particle nonrelativistic classical quantised field interpretation of MAS RFDR has been presented.39 A distinction has been made between the MAS spin Hamiltonian and the associated quantised field Hamiltonian. The interactions for a multispin system under MAS conditions have been described in the rotor angle frame using quantum rotor dynamics. In this quasiclassical theoretical framework, the chemical shift, the dipolar interaction, and rf terms of the Hamiltonian have been derived. The effect of a generalised RFDR train of 7c pulses on a coupled spin system has been evaluated by creating a quantised field average dipolar-Hamiltonian formalism in the interaction frame of the chemical shift and the sample spinning. This derivation shows the analogy between the Hamiltonian in the quantised field and the normal rotating frame representation. The magnitude of this Hamiltonian peaks around the rotational resonance conditions and has a width depending on the number of rotor periods between the n: pulses. Its interaction strength can be very significant at the rz = 0 condition, when the CS anisotropies of the interacting spins are of the order of their isotropic chemical shift differences. The representation of second-order broadening effects in SSNMR by general fourth-degree surfaces has been pre~ented.~'Static SOQ and 'dipolarquadrupolar' interactions have been treated in a unified way. Most experiments involving the fast reorientation of samples such as MAS, variable-angle

'

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spinning, DAS, and DOR and MQ MAS have also been illustrated by using averaged fourth-degree surfaces. The equations of the surfaces have been derived analytically and allow the derivation of most results concerning these experiments. A general formalism has been developed for describing the effects of restricted rotational diffusion on 2H MAS NMR ~pectra.~' The approach is based on the Smoluchowski model that describes restricted rotational diffusion in an arbitrary ordering potential with an arbitrary diffusion tensor. It has been shown that the Smoluchowski model gives a physically more reasonable description of molecular motion than the discrete Markov (jump) model. The models have been shown to be mutually consistent for high ordering potentials and low temperatures provided the diffusion coefficient is sufficiently high. However, for low ordering potentials and high temperatures the discrete Markov model is not a useful approximation and the spectra can only be simulated with restricted rotational diffusion. This is also the case for small diffusion coefficients independent of the ordering potential and the temperature. The formalism is based on finite difference solutions to the stochastic Liouville-von Neumann equation. This defines a linear homogeneous system of coupled parabolic partial differential equations which includes both firstand second-order spatial derivatives. Numerical solutions are very difficult to obtain and some useful finite difference methods have been presented. The results have been elaborated for 2H MAS NMR spectroscopy. Solutions have been obtained both in the presence and absence of rf irradiation and effects of finite pulse width have been included. The method has been applied to the investigation of motional effects on 2H MAS NMR spectra of L-alanineN,N,N-2H3. The orientational dependence of the ordering potential and the quadrupole parameters has been determined from the Smoluchowski model. The activation energies have been found to be temperature dependent. These effects have not previously been observed and give new information on molecular motion in this system. The rotational diffusion results have been compared with the discrete Markov model and it has been found that in this case the two models are consistent. The most important difference is that the Markov model does not map out the orientational dependence of the ordering potential and the quadrupole parameters. Another advantage of the rotational diffusion model is that it is physically more reasonable than the Markov model and that the parameters may be interpreted in terms of molecular properties. A diffusion equation appropriate to the NMR spin diffusion experiments has been analysed using a periodic lattice Effects of disorder in polymer morphology on spin diffusion has been discussed. A simple model to describe heteronuclear spin decoupling in SSNMR under MAS conditions has been pr~posed.~' It is based on a coherent description of two heteronuclear dipolar-coupled spins (I and S) and an incoherent description of the interaction of the I-spin with a large number of other I-spins. The abundant and strongly coupled I-spins are irradiated. The selected I-spin is coupled by a spin-diffusion type superoperator to the I-spin bath, and this coupling is described by a single spin-diffusion rate constant. Such a model

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allows to simulate efficiently the behavior of a spin system under heteronuclear decoupling in the case of CW irradiation as well as in the case of phasemodulated sequences such as TPPM. A general treatment of NMR spectra under MAS conditions has been provided that is applicable both to homogeneously and inhomogeneously broadened lines.& It is based on a combination of Floquet theory and perturbation theory, and allows the factorisation of the spin system response into three factors that describe different aspects of the resulting MAS spectrum. The first factor directly reflects the Floquet theorem and describes the appearance of SSBs, the other two terms give the integral intensities of the resulting SSBs and their lineshapes and depend on the specific features of the considered interaction. From numerical simulations and the analysis of experimental MAS NMR spectra it was found that for typical spin systems, spinning frequencies of the order of the strongest couplings are sufficient to allow the analysis of the SSB intensities within the approximation of two-spin terms. This scaling of the different contributions together with the strong distance dependence of the dipolar interaction thus leads to a considerable simplification in the fast spinning limit and provides the basis for using the dipolar interaction in HR MAS spectra to obtain local structural information. An interesting phenomenon of 'H-13C CP induced by temporary adsorption in nanocapsule dispersions have been reported.45 A special mechanism of CP which is initiated by a temporary adsorption of mobile molecules to the more rigid capsule surface has been proposed. The concept of MQ CP between an I = 3/2 and an I = '/2 spin during MAS have been described.46Experimental and theoretical results for 23Na-'H pairs have been presented that elucidate the transfer mechanism and the beneficial effect of adiabatic amplitude modulations of the CP field. The MQ CP approach has been shown to be beneficial for improving the sensitivity of CP MQ MAS experiments and for detecting dipolar correlations. Procedures for processing data in rotor-synchronised 2D MAS NMR exchange measurements for both structural and dynamic studies have been presented.47 It has been shown that there are two distinct data processing procedures that lead to 2D MAS exchange spectra with purely absorptive crosspeaks and processing of 2D MAS exchange data using both procedures may enhance the information content of 2D MAS exchange measurements. An efficient algorithm for spectral simulations in NMR of rotating solids has been proposed.48 Averaging over the y powder angle has been shown to be generally equivalent to a cross correlation of two periodic functions. This together with the frequency-domain simulation procedure COMPUTE allows to reduce the computation time for spectral simulations by typically a factor of 10-30 relative to the original COMPUTE algorithm. The advantage and the general applicability of the new simulation procedure, referred to as yCOMPUTE, have been demonstrated by simulation of single- and multiplepulse MAS NMR spectra of 3'P-31P and 'H-'H spin pairs influenced by anisotropic CS and homonuclear dipolar interactions. Reintroduction of the quadrupolar interaction under MAS NMR has been

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in~estigated.~' The anisotropy in the dipole-dipole and quadrupole couplings are averaged out in MAS NMR experiments and under conditions of axial symmetry (q = 0), the homologous forms of the respective coupling Hamiltonians have been exploited to show that the quadrupole coupling can also be resurrected by a simple adaptation of a rotor-synchronised pulse sequence previously proposed for the recovery of dipole-dipole couplings. The theory for the recovery of the quadrupole couplings has been developed. Numerical simulations of the quadrupolar (DQ) dephasing observed in the 2H MAS NMR spectra of lipid systems have been used to extract the quadrupolar splitting. A new assignment method based on the periodic ab initio calculation of 23Naquadrupole coupling information using the CRYSTAL95 code has been described and applied to the multisite problem posed by Na~P301oAH20.~' The detailed mechanism of coherence transfer in 2H MAS NMR spectroscopy has been investigated in the presence of molecular motion.51 An elaborate theoretical formalism has been developed that exploits the properties of Lie algebras to characterise the states of nuclear spin ensembles and to identify the allowed coherence transfer pathways. The functional form of the coherence transfer functions has been investigated within the Floquet-Poincare formalism. Based on these principles a general methodology has been described for evaluating the optimum pulse parameters consistent with maximum sensitivity. The intensity and phase distortions induced by the optimum pulse parameters have been discussed and the importance of incorporating these effects in theoretical simulations has been examined. The results have been verified by experimental spectra of polycrystalline thiourea-2H4 that have been analysed in terms of a discrete motional model. A theoretical description of CW high frequency LG CP MAS NMR experiments has been presented.'* The full time-dependent LG CP MAS Hamiltonian has been replaced by its zero order time-independent Hamiltonian in the interaction representation. Carbon signal enhancements of LG CP MAS experiments have been calculated for spin systems consisting of six 'H nuclei coupled to one 13C nucleus. These simulations have been based on Floquet theory calculations, explicitly taking into account the time dependence because of MAS, and calculations based on the zero-order Hamiltonian. The good agreement between these calculations justifies the use of the zero-order Hamiltonian. The time-dependent intensities of the cross peaks in heteronuclear 3C-'H correlation spectra, extracted from 3D LG CP MAS experiments on a natural abundant D,L-alanine sample with increasing CP mixing times, are in good agreement with the theoretical intensities simulated by using the zero-order Hamiltonian. The approximated LG-CP MAS Hamiltonian can be used to obtain structural information about a proton coupled to a single carbon. The simulated intensities of the carbon signals of an isolated l3C-lH group and a 13C-lH group that is coupled to additional protons, measured by LG-CP MAS experiments with increasing CP mixing times, have been compared. This study suggests that the buildup curve of each LG CP MAS carbon signal and its Fourier transformed CP spectrum can be interpreted in terms of a single distance between the observed I3C and

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its nearest proton, if the additional protons are removed from this carbon by at least 1.2 times this distance. Using numerical optimisation procedures it has been shown that it is possible to design composite x rf pulses for MAS NMR spectroscopy by explicitly taking into account the variation of the resonance offset of each crystallite during the application of the rf pulses.53 When using composite rf pulses in experiments such as TOSS, where the delays between the pulses have to be critically adjusted, an optimisation of these delays can lead to the desired performance characteristics. Using molecular cluster models a theoretical ab initio study of the 27Al and 31P NMR chemical shieldings aimed at obtaining short-range structural information on the aluminophosphate oxynitride (AlPON) catalyst system has been ~ n d e r t a k e n .Ortho~~ phosphate-like molecular models with P/Al ratio equal to one and varying N/O ratios were used to simulate the experimentally obtained compositions. The computed NMR chemical shieldings reproduce quantitatively the observed features in the 31PMAS NMR spectra. The comparison of 27Al isotropic chemical shieldings and the experimental spectra allow to conclude that N/O substitution does not happen in the first coordination shell of aluminum atoms. A numeric algorithm has been proposed that is suitable to calculate spectral lineshapes influenced by isotropic and anisotropic tumbling under sample spinning condition^.^^ It is based on the stochastic Liouville equation and a rotational diffusion process described by a stationary Markov operator, An example has been presented demonstrating the potential of off-MAS as a tool to analyse slow tumbling motions. Ab initio H F and DFT calculations, based upon both cluster and periodic modelling approaches, have been reported for the EFG tensor at sodium in NaN02.56Calculations based on different-sized clusters have been compared and it has been shown that resonable agreement with experiment can be obtained for a symmetrical cluster that extends beyond the immediate coordination environment of Na+. It has been suggested that periodic ab initio H F calculations using the standard 6-21G basis set, with suitable basis set optimisation to take into account the cationic nature of sodium, can provide a routine and consistent method for predicting sodium EFG tensor information for ionic sodium compounds. Ab initio HF MO calculations have been applied to the crystalline imidazole and its derivatives in order to examine systematically the effect of possible NH..-N type H-bonding on the nuclear quadrupole interaction parameter^.^^ The I4N QCC and the asymmetry parameter (q) of the EFG were found to depend strongly on the size of the molecular clusters, from single molecule, to dimer, trimer and to the infinite molecular chain, i.e.,crystalline state, implying that the intermolecular N-H. - .N hydrogen bond affects significantly the electronic structure of imidazole molecule. A certain correlation between the 14N QCC and the N-H bond distance R was also found and interpreted on the basis of the MO theory. A re-examination of the observed N-H distances in imidazole derivatives was suggested.

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The use of rotational-resonance experiments to achieve homonuclear polarisation transfer between half-integer quadrupolar spins has been proposed.58A theoretical description for a homonuclear two-spin system in the framework of Floquet theory has been given. Numerical simulations, as well as analytical Floquet calculations, were performed to analyse the behaviour of such a system. It has been shown that while a rotational-resonance experiment at fixed spinning speed leads only to a partial polarisation transfer, an adiabatic passage through the rotational-resonance condition, achieved by a spinningspeed ramp, promotes polarisation transfer for all components of the SOQ broadened lineshape. The assignment of SSNMR spectra has been studied by the use of model systems computed with ab initio methods. 59 Hexabenzocoronene derivative has been used. It has been shown that the combination of experimental NMR data with quantum chemical calculations can be employed as a useful tool in determining the structure of solid-state systems in general, especially where other experimental techniques fail. A theoretical framework for the use of continuously phase modulated rf pulses for homonuclear decoupling in SSNMR has been presented.60Within this framework, new families of decoupling sequences have been derived using numerical optimisation. One of the sequences has been tested experimentally on an ordinary organic solid, and its performance has been compared with standard multiple-pulse sequences. A theoretical and experimental study of the spin dynamics in PISEMA (Polarisation Inversion Spin Exchange at the Magic Angle) to investigate the line-narrowing mechanism has been presented.6' The study focuses on the effect of neighbouring protons on the spin exchange of a strongly coupled spin pair. The spin exchange has been solved analytically for simple spin systems and has been numerically simulated for many-spin systems. The results show that the dipolar couplings from the neighbouring protons of a strongly coupled spin pair perturb the spin exchange only in the second order and has little contribution to the linewidth of PISEMA spectra in comparison to the separated-local-field spectra. The effects from proton resonance offset and the mismatch of the Hartmann-Hahn condition have also been discussed along with experimental results using model single-crystal samples. A selectively 2H labeled osmium dihydride has been studied by 2H MAS NMR.62 It was found that the interference between the quadrupolar and homonuclear dipolar interaction results in a characteristic lineshape of the SSBs. The basic properties of the interference of homonuclear dipolar and quadrupolar coupling on the 2H NMR lineshape were elucidated, using average Hamiltonian theory, and exact simulations of the experiments were achieved by stepwise integration of the equation of motion of the density matrix. These simulations show that it is possible to determine the size of the dipolar interaction and thus the 2H-2H distance from the lineshape of the SSBs.

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4

Experimental Aspects

4.1 New Technique Developments. - It has been demonstrated that throughspace recoupling can be achieved in dipolar coupled quadrupolar spins in the presence of an appropriate rf field.63Experimental and theoretical results for 23Na-23Napairs have been presented that elucidate the experimental conditions leading to homonuclear dipolar transfer. The effect of adiabatic amplitude modulation on spin-3/2 systems has been compared to spin-'/2 cases and applications of this approach in the context of HR MQ MAS for dipolar filtering and correlation have been discussed. Lithium spin-alignment spectroscopy has been presented as an NMR technique for studying slow translational motions in solid and solid-like ionic conductor^.^^ Phase cycling that allows to measure translational correlation functions via the generation of a pure quadrupolar ordered state has been used. Correlation functions of the crystalline electrolyte Li3Sc2(P04)3have been recorded for times ranging from about 0.1 ms to more than 10 s, implying that translational diffusion coefficients smaller than 10- 2o m2/s become accessible. The miscibility of two macroscopic phase separated polymers, a propyleneethylene-diene terpolymer and an atactic polypropylene, has been investigated by a combination of several SSNMR technique^.^^ Carbon-detected proton Tk,('H) and TI('H) in systems conditioned by 2D heteronuclear WISE have been used. Both techniques are sensitive to spin diffusion between phases, with WISE being suited to making it stand apart from the basic relaxation process. These two techniques yield similar but different assessments of the presence and amount of phase separation present. An additional comparison has been made with differential scanning calorimetry and xenon NMR results, which also address this problem. Three exchange NMR techniques have been presented that yield 13CNMR spectra exclusively of slowly reorienting segments, suppressing the often dominant signals of immobile components.66The first technique eliminates the diagonal ridge that usually dominates 2D exchange NMR spectra and that makes it hard to detect the broad and low off-diagonal exchange patterns. A modulation of the 2D exchange spectrum by the sine-square of a factor which is proportional to the difference between evolution and detection frequencies is generated by fixed additional evolution and detection periods of duration z, yielding a 2D pure-exchange (PUREX) spectrum. Smooth off-diagonal intensity has been obtained by systematically incrementing z and summing up the resulting spectra. The related second technique yields a static 1D spectrum selectively of the exchanging site(s), which can thus be identified. Efficient detection of previously almost unobservable slow motions in a semicrystalline polymer has been demonstrated. The third approach, a 1 D pure-exchange experiment under MAS, is an extension of the exchange-induced sideband (EIS) method. A TOSS spectrum obtained after the same number of pulses and delays, with a simple swap of z periods, is subtracted from the EIS spectrum, leaving only the exchange-induced sidebands and a strong, easily detected centreband of the mobile site(s).

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An NMR experiment has been presented which allows the relative orientation of nuclear quadrupole and dipole coupling tensors to be determined.67 The experiment uses a 41 quantum filter to select dipolar-coupled spin I pairs, and the relative tensor orientations have been determined by lineshape analysis. It selects dipolar-coupled spin pairs, even in a multi-spin system, and leads to data which are straightforward to interpret. The same approach has been used to perform a 2D correlation experiment showing which spins are close in space. nuclei with important CSA in disordered When observing spin I = materials, the distribution of isotropic shift can become so large that no accessible spinning rate is able to provide a resolved spectrum. This is the case of 207Pb in glasses where static and high-speed MAS spectra are nearly identical. The possibility of rebuilding an SSB free spectrum using a shifted echo modified PASS sequence has been demonstrated.68This makes it possible to discuss isotropic and anisotropic chemical shifts of lead in phosphate glasses, to characterise its structural role and its chemical bonding state. DQ heteronuclear local field spectroscopy has been used to determine a molecular torsional angle in the metarhodopsin-I, which is the 41 kDa integral membrane protein.69 The result obtained is consistent with current models of the photo-induced conformational transitions in the chromophore. Isotropic NMR spectra of half-integer quadrupolar nuclei using satellite transitions and MAS have been rep~rted.~' A novel MQ MAS spin counting experiment based on the C7 recoupling sequence has been described.71The new experiment is applicable at fast MAS rates and can be used to follow the MQ excitation dynamics with fine time resolution. It has been illustrated by experiments on adamantane at spinning speeds comparable to the nonspinning dipolar linewidth. A new type of fast amplitude modulated pulse scheme has been presented that yields a significant sensitivity enhancement in the TQ MAS NMR spectrum of a spin-512 nucleus.72 Enhancement is achieved by fast phase alternation of the TQ-to-SQ conversion pulse, which transfers TQ-to-SQ coherence in a direct, non-adiabatic manner. REDOR and 8-REDOR experiments for recovering the l3CzH dipolar interaction during MAS NMR have been compared.73It has been found that limited 2H rf power may severely compromise the performance of the REDOR experiment whereas the 6-REDOR experiment can be designed to work well. Results have been presented for an isolated 13C-2H spin pair with a large 2H QCC and for a 13C coupled to three methyl deuterons undergoing fast methyl rotation. New modified spectral editing methods for 13C CP MAS experiments for separating nonprotonated C and CH3 peaks have been reported.74 Examples have been provided for 3-methylglutaric acid, fumaric acid monoethyl ester, and two complex natural products: methyl o-methylpodocarpate and 10deacetylbaccatin 111. The 2D anisotropy-correlated NMR spectra of half-integer quadrupolar nuclei may be recorded by using an exchange sequence in conjunction with

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MAS during evolution and detection, and off-MAS during mixing.75Application to boron oxides has been described, in addition to an analysis of the spin diffusion rates in such materials. The use of double frequency sweeps (DFSs) consisting of two sidebands generated by a time-dependent amplitude modulation of the rf-carrier frequency has been demonstrated in NMR of spin 3/2 nuclei.76This can be used for signal enhancement in both static and MAS spectra, as shown for a number of model compounds. DFSs prove to be efficient for the conversion of TQ-to-SQ .coherence in MQ MAS spectroscopy. This relieves the rf-power requirements and as a result undistorted MQ MAS lineshapes are obtained (demonstrated for the four 23Naresonances in Na4P207). A strategy of designing new heteronuclear broadband decoupling sequences for SSNMR has been presented.77 The new technique involves a phase modulation of the decoupler as a function that is a sum of several cosine terms (abbreviated as CPM m-n for cosine phase modulation with m harmonics and having frequency intervals of ct),.fln,where m and n are integers). Experimental results show that their performances are considerably better than those of the existing decoupling methods, under conditions of moderate spinning rate and decoupling power. The MQ MAS NMR experiment and a sensitivity enhanced variant detecting the SOQ powder pattern through a train of quadrupolar CPMG refocusing pulses have been analysed with respect to the effects of finite rf pulse irradiation and the MAS frequency.78Taking these effects explicitly into account, it is possible to determine optimum conditions for excitation of MQ coherences and reconversion of these into detectable SQ coherences as well as simulate the SOQ lineshape necessary to extract quadrupolar parameters and isotropic chemical shifts. These are important for the exploitation of MQ MAS experiments for quantitative determination of site populations. The various effects have been described analytically and demonstrated by numerical simulations and by 87RbMQ and MQ CPMG MAS experiments on RbN03. The MQ MAS has been used to evaluate the individual lineshape of the A1 signals in the ID 27Al MAS NMR spectrum of zeolite rriaterial~.~~ By the application of these real lineshapes, an improved deconvolution of the 1D MAS NMR spectrum has been achieved. This methodology has been applied to the two tetrahedrally coordinated A1 sites in mazzite zeolite. Changes in the TQ CP matching profiles and 27Al/23NaSOQ lineshapes, have been observed as a function of the I = ' / 2 rf field strength for both static and MAS conditions." It has been shown that only a fraction of the spins in the powder, with specific orientations of the EFG tensor, can match the Hartmann-Hahn condition at the same time, for a fixed I = ' / 2 rf field strength. Magnetic field gradients have proven useful in NMR for coherence pathway selection, diffusion studies, and imaging. Recently they have been combined with MAS to permit HR measurements of semi-solids, where MAS averages any residual dipolar couplings and local variations in the bulk magnetic susceptibility. First examples of coherence pathway selection by gradients in dipolar coupled solids have been presented." It has been shown that when the

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gradient evolution competes with dipolar evolution the experiment design must take into account both the strength of the dipolar couplings and the means to refocus it. Examples of both homonuclear and heteronuclear experiments have been shown in which gradients are used to eliminate the need for phase cycling. The target field method of designing gradient coils has been extended to the case where the gradient producing currents lie on cylinders of a general orientation with respect to the polarising magnetic field.82 This provides a general approach for designing coils that require unusual sample geometries such as those required for MAS applications. A detailed example of a magic angle gradient coil set has been presented. One of the main problems in the performance of the MQ MAS experiment is the poor efficiency of the rf pulses used in converting MQ coherences to the observable SQ signals. As the MQ MAS is an echo experiment this problem can be related to the efficiency with which CW pulses can normally achieve the MQ-to-SQ conversion for different crystallites in a spinning powdered sample. An improved MQ MAS NMR experiment using amplitude modulated pulses has been reported.83These pulses were found to yield MQ MAS NMR signals that are 2-3 times stronger than the ones arising from the usual CW pulse schemes by virtue of a superior efficiency of the TQ-to-SQ conversion. Numerical simulations and experimental results for 23Na and 87Rbhave been presented that corroborate the usefulness of the new approach. Various aspects involved in this MQ-to-SQ conversion have been investigated further,84 in order to devise new experimental schemes that can lead to significant MQ MAS signal enhancements. A new MQ MAS experiment employing amplitude-modulated rf pulses and the mechanisms of operation of CW and of amplitude-modulated pulses have been examined. Experimental results highlighting the utility of this scheme in samples possessing multiple quadrupolar sites with varying quadrupolar anisotropies and chemical shift offsets have been demonstrated. A pulse scheme for achieving HR 'H NMR spectra in solids by coherent averaging of spin-spin interactions called phase-modulated LG has been reported .85 Rotationally induced excitation of TQ coherences has been demonstrated for I = 5/2 nuclei from comparison of the variation of the observed and simulated lineshapes as a function of spin-lock time.86 A new pulse sequence producing purely absorptive lineshapes has been reported. Rotation-induced 5Q-lQ coherence transfer (RIACT) for 5Q MAS NMR experiments has been reported for I7O for compounds with quadrupolar interactions in the range 37 M H z . It ~ ~has distinct advantages over the methods used to excite 5Q MAS NMR hitherto through much reduced sensitivity to the EFG. The 2D 5Q RIACT I7O spectrum of zeolite Na-A has been reported, which is better resolved than the higher field TQ spectrum. A new technique for measurements of dipolar interactions in rotating solids has been presented that combines the capabilities of MQ MAS with the REDOR. 88 This new technique exhibits improved sensitivity toward weak dipolar interactions. WISE NMR with windowless isotropic mixing (WIM)

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has been developed as a method to study the dynamics of polymers and blends.89 This experiment has been designed to measure the dynamics of polymers through the 'H lineshapes that are correlated with the 13Cchemical shifts in 2D NMR experiments. 2D WIMWISE has been used to measure the main-chain and side-chain dynamics in poly(n-butyl methacrylate) and blends of polystyrene and poly(viny1 methyl ether). A new pulse sequence for heteronuclear DQ MAS N M R spectroscopy of dipolar-coupled spin-'/2 nuclei has been introduced." The heteronuclear DQ SSB patterns produced by this experiment have been shown to be sensitive to the heteronuclear distance, as well as the relative orientations of the CS and dipolar tensors. The isolated 13C-lH spin pair in deuterated ammonium formate with I3C in natural abundance has been chosen as a model system, and the perturbing influence of dipolar couplings to surrounding protons on the 13C-'H DQ coherence has been discussed. The pulse sequence can also be used as a heteronuclear DQ filter, hence providing information about heteronuclear couplings, and thus allowing the differentiation of quaternary and CH bonded carbons. A new heteronuclear chemical shift correlation technique of abundant spins (e,g. 'H)with rare spins (e.g. I3C) has been reported?' HR is provided by ultra-fast MAS and high magnetic fields, high sensitivity being ensured by a direct polarisation transfer from the abundant protons to 13C. In a rotorsynchronised variant, the method can be used to probe heteronuclear throughspace proximities, while the heteronuclear dipolar coupling constant can quantitatively be determined by measuring MQ SSB patterns. By means of recoupling, even weak heteronuclear dipolar interactions are accessible. The capabilities of the technique have been demonstrated by measurements on crystalline L-tyrosine hydrochloride salt. 13C ZQ MAS NMR spectroscopy for paramagnetic solids has been proposed to obtain both highly resolved isotropic peaks and accurate values of shift anisotropies by removing line broadening due to bulk magnetic susceptibility (BMS) shifts.92Several pulse sequences for ZQ NMR experiments under fast MAS have been presented which are based on homonuclear J couplings between a pair of 13C spins. For determining paramagnetic shift anisotropies, ZQ SSB (SSB) patterns have been observed which are free from the distortion due to the BMS shifts. To enhance ZQ SSB intensities, a n: pulse was inserted during the tl period of 2D experiments. A selective excitation scheme has also been developed which leads to the improvement of the SNR of ZQ NMR spectra. The 2D PASS experiment is a useful technique for simplifying MAS NMR spectra that contain overlapping or complicated SSB manifolds. The pulse sequence separates SSBs by their order in a 2D experiment. The result is an isotropichnisotropic correlation experiment, in which a sheared projection of the 2D spectrum effectively yields an isotropic spectrum with no SSBs. The original 2D PASS experiment works best at lower MAS speeds (1-5 kHz). At higher spinning speeds (8-12 kHz) the experiment requires higher rf power levels so that the pulses do not overlap. In the case of nuclei such as 207Pb,a

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large CSA often yields too many SSBs to be handled by a reasonable 2D PASS experiment unless higher spinning speeds are used. Performing the experiment at these speeds requires fewer 2D rows and a correspondingly shorter experimental time. New PASS pulse sequences have been implemented that occupy multiple MAS rotor cycles, thereby avoiding pulse overlap.93A version of the multiple-rotor-cycle 2D PASS sequence that uses composite pulses to suppress spectral artifacts has also been presented. These sequences have been demonstrated on 207Pbtest samples, including lead zirconate, a perovskite-phase compound. Different approaches to obtain pure absorption-mode lineshapes in MQ MAS experiments employing a train of 7[: phase-alternating pulses for the MQto-SQ mixing period have been i n ~ e s t i g a t e dFour . ~ ~ pulse sequences have been presented and their improved performance has been experimentally demonstrated by 87RbMQ MAS of RbN03. A simplified method for acquiring pure-phase 2D exchange spectra under slow MAS has been introduced. It combines rotor-synchronised 2D exchange spectroscopy with whole-echo acquisition leading to a simplification in data acquisition and pro~essing.~' The proposed method is applicable to samples where an echo of the FID can be obtained, i.e., where the inhomogenous linewidth is larger than the homogeneous linewidth. This is the case in rarespin spectroscopy of samples with natural isotopic abundance. The usefulness of the new method has been demonstrated, using I3C spectroscopy. Layered paramagnetic compounds, such as La2Li0.5Ni0.504with a perovskite structure, the 7Li NMR spectrum is broadened by anisotropic quadrupolar as well as paramagnetic dipolar interactions. A 2D spin echo (SE) experiment to separate the quadrupolar interaction and obtain a clean quadrupolar spectrum along the w1 dimension has been ~ u g g e s t e dThis . ~ ~ has been demonstrated through 2D SE experiments conducted in static samples as well as those in spinning at the magic angle. A quadrupole coupling constant of 92 kHz has been estimated for paramagnetic La2Li0.5Ni0.504.It has been shown that the same information may be obtained from the C-H dipolar couplings using 2D experiments correlating 3C chemical shifts and H-C dipolar couplings.97 The order parameters of C4-H, and C4-H, vectors calculated from this experiment are in agreement with those previously determined on the same sample from 2H NMR. This experiment opens the way to the simultaneous determination of order parameters of all C-H vectors without the need of specific labelling. New lD, 2D and 3D SSNMR spectroscopic methods designed for structural studies of uniformly "N- and I3C-labeled peptides and proteins in oriented samples have been described.98These methods provide a means of obtaining resolved spectra, sequential resonance assignments, and structural constraints. Experimental results for model single-crystal peptides and amino acids demonstrate that HR 1D 13C spectra can be obtained for signals from carbonyl or carboxyl carbons in uniformly labelled samples by applying phase-modulated selective homonuclear (PSH) decoupling at aliphatic carbon resonances, in addition to heteronuclear proton and "N decoupling. 13C-detected 2D lSN/l3Cchemical shift correlation spectroscopy has been made possible by a

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combination of PSH decoupling and broadband heteronuclear polarisation transfer sequences such as WALTZ-5 CP. Experimental 2D spectra of uniformly 5N- and I3C-labelled AlaGlyGly crystals show that resolution and sequential assignment of (CO)-13Cand I5N NMR signals is possible. Comparisons of experimental spectra and simulations verify the assignments and the accuracy of structural information contained in the 2D spectra in the form of the orientation-dependent (CO)-13Cand 15N chemical shifts. 13C-detected3D spectroscopy has also been demonstrated by adding a 'H-15N dipolar dimension to the 2D methods. Results of experiments at fields of 9.39 and 17.6 T have been reported. Enhancement of sensitivity in I5N NMR by indirect detection through 'H NMR signals under high-speed MAS and high-field conditions has been demonstrated experimentally on two "N-labelled peptides, polycrystalline AlaGlyGly and the helix-forming 17-residue ~ e p t i d e Sensitivity .~~ enhancement factors ranging from 2.0 to 3.2 have been observed experimentally, depending on the 15N and H linewidths and polarisation transfer efficiencies. The H-detected 2D 'H/15N correlation spectrum of AlaGlyGly illustrates the possibility of increased spectral resolution and resonance assignments in indirectly detected experiments, in addition to the sensitivity enhancement. A procedure for structural characterisation based on XRD and on a set of 2D SSNMR experiments has been proposed."' It has been applied to a new gallophosphate oxyfluorinated compound. A set of three NMR experiments RFDR for 19F, DQ for 31Pand HETCOR between I9F and 31P- facilitate the analysis of the topology of homoatomic and heteroatomic sub-networks of fluorine and phosphorus included in the inorganic framework of this material. The efficiency of this combination of NMR experiments in assigning all NMR signals to their crystallographically sites has also been demonstrated.

'

4.2 NMR Parameters: Experimental and Quantum Mechanical Studies. 4.2.1 Spin-'/, Nuclei: Isotropic Shifts and CS Tensors. - Determination of 'H CS tensors is crucial for protein structure determination by SSNMR. The 'H chemical shift is particularly important in spectra obtained on oriented samples of membrane proteins as a mechanism for providing dispersion among resonances that are not resolved with the *H-l5N dipolar coupling and "N chemical shift frequencies. This has been demonstrated with 3D experiments on uniformly "N-labeled samples of Magainin antibiotic peptide and the protein Vpu from HIV-1 in oriented lipid bilayers.'" These experiments enable resonances in 2D 'H-15N dipolar c ~ u p l i n g5N / ~ chemical shift planes separated by 'H chemical shift frequencies to be resolved and analysed. It has been shown that the magnitude and absolute orientation of 'H CS tensors may be determined from polycrystalline powders using CRAMPS by simultaneous evolution under CS and heteronuclear dipolar coupling interactions. lo2 An experimental approach based on the broadband high-order truncating MSHOT-3 homonuclear decoupling sequence has been demonstrated for the H-bonded proton within the 31P-'H-3'P three-spin systems of a powder of KH2P04.

'

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13C - The polymorphic structures of silk fibroins in the solid state were examined on the basis of a quantitative relationship between the I3Cchemical shift and local structure in proteins. lo3 To determine this relationship, 13C chemical shift contour plots for C, and Cp of Ala and Ser residues, and the C, chemical shift plot for Gly residues were prepared using atomic coordinates from the Protein Data Bank and 13CNMR chemical shift data reported for 40 proteins. Possibilities and limitations of iterative lineshape fitting approaches for the complete determination of magnitudes and orientations of N M R interaction tensors in a four-13C-spin system from MAS NMR experiments have been investigated. The model compound chosen for this investigation is the monoammonium salt of maleic acid. Various selectively and fully 3C-labelled versions of this compound permit a stepwise reduction of the number of unknown parameters, necessary to fully describe the four-13C -spin system in the uniformly "C-labelled maleate moiety. It has been demonstrated that assumptions about 'typical' CS tensor orientations, even if not deviating much from the real orientations, lead to severe errors in internuclear distance determinations. The principal values of the 13C CS tensors were measured for coronene and corannulene, both at room temperature and at approximately 100 K.lo5A comparison of the principal values between the room temperature motionally averaged pattern and the low-temperature static pattern provides information about the orientation of the principal axis system of the CS tensor for the bridgehead carbons in these molecules. The results show that the motion is not constrained to simple in-plane rotation, but must also have an out-of-plane component. Quantum chemical calculations of the CS tensors were also completed using both experimental and optimised molecular geometries. 19F - Experimental measurements of 19Fisotropic chemical shifts on a large set of mainly ionic fluoride compounds (from simple metal fluorides to transition metal fluoride glasses) obtained by MAS NMR at 15 kHz have been investigated.lo6First, Ramsey's theory of die chemical shift with MOs obtained by Lodwin's orthogonalisation method has been used to evaluate the isotropic part of the 19Fchemical shift in ionic fluorides for which the crystallographic structure and the atomic radial wavefunctions are known. Assuming that the paramagnetic part of the 19F shielding in a given material is simply the summation of the paramagnetic contributions due to all the cations in the neighbourhood of the considered fluorine, a superposition model of the I9F isotropic chemical shift has been developed. This empirical approach has been applied to complex fluoride compounds of unknown structure and it has been shown that it allows to obtain reliable structural informations. The sensitivity of the 19F isotropic chemical shift to the environment of the fluorine atom has been used to investigate transition metal fluoride glass network^."^ From the chemical shift values, it has been shown that three F sites can be identified shared and unshared Fs between two MF6 octahedra and free Fs which are not implied in these MF6 octahedra. The proportions of these different fluorines in the glasses have been obtained and the connectivity

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of the MF6, octahedra which constitute the networks has been deduced. It has been shown that the structure of the alkali fluoride glasses is quite different from the other fluoride glasses. "P - Local structure of phospholipid/amine polyion complexes in lyophilised powders of dipalmitoylphosphatidic acid/poly-L-lysine and DPPA/PO~Y-Larginine has been investigated.lo* The intermolecular interactions in the polyion complexes were detected by 31P-1HHETCOR. The complete information about the 31P CS tensor components and the mutual orientation of CS tensor with respect to a molecular fixed frame was determined by both CP MAS and spin-echo separated local field measurements. From the ab initio 31P CS calculation, the behaviour of the principal components of 31PCS tensor in the phosphate group in DPPA has been found to be dominated by a change in the electronic state in association with phosphate/amine complexation. The 31PCS surface of phosphinoborane R2PBR2' has been investigated via MO calculations and experimental measurements.' O9 Ab initio calculations of 31P CS tensors were determined for the phosphinoboranes H2PBH2 and Me2PBMe2.Changes in the angle from planarity, i.e. that between the P-BR2' plane and the bisector of the RPR angle, are reflected in the orientations and magnitudes of the three principal components of the 31P CS tensor. To determine the validity of the calculated 31PCS surface, three phosphinoborane compounds with different angles from planarity were studied by 31PSSNMR spectroscopy. The experimental magnitudes of the principal components and orientations of the 31PCS tensor compared well to the calculated predictions. The combined experimental and theoretical results provide a good description of the effects on changes in bond angle on 31PCS as the molecule is distorted from a planar to folded geometry. 31P 1D NMR spectra of a stationary powder sample of a phosphole tetramer containing two phosphorus spin pairs have been obtained at 4.7 T and 9.4 T."' In order to separate 31P-31Pspin-spin coupling from anisotropic CS, 2D spin-echo NMR spectra have been acquired. 31P CP MAS NMR experiments indicate that the two spin pairs of the tetramer are equivalent and each may be treated as an isolated spin pair. Within a given spin pair, the difference between the isotropic chemical shifts of two directly bonded 31P nuclei is 1.7 ppm. They are spin-spin coupled by both the indirect ('J = -362 Hz) and direct interactions (1.80 kHz). The principal components and relative orientation of the two 31PCS tensors have been determined using the dipolarchemical shift technique. Ambiguities in the CS tensor orientation relative to the molecular framework have been resolved using ab initio calculations and simulations of the 2D spin-echo spectra. The spans and skews of the 31PCS tensors for all 4 three-coordinate 31Pnuclei are the same within experimental error, 115 ppm and 0.70, respectively. 3'P CS and spin-spin coupling tensors have been characterised for tetramethyldiphosphine disulfide (TMPS) by analysis of 31P CP NMR spectra obtained for a single crystal and powder samples have been acquired at 4.7 and 9.4 T."' A 2D spin-echo NMR spectrum was obtained to independently determine the effective 31P-31Pdipolar coupling constant. The subtle difference

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between two particular inequivalent phosphorus sites in TMPS were examined using 31P CS tensors obtained from both single-crystal and dipolar-chemical shift NMR methods. lo9Ag - It has been shown that '09Ag NMR signals in AgxCul crystals are shifted to a low field relative to that in solid AgI with decreasing the unit cell constant analogous to 63Cu MAS NMR signals.'12 The observed chemical shieldings were analysed using ab initiu calculations of lo9Ag and 63Cu CS tensors for tetrahedral Ag143- and CuIz- ions. lI9Sn - The effects of powder granule size on parameters such as chemical shift, linewidth, and spin-lattice relaxation have been studied using 19Sn MAS NMR of Sn02 powders.'13 Linewidth showed a general broadening as size decreased, but there was evidence in the anisotropy of the chemical shift and in the spin-lattice relaxation. 199Hg - The 199HgMAS NMR spectra of Hg2X2 (X = C1, SCN, NCO, CH3C02, CF3C02) have been reported.'14 SSB analysis has been used to determine the 199HgCS parameters, Ao and q. In contrast to the case of the corresponding Hg(I1) compounds, the shielding anisotropy has been found to be relatively insensitive to the nature of the X group. This is consistent with the view that the electronic environment of the Hg atom in the Hg(1) compounds is dominated by the Hg-Hg bond. The changes in the 199HgCS parameters from the Hg(I1) to the corresponding Hg(1) compounds, as well as the changes in these parameters in the Hg(1) compounds with changes in X, can be interpreted as variations in the local paramagnetic contribution to the CS tensor. 207Pb - The isotropic chemical shift of 207Pb has been used to perform structural investigations of crystalline fluoride compounds and transition metal fluoride glasses.' Using 207Pb CP MAS with 19F decoupling, it has been shown that the 8iso(207Pb)varies on a large scale (1000 ppm) and that the main changes of its value are not due to the nearest neighbour fluorines but may be related to the number of next nearest neighbour Pb2+ ions. a-PbO and P-PbO have been studied with 207PbMAS NMR.'16 The '07Pb NMR CS tensor in a-PbO is axial, with 81 = 3030 and 611 = -270 ppm. In P-PbO, the 207PbNMR powder spectrum has been represented by a single non-axial tensor with 61 = 2820,822 = 2760 and 833 = - 1000. Using normal-coordinate analysis, the 207PbCS tensor has been represented as a sum of contributions from separate Pb-0 and Pb-Pb interactions. It has been shown that Pb-0 and Pb-Pb contributions are of the same order of magnitude. Pb304 has been studied with 207PbNMR.l17 The 207PbCS tensor of the Pb2+ site has principal values of 811 = 1980, 822 = 1540 and 833 = 1 108 ppm. The CS tensor of the Pb4+site is axial, with principal values 611 = 1009 ppm and 6* = 1132 ppm. The Pb4+-Pb2+spin-spin coupling constant is 2.3 kHz. 207Pb NMR powder pattern analysis has been applied to several leadcontaining inorganic and organic compounds.' l 8 The CS tensors obtained are less susceptible to systematic error than earlier studies employing MAS SSB analysis. Since the lead atom is very sensitive to its local environment, a

'

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correlation between the chemical shift and structure was investigated. An increase in Pb-0 interatomic distance tends to shift the isotropic chemical shift to lower frequency, whereas conversely an increase in Pb-halogen interatomic distance tends to increase the chemical shift. As a consequence of the electronic structure of Pb2+, almost all 207Pb shielding tensors of Pb2+ species have a negative anisotropy. Static and MAS 207PbNMR spectra of a series of lead oxides, including various electronic materials have been reported. * l9 The chemical shift parameters and the spin-lattice relaxation times have been determined. The symmetry of the local environment of the Pb(I1) site and the covalency of the Pb-0 bonds have been determined to be the best indicators of the 207Pb chemical shift parameters. 4.2.2 Quadrupolar Nuclei: Isotropic Sh$s, CS and EFG Tensors. 6r7Li- Two nitrogen ceramic phases, the oxynitride LiSiON and the nitride LiSi2N3, have been studied by 637LiMAS at 7 and 14 T.120 The EFG tensor of both phases has been determined by iterative fitting of the 677Lilineshapes at the two field strengths. Due to the fact that for 7Li the quadrupolar interaction is much larger than the chemical shift interaction, it has been shown that neither the small CSA nor the relative orientation of the two interaction tensors can be determined accurately by 7Li MAS NMR. For 6Li, the two interactions are comparable and the value of these parameters obtained from the fits of the 6Li experimental MAS lineshapes are therefore much more reliable. 637LiMAS NMR has been used to investigate the local coordination environment of Li in a series of xLi20-(1 - x)P205 glasses.12' Both the 6Li and 7Li show chemical shift variations with changes in the Liz0 concentration, but the observed 6Li NMR chemical shifts closely approximate the true isotropic chemical shift and can provide a measure of the lithium bonding environment. The 6Li NMR results indicate that, in this series of lithium phosphate glasses, the Li atoms have an average coordination between four and five. An increase in the 6Li NMR chemical shift with increasing Liz0 correlates with increased cross-linking of the phosphate tetrahedral network by 0-Li-0 bridges. 9Be - Despite the favourable NMR properties of 9Be (I = 3/2), NMR spectroscopy of this nucleus in the solid state remains comparatively unexplored.122An integrated experimental and theoretical study of the Be CS and EFG tensors in bis(2,4-pentanedionato-O,O')beryllium [Be(acac)z] has been presented. Interpretation of the 9Be NMR data was facilitated by XRD results, which indicate two crystallographically unique sites. 9Be NMR spectra acquired at 4.7 and 9.4 T for MAS and stationary samples have been fitted in order to extract the QCC (x), asymmetry parameter (q), and isotropic chemical shift (6iso). The best-fit quadrupole parameters for the two sites were determined to be x(1) = -294 kHz, q(1) = 0.11; x(2) = -300 kHz, q(2) = 0.15. Analyses of the stationary samples also reveal a definite anisotropy in the beryllium CS tensor and allow to place upper and lower limits on the spans of 7 and 3 ppm. This is the first evidence for anisotropic shielding in beryllium. Ab initio calculations of the 9Be CS tensors in B e ( a ~ a c indicate )~ spans ranging

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from 7 to 9 ppm; this represents a substantial fraction of the total known chemical shift range for 9Be (0.5 ppm), which is currently not possible. These findings also demonstrate the current limitations of the experimental techniques and show that no simple correlation appears to exist between the zeolite structure, such as the Si-0-Si bond angles or lengths, and the I7O NMR parameters.486 "F - Interactions between PFA molecules and MFI structure type zeolites have been studied by FTIR and 19F MAS NMR spectroscopies. In the purely siliceous ZSM5 sample, PFA molecules were physisorbed. In the acid samples, the interactions implicated the carbonyl group of the PFA molecules and the Bronsted acid sites of HZSM5. This interaction could be an H-bond or a protonation of the carbonyl group. The complementarity of the IR and I9F NMR spectroscopies has been 1 9 F , 29Si- Pentacoordinated silicon units, SiO4/2F-, have been found by SSNMR experiments in various as-made high-silica zeolites (Beta, SSZ-23, ITQ-3, ITQ-4, ZSM-12, Silicalite-1)that have been prepared in the presence of fluoride ions as mineralising agents.20 23Nu - SOQ broadening has rendered the study of cation sites in microporous materials almost impossible until the recent advent of the 2D MQ NMR experiment. Strong evidence has been given for preferential potassium cation siting.488The framework structure of calcined and dehydrated cancri-

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nite have been characterised by powder neutron diffraction and 23Na NMR spectroscopy. QCC obtained by a simple point-charge model agree well with the simulation of the 23NaMAS spectra.489 23Na,29Si- A variety of SSNMR techniques have been applied in order to assign resonances in the 29Si and 23Na NMR spectra of a new crystalline silicate Mu- 11 to crystallographically distinct silicon and sodium sites, respectively. A preliminary assignment based on the electronegativities has been carried out and 29Si CP experiments confirmed the assignments. The two crystallographically different sodium atoms have been resolved by 23Na TQ MAS NMR, and an assignment for these two sites has been proposed.490 27Al ID and 2D SSNMR techniques, in conjunction with elemental analyses, IR, and powder XRD, establish that high concentrations of A1 have been incorporated into the aluminosilicate MCM-4 1 frameworks, with retention of mesoscopic order.491The nature and concentration of the acid sites of MCM-41 material as a function of Si/Al ratio have been monitored by in situ IR using pyridine as a probe molecule. The thermal stability of aluminium in the framework of these materials has been studied using 27Al MAS NMR spectroscopy.492 Aluminium-containing mesoporous molecular sieves (AlMCM41) with different A1 contents have been prepared and characterised by XRD and MAS NMR studies. 27AlNMR studies reveal that the A1 is present in Td coordination in the samples.493FTIR and multinuclear NMR spectroscopy and catalytic test reaction have been applied to investigate acid sites in mesoporous MCM-4 1 materials.494The distribution of aluminum in MCM-22 zeolite has been studied by 27AI SSNMR!95 Aluminosilicate mesoporous molecular sieves exhibiting excellent structural ordering have been examined by 27AlMAS.496 27Al,29Si IR, NMR, and NH3-stepwise temperature-programmed desorption (STPD) have been used to study the acidity characteristics of dealuminated ZSM-12. 27Al NMR data which indicate mostly the presence of tetrahedral sites for ZSM-12. 29SiNMR data can be described by at least three distinctly different Si sites.497MnAPO- 11, MnAPO-41 and MnSAPO-41, have been prepared and characterised by the techniques of XRD, SEM, TGNDTA, 31Pand 27Al MAS NMR and ESR spectroscopy.498CXN zeolite, a natural zeolite in China, has been modified by ion exchange and calcination. The structural properties have been characterised by chemical analysis, powder XRD, TG/DTA, 27Al and 29Si MAS NMR and nitrogen adsorption.499 Combined 1D and 2D MQ 27AlNMR results together with 29Si NMR data provide interesting information on the siting of A1 in levyne zeolite. It is clearly shown that the distribution of A1 is random in the structure, as the relative intensity of the two tetrahedral species is equal to two, corresponding to the ratio of T I and T2, the crystallographically different tetrahedral sites.500 Combination of 27Al, 29Si MAS and TQ MAS NMR provide interesting information on the siting of aluminium in levyne zeolites. It is clear that aluminium is randomly distributed in the LEV structure during synthesis. Indeed, the relative intensity of the two tetrahedral species is equal to two, which corresponds to the ratio between the crystallographically different

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tetrahedral sites TI and T2?01The application of 27Aland 29SiMAS NMR has provided a direct probe of alkali metal cluster formation and distribution in the alpha-cages of zeolite A.502 27AI,31P- Three different molecular sieves have been characterised using 31P and 27Al MAS NMR spectroscopy and acidity measurement techniques. The synthesised solids were a silicoaluminophosphate sample, a chromium-substituted silicoaluminophosphate sample and a chromium-supported SAPO- 1I sample. Significant differences were observed between the CrAPSO-11 MAS NMR spectra and the spectra for the other two solids.503A layered aluminophosphate, Mu-4, has been investigated by various SSNMR techniques. In particular, 27Al-3'P 2D heteronuclear correlation experiments were useful to evidence the P-A1 connectivities present in this new layered aluminopho~ p h a t e . ~Highly '~ crystalline aluminophosphate molecular sieve AIPO4-41 has been synthesised and have been characterised by SSNMR spectroscopy using 27Al and 31P MAS, 2D 27Al SQ MAS, and 27Al+31PCP MAS techniques. Both 31P MAS and 27Al SQ-MAS spectra of the as-synthesised AlP04-41 exhibit several resonances, all of them being assigned to framework atoms."' 29Si - The sensitivity of ID and 2D MAS NMR experiments to the local order of framework T atoms and energy-minimisation calculations for the geometrical analysis of the complete zeolite framework structure have been combined to provide a most detailed view of their structural properties.50629Si NMR and IR spectra have been measured for as-prepared and annealed porous silicon samples to characterise their structure.507A SAPO- 1 1 molecular sieve was dealuminated to different A1 content. Dealuminated samples have been characterised by catalytic transformation of butene, o-xylene and cumene, IR analysis of adsorbed pyridine and 3 1 Pand 29Si-MAS-NMR. The reaction kinetics and mechanism have been discussed.508SAPO- 1 1 and SAPO31have been characterised 29Si MAS NMR studies.509 A high-quality SiMCM-41 was prepared. The 29Si MAS NMR results demonstrated that the water treatment promoted the wall polymerisation or local atomic arrangement.5'0 The aluminum coordination state of the molecular sieve SAPO-37 has been studied by 27Al MQ MAS NMR spectroscopy. Although the ID 27Al MAS NMR spectra of the aluminum sites show complex patterns, the 2D TQ MAS NMR spectra of SAPO-37 samples, submitted to different treatments, lead to the detection of four distinct framework A1 species in the as-synthesised sample. By use of 'H spin echo editing MAS NMR experiments, a new proton signal at 3.4 ppm has been observed in the calcined H-SAPO-37. By 'H{27Al) and 'H{3'P} spin echo double-resonance experiments, this new signal has been assigned to a kind of framework Al-OH hydroxyls caused by partially broken A1-0 bonds."' The iii coordination state of molecular sieve SAPO-37 has been studied by 27Al MQ MAS NMR spectroscopy, although the 1D 27Al MAS NMR spectrum of the aluminum sites show a complex pattern, the TQ MAS NMR spectra of SAPO-37, submitted to different treatments, lead to the detection of four different A1 species in the as-synthesised ~ample.''~27AlTQ MAS NMR spectroscopy has been applied to study the coordination state of the species giving the 30 ppm A1 NMR signal in the 27AlMAS NMR spectrum

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of activated mordenite materials. From the 27Al TQ MAS NMR measurements it is evident that the broad peak at 30 ppm in the 27AlNMR spectrum of the mordenite calcined at high temperatures comes mainly from the distorted four-coordinated Al? Alumina has been incorporated in the cavities of mordenite and has been assessed on the basis of the data of MAS 27AlNMR measurements and powder XRD. 5'4 Crystalline galliosilicates with the p structure have been synthesised 29Si MAS-NMR and. 71Ga NMR spectra confirm that Ga(1V) is the dominant species and that Ga(V1) formation depends, in part, on the thermal pretreatment applied to the crystals.515 f H , f 3 C , f 5 N- Multinuclear SSNMR has been applied to investigate the use of chloroform as a probe molecule to characterise the basic properties of zeolites. The characteristics of the 'H signals of adsorbed chloroform are consistent with highly mobile molecules leading to an average chemical shift that reflects the interaction of the molecule with the host zeolite through Hbonding with basic framework oxygen atoms. The use of 13CHC13has allowed to establish that the 13Cchemical shift position is also a measure of framework basicity for zeolites X and Y.5'6 H,'7AZ,29Si - The thermal behaviors of five differently prepared samples (A-E) of the very-large-pore aluminophosphate molecular sieve VPI-5 have been investigated. 27Al, 31P and 'H MAS NMR experiments for the two samples yielded almost identical spectra. The 27Alspectra confirmed that all A1 were tetrahedral, the 'H spectra showed that no water was present, and the 3'P spectra contained seven peaks, indicating that the symmetry was much lower The Si/Al ratio of the framethan the expected topochemical space work and the number of defect sites in the framework of zeolites have been investigated by means of SSNMR, 'H MAS NMR showed that the extent of dealumination increased with the number of Bronsted acid sites interacting with the zeolite framework and 29SiMAS NMR showed that at the same time the number of defect sites in the samples i n ~ r e a s e d . ~Variations '~ in the structure and acidity properties of HZSM-5 zeolites with reduction in crystal sizes down to nanoscale (less than 100 nm) have been investigated by XRD, TEM and SSNMR with a system capable of in situ sample pretreatment. As evidenced by a combination of 27AlMAS, 29SiMAS, CP MAS and 'H MAS techniques.519The variations in structure and acidity on the internal and external surfaces of HY zeolites modified by MgO and S O 2 have been studied by MAS NMR, together with the selective adsorption of perfluorotributylamine. 27Aland 29SiMAS NMR spectra revealed that the modifications led to significant changes of the framework owing to the migration of some nonframework Si and A1 into the framework in the course of the modifications. 'H MAS NMR spectra showed that the modifications decreased the acidity of the zeolite surfaces.520 7Li,27AZ,f33Cs- A study on the modification of the surface acidity of pzeolite exchanged with Cs and Li has been carried out by means of XRD, 27Al, 133Csand 6Li MAS. The 133Csand 6Li MAS technique suggested that Cs cations were located in accessible sites, while Li cations were inaccessible and possibly located in cavities within the channels netw01-k.~~'

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27Al,2ySi,3'P - The hydrothermal transformation of silico-aluminophosphate gel with cyclohexylamine to SAPO-34 has been examined. The hydrothermal crystallisation products of the SAPO have been investigated by XRD, FTIR, nitrogen and water adsorption, thermogravimetric analysis, surface analysis and 27Al, 31Pand 29Si MAS NMR. The 27Al MAS NMR signal of tetrahedrally coordinated A1 observed in the silico-aluminophosphate gel without the organic template was changed to octa-, penta- and tetrahedrally coordinated aluminium upon the addition of the cyclohexylamine template to the SAPO gel. After 3 h of hydrothermal treatment at 473 K however, the 27Al MAS NMR signals of the octahedral and pentacoordinated aluminium were removed. This has also been confirmed by 31Pand 29SiMAS NMR.522 2uSi,27AI,i'3Cd,207Pb- NMR studies of Pb- and Cd-exchanged LTA zeolites have been reported. 523 '3C, 23Nu,27A1,29Si- Multinuclear MAS NMR techniques were used for investigation of surface reaction of Y-type zeolites with CFCs and HCFC.524 27AZ,2ySi,23Nu, '33Cs- The NaY zeolites exchanged with other alkali cations have been characterised by HR SSNMR. The chemical shifts of the various Si(nA1) n = 0, 1, 2, 3 and 4 configurations show particular changes as a function of the alkali c o u n t e r - ~ a t i o n s . ~ ~ ~ 27A1,2ySi, 7' Ga - A wide range of sodalite framework materials, M&r"'04]6& where T = Al, Ga, Si, T' = Be, Al, Si, Ge, have been characterised using MAS NMR spectroscopy. Structural parameters, such as functions of the framework T-0-T' angle, correlate linearly with the determined chemical shift values and provide relationships, as a function of T', which will facilitate characterisation of more complex zeolitic compounds containing such species.525Gallioaluminosilicate hydrogels were prepared at different temperatures to yield crystals with the faujasite or gmelinite structure. This finding has been proved by 27Al,71Gaand 29SiMAS NMR spectroscopic results. The 29SiNMR spectra of samples exchanged are poorly resolved and their quantitative analysis indicates substantial loss of A1 and Ga from the faujasite framework. Consistent with this interpretation, 27AI MAS NMR spectra reveal large amounts of octahedrally coordinated species. Moreover, the 71Gaspectra show no evidence of tetrahedrally coordinated framework Ga.526 29 Sz, "Gu - The hydrothermal transformation of galliosilicate hydrogels were prepared from colloidal silica and sodium gallate mixtures, yields crystals with the faujasite or natrolite structure. Raising the crystallisation temperature causes the formation of analcime crystals. 71Ga MAS NMR spectra have shown that in both GaY and H(Ga,La)Y, no extraframework Ga(V1) species are present. 527 5.10 Surface Science and Catalysis. - A VT isolated flow and large-volume

MAS NMR probes for heterogeneous catalysis studies has been developed.528 ' H - The chemisorbed state of hydrogen on partially reduced Ag+- exchanged Y zeolite has been studied by means of 'H MAS NMR spectroscopy, When AgY was reduced with hydrogen, a peak appeared at - 0.1 ppm besides

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peaks at 4.6 ppm and 4.0 ppm. The latter two peaks have been attributed to bridging hydroxyl groups in sodalite cages and supercages, respectively. The peak at -0.1 ppm has been ascribed to atomic hydrogen species adsorbed on cationic silver clusters.529The isomerisation of 2-methyl-2-pentene has been studied on boron phosphate catalysts with different P/B ratios. 'H MAS NMR has been employed to provide supplementary evidence of the variation in acidity with composition.530 CdS nanoparticles precipitated from aqueous solution have been studied by 'H NMR. The nanoparticles were deliberately not capped by any surface termination agent. The samples had a porous structure. 'H, "3Cd MAS NMR spectra revealed that there are three abundant proton species in nanoparticle samples prepared with an excess of Cd, having different chemical shifts: a relatively narrow peak due to OH groups and two broader lines resulting from adsorbed water molecules with different chemical environments. The exchange between different lines has been studied by 2D exchange MAS spectroscopy. The analysis of the 'H-'H elation spectra as a function of the mixing time led to the conclusion that the nanocrystalline surface is covered by clusters of water.531Substitution of the protons in 12tungstophosphoric acid by monovalent cations of the Groups 1B and 3B, in particular Ag' and Tlf , respectively, produces high surface area microporous solids whose morphological properties and numbers of residual protons are dependent upon the relative amounts of the preparative reactants. 'H MAS NMR and the isomerisation of 1-butene have been employed to demonstrate that both the numbers of protons and the distribution of the acidic strengths present.532 Co-Nb(2)Q(S)-Si02 catalysts have been examined by 'H MAS NMR.533 'p2H - The regioselective 'H/2H exchange taking place between isobutane and acidic OH groups of strong solid acids has been investigated both by in situ 'H MAS NMR with perdeuteroisobutane and H-ZSM5.534 1H,5'V - A series of vanadia catalysts supported on sol-gel derived mixed oxide A1203-Ti02 have been synthesised with a V205 varying content. The catalysts have been characterised by employing 51V,27Al,'H MAS NMR. In the calcined catalysts 51V NMR studies indicated the peaks corresponding isolated and distorted tetrahedral vanadia species at low V2O5 contents and octahedral vanadia species at high vanadia loadings. The 'H MAS NMR studies showed the presence of terminal and bridged hydroxyl groups of alumina and titania.535A series of supported vanadia catalysts with different V205contents were prepared using the ZrO2-SiO2 support. 5'V NMR studies indicated the presence of tetrahedally coordinated vanadate species at lower vanadia contents and octahedral vanadyl species at higher V2O5 loadings. 'H MAS NMR spectra showed peaks corresponding to hydroxyl groups of zirconia and silica.536Alumina-silica mixed oxide has been used as a support for dispersing and stabilising the active vanadia phase. The catalysts have been characterised employing 5'V and 'H MAS NMR. 51VSSNMR studies on the calcined catalysts showed the peaks corresponding to both tetrahedral and distorted octahedral vanadia species at low vanadia loadings and with an increase in V205 content, the 51Vchemical shifts corresponding to amorphous

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V2O5 like phases have been observed.537A series of catalysts with varying vanadia contents have been characterised employing 51V,'H MAS NMR. 51V NMR studies indicated the presence of tetrahedral vanadate species at all the loadings. The 'H MAS NMR studies showed the peaks corresponding to hydroxyl groups of alumina and ~ i r c o n i a . ~ ~ ' 2H - The effect of carbon modifications and oxygen surface groups on the hydrophobicity of activated carbons has been described. 2H SSNMR experiments showed that the presence of toluene decreases the mobility of water, either due to a stronger interaction with the surface sites or to an enhanced adsorption in the smallest pores.539 " B - The acidity and surface structure of pure y-alumina and a berated alumina (AB), containing a low amount of boria have been investigated by a variety of techniques. The coordination states of AB, and the distribution of hydroxyls on the surface, have been studied by "€3 MAS NMR. Partially hydrated AB has trigonal boron on the surface, as found in B2O3, and a small part of the boron nuclei exhibit "B resonances narrowed by exchange with water. In situ 13CNMR has been also used to study the acidlbase strength.540 ' I B, 27AZ- Boria-alumina mixed oxides have been studied by MAS NMR.54' 13C - The temperature dependent effective catalytic pore size can be determined by the comparison of I3C MAS spectra of heterogeneously catalysed, shape selective reactions and data collected from GC analysis of the product stream of reactions under similar reaction conditions.543 Powder neutron diffraction and 13C MAS methods have been used in the structural parameters determinations of the NaX and NaY zeolites with chemisorbed methyl groups CD3 or CH3.543The mechanism of propane conversion into butanes over H-MFI as catalyst has been investigated using controlled atmosphere I3C MAS NMR spectroscopy. The labelled reactant was propane 2-13C. The nature of the primary labelled products has been found to depend on the propane partial pressure.5+' I3C MAS NMR have been performed in situ to investigate the mechanism of n-hexane conversion over Pt/alumina, Pt/ceria and Ptlceria-alumina composite oxides, prepared by laser assisted Pt deposition. n-Hexane conversion has been studied at 653 K. n-Hexane 1-13C was used as the labelled reactant.545 13C ~) NMR spectra and relaxation parameters (7''('H), TI('3C), T1(, * H), 7 ' ~have been measured via CP MAS and DP MAS techniques as a function of acetone loading level in acetone/Si02 samples at 25 "C. Tl('H)and T1,('H) values have also been measured via 29Si CP MAS experiments. Peak positions and relaxation parameters are averaged by rapid exchange between different acetone interaction sites on the surface.546 f3C, 'F - The adsorption of benzenoid aromatic compounds on porous silica has been studied by 13C and 19F NMR. Small chemical shifts between the resonances of methyl groups or fluorine substituents in the microcrystalline bulk material and the adsorbed species can be used to monitor the degree of adsorption.547 f3C,2ySi- The molecular metallasiloxanes Mo((NBu)-Bu-t)2(OSiMe& and Ti(OSiMe3)4 have been studied by 13C and 29Si CP MAS NMR.548 Meso-

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porous titania-silica mixed oxides with covalently bound 3-chloropropyl, 3acetoxypropyl, and N,N-dimethyl-3-aminopropylgroups have been characterised by 29Si and 13C NMR. 29Si MAS NMR measurements indicated covalent incorporation of the modifying Titania-silica mixed oxides with covalently bound methyl or phenyl groups have been studied by 29Si and 13CMASNMR.549 '3C,31P- 31Pand 13C MAS NMR spectra have been used to characterise pure bisphosphonates and also bisphosphonates adsorbed on hydroxyapatite. The molar ratio of phosphonates adsorbed on hydroxyapatite determined by 31Pspectra without CP is approximately two times larger for geminal bisphosphonates than for o1,w-bisphosphonates and phosphonoacetic acid. 13C CP MAS spectra of pure and adsorbed bisphosphonates recorded in two cases for identification of adsorbed compounds give additional information about the state of adsorbed compounds.550 15N - NaX and CaX zeolite samples were dehydrated under controlled conditions and 15N2was adsorbed on the zeolites. The water content has been determined by quantitative 'H-NMR. The gas and adsorbed nitrogen phases were fully characterised by 15NNMR. The partition coefficients and relaxation times Tl and T2 of the N2 phases were then studied as a function of temperature. "N-NMR measurements were performed under static and MAS conditions. Two different types of adsorbed nitrogen molecules were also found on the zeolites.55' I9F - KF/alumina catalysts containing different amounts of KF have been characterised by 19F MAS NMR to elucidate the catalytically active sites, whose appearance varies much with the pretreatment temperature. Although the main species containing F was K3AIF6 formed by the reaction of K F with alumina, which has been observed by both XRD and I9F MAS NMR, it was not related to the formation of active sites.552Some reactions that control the dissolution of bayerite [P-Al(OH)3] and boehmite [y-AlOOH] have been identified by comparing the adsorption chemistry, the dissolution rates, and 19F spectra of the reacting surfaces. The 19Fspectra of bayerite distinguish two sites for fluoride reaction that vary in relative concentration with the total adsorbate density. One resonance at - 131 ppm has been assigned to fluoride bridges and the other resonance at - 142 ppm has been assigned to fluoride at terminal sites.553 23Na,5' V - Sodium-doped binary vanadia-titania catalysts have been studied using modern HR SSNMR techniques, including fast MAS and 2D TQ MAS NMR. NMR and powder XRD data brought together allow identification of sodium- and vanadium-containing species formed in the course of the preparation of the catalysts.554 27At - 27Al MQ MAS has been used to characterise the oxide Mo-P-A1 hydrotreating catalysts.55527AlNMR MAS has been used to study the surface aluminium compounds forming during the treatment of supported Tic14 MgC12 catalyst with organoaluminium compounds of various composition.556 By means of 27Al TQ MAS NMR and 27Al{19F)WISE MAS NMR, three different Al-F sites on the surface of fluorinated y-alumina have been detected.

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Three 19F resonances at 9, 20, and 33 ppm (from (36) correlated to 27A1 resonances in the octahedral range. While the positions of the maxima in the 27Al dimension were ill-defined due to the inherently low efficiency of the 27Al{'9F) CP MAS process, the centre of gravity of the lines shifted significantly upfield in that dimension with increasing wt.% F. At F contents above levels corresponding to the full fluorination of the y-A1203surface, neoformation of an AlF3. 3H20 phase was also evidenced 19F resonance at - 8 ppm and with an 27Alresonance at - 17 ppm.557 27A1,29Si- An increasing demand for 2,6-dialkylnaphthalene has spurred interest in shape-selective naphthalene alkylation. This process has been examined by other techniques including 27Aland 29SiMAS NMR.558 B,27Al,3fP - The synthesis of isoprene from the dehydration of 2-methylbutanal has been described using boron phosphate, aluminium phosphate and mixed boron/aluminium phosphates as catalysts. It has been investigated by 31P,27Al, and I'B MAS NMR spectroscopy shows that B and A1 are in the same lattice in these mixed phosphate catalysts.559 29Si- A series of highly crystalline silicoaluminophosphates having the AFI topology (SAPO-5) have been synthesised with varying silicon. These have been studied by the 29Si MAS NMR technique.560X-Ray amorphous ZnO nanoparticles homogeneously dispersed in a silica matrix were evidenced in ZnO-Si02 nanocomposites obtained by a sol-gel method. Through a comparison of the 29Si MAS NMR data of the nanocomposites and silica samples, obtained by the same method, it was possible to observe that reaction occurs between ZnO and silica on heating, which causes a depolymerisation of thehost matrix with the formation of low condensation 3 f P - Zirconium-pillared layered phosphates were prepared from a-zirconium phosphate and zirconium phosphate phenylphosphonate by colloid manipulation method. IR and 31PMAS spectroscopic studies show that P-OZr linkages are formed after calcination as a result of this strong interaction.562 The room temperature decompositions of the nerve agent simulant diisopropyl fluorophosphate sorbed on y-alumina, polydivinylbenzene, and Ambergard, have been studied in situ using 31P MAS NMR.563 Ruthenium complexes having bidentate phosphine ligands were incorporated into an amorphous silica matrix via chemical anchoring using a silylether bridge. The integrity of the complexes after immobilisation has been confirmed by CP MAS 31P NMR.564 Different compositions of Calo.5-xC~,(P04)7 (0 5 x 5 1) and Calo-,/2Na,Cuo.s (PO& (0 5 x 5 1) belonging to the Whitlockite-type structure have been characterised. 31P MAS NMR investigations of Ca10.5(P04)7 and C a 1 o N a ( P 0 ~showed )~ that the occupancy level of the Ca sites does not modify notably the symmetry of the The heterogenisation of the zwitterionic Rh(1) catalysts (sulfos)Rh(cod) and (sulfos)Rh(C0)2 [sulfos = -03S(C6H4)CH2C(CH2PPh2)3; cod = cycloocta- 1,5diene] has been performed by controlled adsorption on partially dehydroxylated high surface area silica. Experimental evidence of the -S03.-.HOSiinteraction with silica has been obtained from CP MAS 31PNMR The hydroformylation of styrene over Rh/Si02 has been studied by FTIR and

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31PCP MAS NMR.567A new family of vanadium phosphorus oxides (VPO) catalysts have been identified by XRD and 31PNMR. 31PNMR by spin echo mapping and 3'P MAS NMR have confirmed an interaction of the VPO precursor with Nb and of the NbPO amorphous material with V.56831PMAS NMR characterisation of zirconium phosphate/phosphonate (ZP) multilayer assemblies grown on SiO,. The reaction of silica with excess POC13 and treatment with collidine results in both physisorbed and chemisorbed H,P04+3) being present at the SiO,, surface was reported. The 31P NMR spectrum of zirconium phosphate grown from silica shows no residual Clcontaining species, indicating essentially complete hydrolysis.569The change in chemical shift of a chemisorbed probe molecule is an informative tool for the characterisation of solid acids. This work demonstrates that the 3 1P isotropic chemical shift of the probe triethylphosphine oxide (TEPO) can be used to identify the acidity of multiple acid sites on a surface.570The bonding of triethylphosphine oxide to the acid site of a surface has been shown to produce a systematic change in the 31Pisotropic chemical shift that is proportional to the acid strength of the adsorption site.571 .5I V - Vanadium-cerium oxide samples with different V/Ce atomic ratios have been characterised by XRD and "V MAS NMR. 5'V MAS NMR study has shown the presence of different V sites in solids.572Wideline "V SSNMR spectra of supported V205-W03/Ti02 catalysts were obtained under ambient conditions.573Milling of V2O5 in a ball mill increased the surface area. After milling in a ball mill it has been characterised by the combination of wideline and MAS 51VNMR techniques together with theoretical simulations of NMR spectra.574The fresh catalyst V205-W03/Ti02 and catalyst have been studied by the 51VSSNMR spectroscopy in static and MAS conditions. According to 51VNMR in both samples the majority of vanadium sites are in a distorted octahedral environment similar to that in V205.572 fH,27AZ,29Si- The interaction between Mo species and a conventionally microsised and particularly nanosised HZSM-5 support has been studied by HR SSNMR techniques. As proved by 27Aland 29SiMAS as well as CP MAS NMR investigations, this interaction was so strong that the framework aluminum of both microsised and nanosised HZSM-5 zeolites could be extracted. With increasing Mo loading, more nonframework aluminum, resonanced at 30 ppm, appeared in the 27AlMAS NMR spectrum of the Moloaded nanosized HZSM-5 catalyst.575 1H,2H,3'P- Spectroscopic techniques in controlled atmosphere, such as 'H, 2 H and 31PMAS and inelastic neutron scattering (INS), have been used to investigate the effect of dehydration on structural modifications and acidic properties of solid 12-tungstophosphoric acid H3PW 1 2 0 4 0 and its cesium salt C S ~ . ~ H ~ . ~ P WMAS ~ ~ Ospectra ~ O . have been recorded as a function of the degree of dehydration/rehydration and allowed to characterise the protonic species present .576 5.11 Inorganic and Other Related Materials. - ' H - 'H MAS NMR at high magnetic fields and high spinning speeds provides a powerful means of

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identifying the different proton sites in smectites and affects information on the octahedral nature.577 'H MAS NMR experiments have been used to characterise the non-acid protons of the anions in polycrystalline paratungstates by means of the measured isotropic chemical shift values. The investigation of various hydrates of ammonium paratungstate allows a direct proof of protons in NH4 ions and in water molecules while protons of the anions are not detectable. However, for both the potassium and the sodium paratungstates 'H MAS NMR investigations detected the protons of water molecules and the non-acid protons of the paratungstate anions. Additional 'H broadline NMR experiments at 173 K support the interpretation of the results.578 6'7Li- LiMn02 with orthorhombic structure was synthesised, a spectrum of solid-echo 'H-NMR on y-MnOOH became diminished, while a spectrum of solid-echo 7Li-NMR emerged with three different kinds signals which is accompanied by SSB at around 37 ppm, 74 ppm and 1 ppm, respectively.5796Li and 7Li MAS NMR and in situ XRD have been used to study lithium manganate cathode materials (Li,Mn204, 0 < x 5 1) during and following charging and discharging. Only one major local environment has been observed by 6Li MAS NMR from lithium in the tetrahedral sites of the spinel structure, the resonance shifting by no more than 8 ppm in this range (from its original position).580Samples of Li,CoO2 (0.5 x 1) have been prepared by electrochemical deintercalation from high temperature LiCoO2 and are characterised by XRD, electrical measurements and 7Li MAS NMR spectroscopy. 7Li MAS NMR suggests that the metal-non-metal transition is the driving force for the existence of the biphasic domain.581 7Li MAS NMR studies have been performed for both the spinel compounds before and after Li+ i n t e r ~ a l a t i o n . ~ ~ ~ 13C - Jahn-Teller distortion in the c60- ion has been studied by 13C NMR spectroscopy in the solid samples KC60(THF) and KCm(THF), (8 < x < 1). Solvating the K+ ion by THF can change electronic states of c60- ion.583The microstructure of carbonaceous matter prepared from copper acetylide, by a coupling reaction promoted by air, hydrogen peroxide or Cu(NH3)?+ oxidation has been studied by SSNMR, FT-IR and Raman spectroscopy. SSNMR provides additional evidence on the presence of small cumulenic moiety 'allenic or cumulenic carbyne (polyethylenediylidene). SSNMR in conjunction with Raman spectroscopy show sp3-hybridisedcarbon moieties known also as 'diamond-like carbon', formed by a crosslinking reaction of acetylenic carbyne chains; additionally sp2-hybridised carbon atoms have been detected and assigned to amorphous and graphitic carbon.584 I7O - Three- and five-quantum 1 7 0 MAS NMR experiments are used to resolve fully the three crystallographically distinct oxygen species in forsterite (Mg2Si04). The chemical shift and quadrupolar parameters extracted from these spectra are compared with the literature values obtained using conventional 170MAS and DAS NMR.585Powder XRD and electron single-crystal diffraction of crystals of Li2TiOSi04 and Li2TiOGe04 showed them to be tetragonal, space group. I7O NMR spectra of the two compounds, isotopically enriched with 170,showed peaks due to the apical titanyl, Ti-0, and basal, bridging, Ti-0-Si or Ti-0-Ge, oxygen atoms of the title compounds.586

7: Solid State N M R Spectroscopy

28 I

23Na - The structure of NASICON-type compounds, N ~ ~ + , S C , T ~ ~ - ~ ~ ( P O ~ ) (0 5 x 5 2), and the dynamics of Na+ have been investigated by 23Na NMR spectroscopy. It was found that the 23Na 1D and 2D MQ MAS spectra depend on the Na concentration, suggesting strongly that the Na+ ions are distributed between two crystallographically nonequivalent sites, one is a special position with axial symmetry, and the other a position of low symmetry.587 27AZ- The kaolinite-mullite reaction process has been studied by SQ and TQ 27Al MAS NMR. All materials have also been characterised by 29Si MAS NMR and powder XRD. It has been shown that in order to record reliable 27AlMAS NMR spectra it is important to use very fast sample spinning rates. The conflicting values previously reported in the literature for the four-, fiveand six-coordinated A1 metakaolinite populations are due to the relatively modest and different MAS rates and magnetic fields used. TQ 27Al MAS NMR provides hard evidence for the presence in metakolinite of distorted (and distributed) four-, five-, and six-coordinated A1 local environments.588 The structure of Bi2A1409 has been refined using a combination of X-ray and HR neutron powder diffraction. Iterative simulation of the 27AlSSNMR data confirms the presence of two A1 sites, one octahedral and one tetrahedral, in an approximate 1:1 ratio.589 Investigations on the hydrothermal formation and the crystal structure of nitrate cancrinite have been carried out in the system Na~O-xSiO~-Al~O~-NaNO~-H~O, 1 < x < 6. Structural investigations of nitrate cancrinite have been performed by IR and MAS NMR spectroscopy of the nuclei 29Si,27Aland 23Na.The results confirmed the alternating Si, A1 ordering of the alumosilicate framework for a Si/Al ratio of 1.O for all samples, independent of the WA1 ratio of the educts as well as the temperature and pressure of crystallisation. A distribution of the quadrupolar interaction of the sodium cations caused by the enclathrated water molecules and motional effects can be suggested from the 23Na MAS NMR spectrum.590A series of bentonite illite/smectite with a range of Fe content and illite composition has been analysed by means of FTIR, 27Al MAS NMR and theoretical calculations.The calculated distributions have then been analysed in comparison with the experimental 27AlNMR results.591 29Si - The microstructure of silicon carbide has been characterised by XRD and 29SiSSNMR.592 "P - A 31P SSNMR study of TiP207 has been reported. The combined constraints from 1D and 2D 31P MAS NMR experiments probing throughbond P-0-P connectivities via homonuclear J coupling interactions, positively identify the cubic space group. 2D SQ-DQ correlation 31P MAS NMR experiments subsequently lead to complete assignment of the distinct crystallographic P sites in the structure of Tip207 to the corresponding eight resolved resonances in 31PMAS NMR spectra of TiP207,while 31PZQT and C7 DQT MAS NMR experiments confirm that Tip207 and the cubic phase of SiP207 are i s o s t r ~ c t u r a l The . ~ ~ ~compound P215+Al14 - and the novel compounds P2J5+Ga14- and P215+In14-have been prepared in two different ways either from PI:, and E13 or from P214and 12/E13(E = Al, Ga or In). The products have been characterised by 31PMAS SSNMR, Raman and IR spectroscopy.

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The 31P MAS SSNMR spectra are compared with NMR studies of related PI4+salts and alkylphosphorus tetra iodide^.^^^ '13Cd - Semiconductor nanocrystals doped with transition metals has been studied 13Cd MAS spectra; the observed paramagnetic shift and decreased longitudinal relaxation time are consistent with Mn incorporated in the quantum dots.595 13C,29Si,31P - The SSNMR characteristics of' unsupported and silicasupported H-4[SiMo12040] and H-3[PW 120401 heteropoly acids after (i) calcination or (ii) reaction with 2,3-butanediol are described. The supported and unsupported acids have been studied by 29Sior 31PMAS NMR spectroscopy and the supported catalysts, which became steady-state in a 2,3-butanediol flow, have been probed by 13C CP MAS NMR measurements as well. The position of Si signal corresponding to the heteropoly acid in H4[SiMo12040]1 Si02 could be distinguished from the Si signals of the support. The 31PNMR spectrum of H3[PW12040]revealed that the material contains two species: one is somewhat dehydroxylated, but fully protonated, the other has been further dehydrated and probably proton deficient too.596 23Na,31P - 23Na and 31P MAS and powder XRD have been used in combination to study the structure of the model phosphate phases NaMg(P03)3 and NaZn(P03)3. The 23Na and 31PSSNMR data confirm the similarities between the two structures, while the 23Na also suggested three sodium sites in a 2: 1:1 ratio, consistent with the proposed crystallographic

'

27Al, 29Si- MAS NMR studies on building stones from historical monuments have been reported. 7Li,23Na- Two new non-metallic filled beta-manganese phases M2Ga~Telo (M: Li, Na) have been characterised by MAS NMR, including MQ NMR. 6

Molecular Dynamics Studies

6.1 Organic Solids: - Hydrogenated oligocyclopentadiene has been investigated by means of 'H and 13C NMR in the solid state.598The dynamic behavior has been investigated through the measurements of Tip( 'H) and T,('H), as well as Tl(l3C), highlighting the presence of two major motional processes. 2H NMR techniques have been employed to study phenyl ring dynamics of enkephalin molecules and behaviour of bound solvents in the crystalline the methyl group dynamics in aspirin and in the inclusion complex aspirin/P-cyclodextrin,600 symmetry, disorder, and dynamics in solid crown ether complexes6" and the molecular motional modes in sucrose octapalmitate.602 Dynamics of the benzene and pyridine p-tert-butylcalix[4]arene inclusions has been studied using wideline 2H NMR lineshapes and "N NMR CS tensor components.603The absence of short contacts rules out a C-H-e-Nhydrogen bonding interaction of the host to the guest.

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Dynamics of aromatic ring flipping in solid 4,4'-diphenoxydiphenylether has been studied by 13Ctime-reverse ODESSA NMR?" Conformational dynamics in the solid state has been studied by 13C techniques for S-alkyl-1,4-dithianiurn salts (a persistent solid degradation product of rnu~tard),~''triisopropyl(aryl)silanes,606 triisopr~pylsilanes,~~~ tris(trimethyl~ilyl)methanes,~~ homochiral dicarvone6w and 9-tert-butylanthracene6" Tautomerism and H-bonding of benzylideneanilines in the solid state has been studied by 13C CP MAS.247Molecular dynamics of Cm in cocrystals of c6()and p-bromocalix[4]arene propyl ether6" and of ferrocene in deoxycholic acidferrocene inclusion compound have been studied by I3C NMR.612 "N CP MAS has been employed to study the kinetics of degenerate intermolecular triple proton and deuteron transfers in the cyclic trimers of "N-labeled polycrystalline 4-nitropyrazole and 4-bromopyra~ole~'~ and the proton transfer thermodynamics and dynamics and the proton locations in polycrystalline 'N-labeled porph ycene .614 The rate of ring inversion of fluorocyclohexane in its thiourea inclusion compound has been studied using 19F NMR, 2D EXSY and selective polarisation inversion, triple-channel 3C-('H,'9F) spectra. The superiority of the 19F measurements over the use of 13Cspectra has been emphasised?

'

6.2 Organometallics and Coordination Compounds. - H-migration and carbony1 mobility in (NMe4)4-x[H,Ni12(CO)21]- S (x = 1, S = Me2CO; x = 2, S = 2THF) have been studied VT 'H and 13CSSNMR.616 Wideline 2H NMR has been used to probe the motion of the q2-dideuterium ligand in the solid state.617 Representative examples of large-amplitude dynamic phenomena in various classes of solid metallorganic compounds and their study by VT 13C CP MAS have been presentede618Dynamics of monohaptocyclopentadienyl rings of hafnium and titanium tetracyclopentadienyl in the solid state has been I D and 2D 13Cand 29SiMAS NMR have investigated by 2D 13CCP been employed to demostrate that crystalline (Ph3Si)Si(SiMe3)3 lacks all molecular symmetry and that internal 2 d 3 reorientation of all three crystallographically inequivalent SiMe3groups in the molecule occurs.62o Modes of molecular reorientation in solid C(SnMe3)4 have been investigated by 1D and 2D 13C and 'I9Sn static and MAS NMR.621The pseudo-five-fold disorder previously observed by single-crystal XRD has been shown to be dynamic. A dynamic-disorder model where each tin atom in a C(SnMe3)4 molecule occupies the twenty sites of a nearly perfect pentagonal dodecahedron with equal probability has been suggested. Intermolecular H-bonds of the type N-H-q-Nin crystals of imidazole and its 4-substituted and 4,5-disubstituted derivatives have been studied by 15N CP MAS NMR and an ab initio calculation.622 6.3 Biological and Biomedical Applications. - 'H Tl/Tl, and 13C CP MAS studies have been used to study molecular dynamics of poly(1actide-coglycolide) controlled pharmaceutical release polymers.623Results suggest that

284

Nuclear Magnetic Resonance

around the broad glass transition at about 50°C slow polymer backbone motions (on the 10 to 100 ps timescale) become significant. Lipid dynamics in the annexin V - membrane complex has been studied by 2H NMR.624Details of backbone motions in a crystalline protein have been derived from field-dependent 2H NMR relaxation and lineshape analysis.625 2H NMR has also been applied to study dynamic properties of phospholipid model membranes626and mobility of the tyrosine side chain in Bombyx mori and Samia Cynthia ricini silk.62713CNMR studies of dynamic properties of 14 residue antifreeze glycopeptide have been reported.628 Effects of hydration on molecular mobility in dormant Bacillus subtilis spore samples have been investigated by 31Pand 13C SSNMR.629Superslow backbone dynamics of the protein barstar and the polypeptide polyglycine has been studied by time-reverse ODESSA technique that can detect reorientation of nuclei carrying anisotropic chemical shift tensors.630Experiments have been performed on carbonyl 13C in polyglycine and backbone "N nuclei. Two exchange processes have been observed in the experiments: molecular reorientation and spin diffusion. It has been shown that the application of MAS exchange spectroscopy provides new opportunities in studying slow biomolecular dynamics that is important for the biological function of proteins. 6.4 Polymers. - Temperature dependent 2H quadrupole echo lineshapes have been reported for polyamidoamine dendrimer salts.63 The spectra are characteristic of amorphous materials undergojng broad glass transitions. The estimated average H-bond lengths are 2.2 A. Three-fold rotation and asymmetric cone libration model has been used to explain the observed temperature dependent asymmetry parameters of terminal ND3+groups. I3C2D exchange NMR has been applied to investigate dynamic alternation among three forms of H-bonds in the poly(acry1ic acid)/poly(ethylene The 13C CP MAS signal for carboxyl carbon in the complex is split into three peaks corresponding to three forms of H-bonds: the complex form, the dimeric form and the free form. These three peaks coalesce into a single peak at temperatures above 346 K, showing that dynamic alternation of Hbonds occurs in the complex. 2D exchange NMR spectra directly show that the dissociation-association of H-bonds occurs at temperatures, higher than Tg.The exchange rates are on the order of Hz and have a wide distribution. It was found that the H-bonding dynamics is coupled to the segmental motion of poly(acry1ic acid) in the complex. I3C NMR has also been employed to study the local motions in arylaliphatic copolyamides by following the temperature dependence of 3C CSAs and * 3C-1H dipolar couplings.633

6.5 Microporous Solids and Related Materials. - 'H MAS tecniques have been used to study the dynamic properties of acidic protons in metallosilicate molecular sieves.634Results of the first pulsed field gradient NMR studies of H-.diffusionin zeolites have been presented and compared with corresponding measurements by quasielastic neutron scattering.635The results of the two

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285

techniques were in satisfactory agreement. The H-diffusivity is generally found to decrease with decreasing free apertures of the zeolite pore structure. HR 'H MAS NMR investigations (including 'H NOESY NMR and 'H-I3C cross-relaxation studies) of the mobility of 1-butene and 1-pentene molecules adsorbed in zeolite NaX have been presented.636The mobility of the adsorbed species can be characterised by a fast libration motion of the molecules and an overall reorientation-translation motion with a correlation time which is comparable to the lifetime of the molecules in a cavity with respect to their jump into a neighbouring cavity. 2H NMR spectroscopy have been used to examine the structure and rotational dynamics of CD3 groups pointing into the unidimensional channels of the 0-polymorph of the hybrid organichorganic microporous solid aluminium methylphosphonate, A12(CD3P03)3.637 1D and 2D exchange 13C NMR techniques have been used to show that npentane translation in zeolite ZK-5 occurs by hopping between neighboring a and y cages.638Owing to the structure of the ZK-5 pore network, these techniques give direct information about the translational part of molecular intercage motion, which represents the elementary step of sorbate selfdiffusion. 6.6 Other Materials. - The dynamic behaviour of n-octylammonium ions intercalated into tetrasilicicfluormica has been investigated by measuring H and 2H NMR lineshape and Tl('H) measurements. Uniaxial rotation of cations has been found to take place above ca. 200 K among non-equivalent potential wells made by clay sheets.639 2H and 15N NMR has been used to measure the 2H EFG and 15N CS tensors in solid hydroxylammonium chloride and to study the NH3 and OH dynamics. There was no evidence for OH reorientation up to 405 K, indicating a rather strong OH...Cl H-bond.640The rotational dynamics of P4O6S and P407 in the solid state have been studied by means of 31Pspin-echo and 31P MAS NMR.64' All spectra have been simulated to confirm the type of the motion and to extract the time scales as a function of the temperature. Good agreement between experimental and theoretical data was obtained on the basis of a three-site jump model. 31P NMR measurements have been reported for solid Li3P7 under both nonspinning and MAS conditions.642At low temperatures the spectra correspond to a static situation, exhibiting a superposition of three subspectra due to the epical, equatorial, and basal P atoms in the P7-cage. Analysis of these spectra provided information on the CS tensors of the various P atoms. Their orientations in the molecular frame have been obtained from quantum mechanical calculations. At high temperatures a bond shift rearrangement model (similar to the Cope rearrangement process in bullvalene) has been used to interpret the lineshape changes. 7 Phase Transitions.- H - 'H TI and TI, have been measured as a function of temperature in order to investigate order-disorder phase transitions in

286

Nuclear Magnetic Resonance

layer crystals with a rotator phase, (n-CsH11NH&ZnC14 and (nC12H25NH3)2ZnC14.643 The highest-temperature solid phase in both compounds was found to be the rotator phase, where rod-like cations perform uniaxial reorientations about the molecular long axes accompanied by conformational disordering and translational self-diffusion of the cations. These rotator phases have been shown to be quite analogous to those reported in n-alkylammonium chlorides. (n-C5H 1NH3)2ZnC14undergoes four structural phase transitions, while Cl2HZ5NH3)2ZnCl4 exhibits a single transition above ca. 120 K. All of these transitions have been shown to be of order-disorder type. 'H NMR measurements have been performed to study thermally induced rearrangement of H-bonded helices in 4-isopropylphenol below the melting point.644 Energetically inequivalent Me group reorientations have been observed in differently prepared samples and this suggested that a high-temperature polymorph occurs below the transition point as a metastable phase. It has been proposed that thermally induced molecular rearrangements enable proton transfer in H-bonds and this stimulates protonic conduction. f 3 C - I3C MAS NMR studies have been untertaken to study the low temperature phase transition in fullerene C60.645 At the phase transition near 262 K, the T1(I3C)in static samples of pure c60 decreases from 31 to 0.8 s as the temperature decreases by less than 8 K, while XRD patterns change little. Under MAS T I (13C) decreases from 46 to 0.8 s over only 2 K, while the onset of the phase transition is lowered to 255 K. It has been suggested that uniaxial cage motion cannot average the CSA to the value observed in static samples, and nutation must be present. The deoxycholic acid inclusion complex 2DCA:ferrocene has been shown to undergo a 'gradual' phase transition above ambient temperature, 'completed' by similar to 360 Ka6 The phase transition has been characterised by using XRD and 13CC P MAS. The transition, most clearly observed in the positions of the DCA molecules, is largely dominated by the dynamic behaviour of the guest ferrocene molecules. f3C,"N - Polymorphism and phase transitions in 2-(2,4-dinitrobenzyl)-3methylpyridine have been investigated using single-crystal 13Cand "N NMR. The spectral observations have been interpreted in terms of a distribution of mesoscopic domains within the crystal, differing in their local pressure. The presence of dynamic processes on time scales shorter than minutes has been excluded by both NMR measurements and tautomerisation kinetics.a7 f 5 N - The phase transitions within the three phases of potassium nitrate (a-, p-, and y-KN03) have been investigated in detail by VT "N MAS. The stability of the metastable y-KN03 phase has been found to be highly influenced by the thermal history of the sample. The spectral appearance for the room-temperature a-KN03, phase has been found to change after excursions of the sample into the high-temperature f b K N 0 3phase.648 *'Si - The different polymorphs of Y&07 have been studied by 29SiMAS NMR. 29SiNMR spectra for the a,p, y and 6 polymorphs are consistent with accepted structural data from the point of view of the number of sites, the populations and the chemical shifts.649

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31P - The order-disorder ferrielectric-paraelectric transition in lamellar CuInP2S6 has been studied using VT 31P MAS NMR.650 Two centrebands have been observed at the lowest measured temperature while only one has been detected at the highest temperature. The former two represent the inequivalent positions for the P atoms of the P2s6 group which reflect the antiparallel displacements of the polar Cu and In sublattices in the ferrielectric phase. The latter corresponds to the appearance of a 2-fold axis through the P-P bond as the Cu ions undergo double-well hopping motions, and the In ions occupy on-centre sites in the paraelectric phase. The presence of the ferrielectric type resonance in the paraelectric regime has been ascribed to the nucleation of polar order, while the persistence of the paraelectric signal well below the transition temperature implies residual hopping motions occurring in the ferrielectric regime. 7fGa - The melting-freezing phase transition of gallium confined within Vycor glass has been studied by 71GaNMR. A single broad 7'Ga NMR line corresponding to melted gallium was observed in contrast to lineshapes found until now for liquid gallium in porous matrices. A difference between results obtained using the three methods was explained by formation of various confined solid gallium modification^.^^' f33Cs- Crystalline Cs'( 18-crown-6)2e- is a linear chain Heisenberg antiferromagnet undergoing a slow, irreversible transition above 230 K from a crystalline low temperature phase to a disordered Curie-Weiss paramagnetic high temperature (HT) phase. A 100 ppm diamagnetic shift of the 133CsMAS NMR peak accompanies this transition. The HT phase undergoes an additional first-order reversible transition upon cooling below 220 K accompanied by the observation of two peaks in the 133CsNMR spectrum.652

8

I n situ Reactions

8.1 Polymerisation. - The solid-state formation of a centrosymmetric cage dimeric 4-aryl- 1,4-dihydropyridine has been monitored by 3C CP MAS NMR.653The spectra of the photodimerising derivatives show the changes in symmetry during the reaction and the reaction progress and clearly reflect conformational properties of the molecules. Starting from centrosymmetric pairs of monomeric molecules, dimerisation proceeds to non-symmetrical syndimers, that subsequently cyclise to centrosymmetric cage dimers. The thermally induced solid-state polymerisation reactions in sodium chloroacetate and sodium bromoacetate, leading to poly(hydroxyacetic acid) (polyglycolide) and NaCl and NaBr, respectively, have been studied by isothermal in situ 23Naand 13C MAS NMR with a time resolution of the order of 5 to 25 min. For sodium chloroacetate, there is no evidence for the involvement of intermediate phases during the reaction whereas this cannot be excluded for sodium bromoacetate. The mechanistic and kinetic information obtained from in situ solid-stale NMR investigations has been compared and contrasted with

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information obtained from other in situ probes of the polymerisation reactions in these materials.654 8.2 Heterogeneous Catalysis. - An isolated flow MAS NMR probe have been developed to enable simultaneous observation of events occurring on a catalytic surface with characterisation of the effluent gas using an external analytical instrument.655The probe uses ceramic ball bearings to support the rotor. The spinning and spectroscopic capabilities of the probe have been proven by obtaining a 13CCP MAS spectrum of hexamethylbenzene at 2 kHz. The flow capabilities of the probe have been demonstrated by observing methanol adsorption onto HZSM-5. The VT capabilities have been shown by monitoring the change in the 207Pbchemical shift of Pb(N03)2 with temperature. Temperatures >300 "C have been achieved for the probe. The reaction of methanol to dimethyl ether on zeolite HZSM-5 has been used to demonstrate the ability of the probe to study heterogeneous catalysis reactions in situ. Under flow conditions only dimethyl ether was observed. In previous studies under sealed (i. e. batch) conditions, an equilibrium was observed between methanol and dimethyl ether. A new technique has been introduced allowing simultaneous in situ MAS NMR investigations of hydrocarbon conversions on solids under flow conditions and on-line gas c h r ~ m a t o g r a p h y .The ~ ~ ~new in situ technique has demonstrated its advantage for a simultaneous investigation of compounds with a long residence time on the catalyst surface and of compounds rapidly leaving the catalyst surface. In situ 13C MAS NMR techniques have been employed to study the conversion of methanol to dimethyl ether on zeolite HZSM-5,657the methanol-to-gasoline conversion,658the formation of methyltert-butyl ether on zeolites HY, HBeta, HBetdF and HZSM-5,659the photocatalytic oxidation of ethanol over two Ti02-based catalysts,660the photocatalytic oxidation of ethanol over a TiOz-coated optical microfiber catalys661 t and the propene polymerisation under the conditions of continuously flowing propene through a supported Ziegler catalyst.662

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539 540 54 1 542 543 544 545

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8 Multiple Pulse NMR BY 1. BARSUKOV

1

Introduction

The efforts of the pulse-sequence development in the reviewed year were concentrated in a small number of areas. Most of them can be described as an optimisation of the existing sequences for application to larger molecules or a search for NMR parameters which have not been measured before. TROSY method was incorporated into more double- and triple-resonance sequences, providing substantial sensitivity enhancement. Multiple-quantum protoncarbon coherence was used instead of the single-quantum one to improve the performance of the constant-time experiments. The sequences to measure a large number of different cross-correlated relaxation rates, residual dipolar couplings and couplings across hydrogen bonds were presented and optimised. A substantial fraction of the covered parameters either can not be evaluated for proteins of a reasonable size or do not provide valuable information. So the application NMR spectroscopist faces a problem of selecting the right experiment suitable for the system. As before the new and improved experiments are summarised in Table 6.1 at the end of the chapter.

2

Shaped Radiofrequency Pulses and Solvent Suppression

2.1 Adiabatic Pulses. - The pulses offer wide bandwidth as well as low sensitivity to rf inhomogeneity and non-optimum parameters. The use of adiabatic pulses for transverse magnetisation refocusing had been limited due to large phase distortions introduced by a single pulse. Zweckstetter and Holak' demonstrated that a pair of adiabatic pulses produce clean refocusing over a wide frequency range with no phase distortions. The incorporation of the refocusing pairs into 2D "C-lH HSQC, 3D HCCH-TOCSY and 3D CBCA(C0)NH experiments was described. The use of the adiabatic pulses in the HSQC experiment provides a way of recording fully refocused spectrum over the complete I3C range. Additionally, the absence of linear phase correction produces spectra with a flat base-line. The gain of a factor of 1.8 was reported for some regions of the constant-time HSQC spectrum. Adiabatic pulse decoupling has a profile with sharp edges and covers wide frequency range at relatively low power. These make it an ideal choice for Nuclear Magnetic Resonance, Volume 30 $Y The Royal Society of Chemistry, 2001

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homonuclear decoupling. The main drawback of the method is the presence of cyclic irradiation side-bands. In homonuclear cases the side-bands are mainly caused by the direct irradiation of the decoupling field. The side-bands are anti-symmetric and do not change the intensity of the main component. When an additional initial decoupling is introduced with the same root-mean square of the rf field, the phase of the side-bands has a cyclic dependence on the time of the initial decoupling. By adding together FIDs registered with different time of the initial decoupling, the side-bands are effectively reduced.2

2.2 Solvent Suppression. - The efficiency of water suppression often suffers from a residual water signal coming from a region away from the centre of the rf coil. The ways to improve the performance of the WET method in that respect was discussed by Zhang et aL3 They demonstrated that the sequence can be optimised by replacing the 90" recording pulse with a 270" pulse in every fourth scan. 3

NOE, Chemical Exchange and Relaxation

3.1 NOE and Chemical Exchange. - A large number of methods have been developed for detection of NOEs between bound water and macromolecules. Many of them have problems distinguishing interactions with water molecules from those involving protons with a chemical shift coinciding with the water chemical shift. Even the use of isotope filtering techniques may fail due to. incomplete '3C labelling of a protein. The problem of isolation of NOEs to the water molecules in fast exchange with the bulk solvent was addressed by Melacini et aZ.4The new sequence relies on a relaxation filter to remove NOEs from proton pairs within a macromolecule. The filter was implemented as a constant-time proton evolution in a NOESY experiment. Fast transverse relaxation of protons in the macromolecule substantially decreases the intensity of the corresponding cross-peaks, with further attenuation coming from the evolution with homonuclear couplings. The sensitivity of the experiments was enhanced by a factor of J 2 with the quadrature-free indirect detection while positioning the offset on the water resonance. This also allowed an easy implementation of the water flip-back scheme. The radktion damping in the constant-time period was controlled with gradient pulses. The variants of the experiment for on- and off-resonance ROESY were described, as well as 'H-I5N HSQC-NOESY sequence with additional I3C filter. A set of experiments for detection of the intermolecular NOEs with water and organic solvent molecules were presented by Liepinsh et al.* Selective water excitation was achieved with induced radiation damping. A combination of selective pulses and a relaxation filter was used for the selective excitation of small molecule signals. The usual assumption in the analysis of ROESY data is the same tilt angle of the spin-lock field for the interacting spins. Under this condition ROE and NOE effects cancel each other when the tilt angle equals to 35.5". Cutting et

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al. checked the conditions for the cross-relaxation cancellation when two interacting spins have significantly different tilt angles of the effective field. Selective 'H-15N cross-polarisation was used for the selection of the longitudinal magnetisation of an HN proton followed by adiabatic application of the spin-lock field. The dependence of the selective cross-relaxation between two amide protons was monitored as a function of spin-lock field. The experimental dependence for different proton pairs agreed well with theoretical predictions. The difficulty of introducing flip-back method into NOESY experiments is caused by the phase changes required for the indirect quadrature detection. Grueschues and Ferretti7 applied a 45" water selective pulse with a combination of gradient pulses to induce refocusing of the water magnetisation along the same axis for both real and imaginary scans at the end of evolution time of the NOESY experiment. This allows a straightforward implementation of the flip-back method into NOESY as well as ROESY experiments. The pulse sequences for NOESY-HSQC, NOESY-HMQC and ROESY-HSQC, ROESY-HMQC experiments were presented. Significant intensity enhancement was observed for cross-peaks of exchanging and non-exchanging protons. A method for selective measurement of spin-diffusion-free NOE was presented by Harris et a1.' Excitation sculpting was used to select the magnetisation of the parent proton, During the mixing period the magnetisation of the parent and the target protons were selectively inverted to remove spin-diffusion contributions into cross-relaxation. The best suppression of the spin-diffusion was achieved with a pair of selective pulses. The experiment allows only the measurement of the cross-relaxation within a single proton pair and the sensitivity is low due to the use of long selective pulses. The sensitivity of 1D transient NOE experiments in small molecules was improved by the replacement of the relaxation delay with the reverse NOE sequence.' In that period the magnetisation of the source proton is selectively restored by the cross-relaxation with other protons. The use of reverse NOE allows the complete removal of the relaxation delay, thus increasing the repetition rate of the experiment. Sensitivity gains of 30% were demonstrated. The experiment, normally used for 5N longitudinal relaxation time measurements, was applied by Vialle-Printerms et al. l' to detect exchange crosspeaks between free and bound forms in slow exchange. The magnetisation transfer was achieved via 15N nuclei, rather than 'H, which allowed reduction of chemical exchange and relaxation loses during the mixing period.

3.2 Relaxation Time Measurements. - Measurements of 3C relaxation rates is a particularly important source of information on nucleotide dynamics due to the limited number of 15N sights. The contribution of the 13C-13C interaction into the relaxation rate 'can be substantially reduce by the use of 15% random 13C labelling, as shown by Boisbouvier et al. * The authors presented a set of optimised experiments for measurements of 13C spin-lattice, spin-spin relaxation rates and relaxation rates of the individual 3C multiplet compo-

'

3 14

Nuclear Magnetic Resonance

nents in nucleotide bases. The relaxation losses at the preparation stages of the experiments were reduced by the use of spin-state selective methods retaining the slower relaxing multiplet component. The residual signals from I3C-l3C groups were removed with C-C filter. Both carbon and proton steady-state magnetisations were utilised to improve the sensitivity. Spin-lattice and spinspin relaxation experiments have the same layout: S3E element is used to select the narrow component followed by C-C filter, 13C evolution and the relaxation period. Individual relaxation rates were measured in the experiments where relaxation period preceded C-C filter and 13C evolution. When relaxation of the broad component was of interest its coherence was transferred to the narrow component at the end of the relaxation period. The narrow component was observed in the TROSY scheme. The CSA-dipolar cross correlation can be deduced from the relaxation of the individual components. Characterisation of methyl group dynamics in I3C-labelled proteins is complicated by the presence of 3C-'3C J-couplings and 'H-I3C dipole-dipole cross-correlation. The difficulties can be overcome by expressing proteins in D20 medium with 3-13C-pyruvate as a carbon source.12 This leads to the production of proteins containing I3CHD2 groups and the rest of the sidechain unlabelled. The transverse carbon relaxation can be measured with the proposed experiment selectively detecting signals from 13CHDzgroups. Straightforward implementation of the TROSY method into the experiments for measurement of 15N relaxation rates were described by Zhu et a l l 3 The sequences offer higher sensitivity and resolution than non-TROSY versions. The evaluation of I5N T1/T2ratio from the 15N steady-state magnetisation in the presence of off-resonance rf irradiation was described by Guenneugues et a1.l4 The pulse-sequence for the measurement of the "N steady-state magnetisation was presented. To shorten the time for reaching the steady-state, proton are spin-locked before the nitrogens. The nitrogen irradiation is switched on abruptly and off in the adiabatic fashion. After that the I5N magnetisation is transferred to the directly bound protons through INEPT steps for observation. The accuracy of the method is limited by the hardware characteristics. One-dimensional methods for evaluation of selective transverse and longitudinal relaxation rates of HN and H a protons in 15N labelled polypeptides were discussed by Millet et a l l 5 The selective proton detection and inversion was achieved with two-way selective heteronuclear cross-polarisation, while for the semi-selective inversion the BIRD sequence was used. The experiments allow quick characterisation of proton relaxation properties in a small number of critical sites. 3.3 Cross-Correlated Relaxation Experiments. - Cross-correlated relaxation involving protein backbone nuclei is an effective probe of local conformation. A general theoretical analysis of the effect of cross-correlated relaxation on the relative intensities of multiplet components of zero-quantum and doublequantum components was presented by Reif et a l l 6 Both dipole-dipole and

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dipole-CSA mechanisms were considered. The authors described a mathematical formalism for manipulation of different cross-relaxation components. The angle dependence of the cross-relaxation was presented. That dependence can be used to deduce local conformation of a polypeptide. The effect of other relaxation sources on the measured cross-relaxation rate was considered. An HNCOCA-based pulse-sequence for measurement of the 13Cari-'Hai(dipole)/I5Ni+1-'HNi+1 (dipole) cross-correlated relaxation rate was presented as an illustration and the evaluation of the $-angle from the combined use of the above rate and the 15Ni+l (CSA)/'3C"i-'H"i (dipole) rate was described. The use of two cross-correlated relaxation rates allows to determine the dihedral angle unambiguously. Several authors presented pulse-sequences for the measurement of various cross-correlated relaxation rates and then compared them with the rates theoretically deduced from the known protein structure. The majority of the experiments induces zero- and double-quantum coherences between protein backbone nuclei of interest and follows their evolution under the effect of cross-correlated relaxation. These effects are registered either as the specific coherence transfer or as differences of the overall relaxation when certain mechanisms are suppressed. Most of the methods are straightforward modifications of triple-resonance experiments correlating the nuclei of interest. The small protein ubiquitin was often used to test the methods. The methods are briefly discussed below. The dihedral angles cp and JI can be evaluated from cross-correlation dipoledipole relaxation between 15N-'HN and 13Ca-'Harvectors belonging to the same or sequential residues, respectively. These effects are measured simulta' ~ sequences are neously in the experiment proposed by Pelupessy et ~ 1 . The designed to excite simultaneously two-spin coherences involving intraresidue and sequential Ca and N nuclei and measure their evolution under crosscorrelated relaxation. The rate of cross-correlated relaxation is calculated from the cross-peak intensity ratio in two complementary experiments. The cp and JI angles are deduced from the measured rate using independently evaluated order parameters and overall correlation time. The $-angle related crosscorrelated relaxation rates - 15N-'HN (dip~le)/'~C"-'H*(dipole) and I3C' (CSA)/l3CU-'H" (dipole) are available from HNCO-based experiment^.'^^'^ Chiarparin et ~ 1 . ~presented ' an HNCA-based experiment to measure the 1 3 a 1 a r C i- H i (dipole)/"CC";+I-'H"i+ (dipole) cross-correlated relaxation. The rate depends on the dihedral angle between the C-H vectors, which is indicative of the protein secondary structure. Many experiments on measurement of cross-correlated relaxation rates are based on the existence of direct scalar coupling between the nuclei involved in the coherence and a dipolar-coupled spin. These experiments fail if the coupling is small, as in case of 13C'-'HN (dipole)/13C' (CSA) cross-correlation within a peptide group (2J(HN,C')coupling). The difficulty can be overcome by resolving the multiplet components with E.COSY method.2' The experiment is based on HNCO sequence. The doublet components of the antiphase 13C' coherence, corresponding to the different states of 'HN, evolve with

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different rates due to 13C'-'HN (dip~le)/'~C' (CSA) cross-correlated relaxation. The components are resolved in "N dimension with large one-bond coupling 'J(HN,N). The cross-correlated relaxation rate is evaluated from the multiplet component intensity change between two complementary experiments. The rate measured for ribonuclease binase showed no correlation with the protein secondary structure. Another set of modifications of HNCO experiments allowing the measurement of 13C(CSA)/"N (CSA) and 'HN-"N (dipole) /'HN-I3C (dipole) crosscorrelated . relaxations was described by Pellecchia et a1.22 The rates are evaluated from deferential relaxation of l5N-I3C double-quantum and zeroquantum coherences. The 13C' (CSA)/"N (CSA) cross-correlated relaxation rate is measured directly in one of the proposed experiments and then used to extract the 'HN-"N (dipole) /'HN-13C (dipole) rate from the experimental data that depends on the combined relaxation rate. The magnitude and orientation of 13C CSA can be evaluated from the field dependence of the longitudinal 13C--l H (dipole)/' 3C (CSA) cross-correlated relaxation and a linear combination of longitudinal relaxations of 'H, 13Cand 'H-I3C two-spin order.23The cross-correlation rate is derived from the rate of conversion between longitudinal carbon magnetisation and 'H-' 3C two-spin order. The authors described pulse-sequences for measurement of the interconversion rate by following either carbon magnetisation or the two-spin order. The method was tested on the 22-residue peptide selectively 13C-labelled in the ortho position of the single Tyr ring. In the axial symmetry approximation the CSA tensor has a magnitude of 156 ppm with the symmetry axis making an angle of 23' to the I3C-lH vector. These values are in a good agreement with the solid-state NMR data. The 'HN (CSA)/''N (CSA) cross-correlated relaxation can be measured in an HSQC-based constant-time e ~ p e r i m e n tA. ~multiple-quantum ~ term 2N,H, is generated and its conversion into 2N,,H,,during free precession is monitored. The conversion occurs due to differential relaxation of DQ and Z Q coherences. The contribution of the 'HN (CSA)/ 15N (CSA) cross-correlated relaxation into the differential relaxation is proportional to the square of the magnetic field and can be extracted by performing the experiment at different field strengths. The use of cross-correlated relaxation rates for the analysis of protein backbone dynamics was described by Carlomagno et al. 25 They presented HNCO-based experiments for evaluation of combined C-CU (dipolar)/N-Ca (dipolar) and N-HN (dipolar)/C-HN (dipolar) as well as C' (CSA)/C'-C' (dipolar) and C (CSA)/N-HN (dipolar) relaxation rates from differential intensities of zero- and double-quantum multiplet components. The rates were interpreted in terms of Gaussian axial fluctuation model of motion. The results for ubiquitin suggested the possibility of concerted motions. Richter et a1.26presented a method to derive tortional angle information for rybosil moieties of nucleotides from CH-CH dipole-dipole cross-correlated relaxation rates. The rates are measured in the forward-directed quantitative T-HCCH-TOCSY experiments from the cross-correlated relaxation effect of

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double- and zero-quantum I3C-l 3C coherences. The pulse sequence utilises a combination of isotropic and longitudinal mixing to select forward-directed coherence 2Ci'xC(i'+ (i = 1,2,3) while suppressing backward-directed coherence 2Ci&,- l)z. The coherence is converted into the mixture of double- and zero-quantum coherences 4Hi'zCi'xC(i'+1)y and the conversion into the coherence 4H(it+l)zC(ir+&irY is monitored. The cross-correlated relaxation rate is obtained from the cross-peak ratios in two separate spectra. The rate depends on the dihedral angle between the interacting vector. The combined information for different CH-groups within the sugar ring is sufficient to define the pucker mode.

4

Coupling Constants Measurements

4.1 Scalar Couplings. - A simple method for evaluation of 3J(HN,H") The couplings in "N-labelled proteins was described by Perni et al.27128 method is an extension of CT-HMQC-HA and CT-HMQC-HN experiments, addressing the problem of limited digital resolution of the parent experiments imposed by the use of constant-time delay both for chemical shift and coupling constant evolution. The modified sequences separate the two evolutions. HMQC and HSQC versions of the method are presented, where a proton spinecho period follows the first excitation pulse. During the period the 'HN coherence either evolves with 3J(HN,H") or the coupling is removed with selective homonuclear decoupling. The coupling constant is evaluated from the intensity ratio between decoupled and undecoupled spectra. A quantitative J correlation experiment HA[HB,HN](CACO)NH for the simultaneous measurement of 3J(HN,H") and 3J(Ha,HP)couplings was proposed by Lohr et a1.29The experiment is a combination of a homonuclear inphase COSY and the H(CAC0)NH triple-resonance experiment. The initial INEPT element of the triple-resonance experiment is additionally used to transfer coherence over homonuclear 3J(HN,H") and 3J(H",HP). As the result two additional cross-peaks are observed in the spectrum at chemical shifts of HN and HP protons. The intensities of the cross-peaks depend on the values of the coupling constants; those can be calculated from the ratios of the crosspeaks and the auto peak at the chemical shift of H" protons. All the peaks required for the coupling constant evaluation are well resolved. The sensitivity of the experiment was optimised with the use of multiple-quantum 'Ha-13Ca coherence. A modification of 'H-13C CT-HMQC-J experiment for measurement of proton-phosphorous coupling constants 3J(H3'i,Pi+l) and 3J(H5'i/5''i,Pi) in 13C labelled nucleic acids was described by Hu et a1." The couplings are determined from the ratio between cross-peaks in 31Pdecoupled and undecoupled spectra, similar to other quantitative J-modulation experiments. Szyperski et al. 31 used a straightforward spin-echo-difference constant-time 'H-13C HMQC experiment for the evaluation of 3J(H3'i,Pi+l) and 3J(C4'i,Pi+l) couplings. A single reference experiment is required for both constants. This

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experiment is compared with the experiment where either 3J(H3'i,Pi+1) or 3J(C4i,Pi+') couplings are active. The use of the multiple-quantum coherence reduces relaxation loses. Multiple-bond I3C--lH couplings in small molecules can be easily resolved in the accordion-type refocused INEPT e ~ p e r i m e n tThe . ~ ~ position of the 180" pulses in the first INEPT sequence is systematically changing to produce simultaneous evolution with chemical shift and scaled coupling constant. As the result cross-peaks have antiphase doublet structure in the 3C-dimension with large enough multiplet component separation to measure coupling constants accurately. The version of the experiment suppressing direct 3C-1H coupling was also presented. A similar method of the accordion-type change of the additional homonuclear spin-spin coupling evolution delays in the HMQC experiment (PPJ-HMQC) was used by K ~ z m i n s k ito~ resolve ~ ? ~ ~ homonuclear splitting. The application of the method to measure 3J(HN,H") couplings in peptides and homonuclear couplings in saccharides was demonstrated.

'

4.2 Couplings across Hydrogen Bonds. - Measurements of small coupling constants IhJ(HN,N)across hydrogen bond in nucleotides with the E.COSY method are often difficult at high molecular weight due to extreme broadening of one of the doublet components. Then, a quantitative method monitoring the intensity change of a slowly relaxing TROSY component is an answer. Pervushin et al. 35 described a ZQ-TROSY experiment where cross-peak intensities are modulated by the coherence transfer over hJ(HN,N)coupling. The coupling constant is evaluated from the intensity difference between decoupled and undecoupled spectra. The use of TROSY component is essential, as small values of the coupling require long delays for efficient coherence transfer. The use of TROSY-based detection method and highdegree deuteration allowed Wang et al. 36 to observe "J(N,C') connectivities in a 30 kDa protein. The experiment used is essentially TROSY-based HNCO with 15N-13C' transfer time optimised for detection of small coupling constants. The average 15NTROSY T2 relaxation time for the protein was 169 ms at 750 MHz. A modification of the TROSY-HNCO experiment was used by Meissner and S ~ r e n s e n ~to~ .evaluate ~* 3hJ(Ca,HN),2hJ(C',HN)and 3hJ(C',N) couplings in proteins. Cross-peaks have E.COSY-type multiplet structure with the separation by the 'J(Ca,C') coupling in one the C' dimension and the coupling across the hydrogen bond in the HN dimension. The 3hJ(Cm,HN) couplings measured for chymotrypsin inhibitor 2 ranged from 0 to 1.4 Hz. The detection of through-hydrogen-bond couplings 2hJ(HN,C')in proteins is difficult due to the presence of larger through-covalent-bond couplings 'J(HN,C') and 3J(HN,C'). The evolution with the couplings greatly reduces the efficiency of the coherence transfer through hydrogen-bond couplings. Cordier et a1.39used band-selective pulses in 'H-I3C HMQC experiment to restrict the transfer to individual 1HN-13C'pairs. The 6.6 ms sinc 180" inversion pulses with a bandwidth of f 5 0 Hz were sufficient for successful hydrogen-bond detection.

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Experiments for quantification of 3hJ(N,C'), 2hJ(N,N) and lhJ(H,N) coupling in nucleic acids were presented by Dingley et al.40941In the case of 3hJ(N,C')a long-range H(N)CO experiment, used for proteins, was modified to avoid magnetisation loses due to relatively large 1sN-13C intranucleotide couplings. This was achieved with band-selective pulses restricting the coherence transfer to 15Nl and 13C6spins. The 2hJ(N,N)coupling was measured in the H(N)N-COSY experiment modified to allow simultaneous detection of both amino and aromatic 15N resonances. The lhJ(H,N) coupling is available with the simple quantitative JHN 'H, "N-HSQC e~periment.~' Two complementary 2D experiments HN(N)-TOCSY and CP-H(N)CO(NN)-TOCSY were used to detect and measure 4hJ(N,N)couplings across NH. ..O=C hydrogen bonds in a G-G.G.Gtetrad42The 15N-15N magnetisation transfer was achieved with cross-polarisation and the coupling constants were evaluated using the analytical expression for the coupling constant dependence of the isotropic mixing efficiency. In the CP-H(N)CO-(NN)-TOCSY experiment magnetisation is additionally transferred to the 13C' nuclei for indirect detection. The average value of the measured couplings was 0.136 Hz. 4.3 Residual Dipolar Couplings. - Residual dipolar couplings in oriented systems are widely used to derive long-range order restraints in proteins. The size of a system where the couplings can be measured with a sufficient precision is increased to 30-40 kDa range when perdeuterated samples are available. In such cases it is essential to utilise TROSY-based methods, as demonstrated in several papers.4347 Yang et al.43presented a set of TROSYHNCO-type experiments for measuring lHN-l5N, 15N-13C', H -l 3 C', 3C'-1'C* and 'HN- 3C* dipolar couplings in "N,' 3C,2H-labelledproteins. The 'HN-I5N coupling is obtained in two complementary experiments selecting individual multiplet components. The other couplings are measured pair-wise in E.COSY spectra: 1D(N,C")/2D(HN,C) and 'D(C',C*)/ 3D(HN,C"). The experiments included active suppression of anti-TROSY component and removal of the antiphase component due to 'J(HN,C') coupling. The precision of the measurements, estimated from repeated experiments for a 370-residue protein, was 1.1-0.4 Hz, depending on the experiment. Permi et aLU7 used spin-selective HNCO-based experiments to measure all dipolar couplings within a segment including peptide group and neighbouring C* atoms: 1HN-13Ca,15N-13Ca, H -l 3Ca i - 1 , 15N-13Cai-~, 1 N 13 H - C'i- 1, 1sN-13C'i-1 and 13C'i-I-13C*i-1 (all numbering is relative to the HN group used for the detection). The multiplet components corresponding to a and Q states are registered in two complementary experiments. The inter- and intra-residue signals are detected in separate experiments utilising 'J(C',C*) coupling for the selection. Three experiments were proposed. The HN(C0-a/P-NC*-.J)-TROSY experiment provides 1 N 13 CL 15 H - C / N-13C* or 1HN-13C*;-1/1sN-13Cn;_1 couplings, measured in a pairwise manner; the HN(a/P-NC'-J)-TROSY experiment provides 1 N 13 H - C'i - 1/'5N-13C'i- 1 available simultaneously; and the HN(C0-a/Q-C'C*J)-TROSY experiment provides 13C'i- 1-'3Cui- coupling. The experiments

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Nuclear Magnetic Resonance

have a smaller number of cross-peaks than the E.COSY versions, but suffer from the reduced sensitivity that restricts their applications to proteins below 20 kDa. The modified versions of the experiments were presented by the same group a little later.45 Spin-state selective experiments are widely used for the evaluation of the 'J(HN,N) couplings. The optimum set-up of such experiments depends on the For small proteins both size of the protein as discussed by Lerche et TROSY and anti-TROSY components are narrow and the coupling can be derived from the comparison of TROSY and anti-TROSY sub-spectra. In large proteins the anti-TROSY component is too broad to be used for accurate measurement and has to be replaced by the component evolving as TROSY in one dimension and decoupled in the other. The sequences selecting different combinations of TROSY and non-TROSY multiplet components are described. Kontaxis et al. 49 systematically studied the effect of large correlation times on the precision of the measurement of one-bond couplings 'J(N,HN) and 'J(N,C'). For large proteins they recommended to derive couplings from the difference between TROSY and decoupled HSQC or from a 3D TROSYHNCO and the proposed J-scaled TROSY-HNCO. Partial orientation of a protein sample causes substantial broadening of 'H resonance due to residual 'H-'H dipolar coupling. This decreases the sensitivity and accuracy of the experiment on measuring 'H-"N residual dipolar couplings. Band-selective homonuclear decoupling with adiabatic WURST-4 pulses during acquisition effectively removes homonuclear residual dipolar couplings.50 Additional sensitivity improvement comes from using bandselective 'H pulses in the INEPT steps refocusing dipolar couplings between amide and aliphatic protons. Cai et al? used modified Soft HNCA-E.COSY experiment to evaluate residual dipolar couplings between HNi-Huj and HNiH"i- protons. The cross-peaks have E.COSY structure with the separation by the large one-bond coupling 'J(Ca,Hu). Both sign and magnitude of the couplings were determined. A modification of JHH-TOCSY experiment (signed COSY) allowing the determination of the sign of the homonuclear residual 'H-'H dipolar coupling in uniformly '3C/'5N-labelled proteins was described by Otting et al. 52 The evolution with the direct 'J(C,H) coupling and homonuclear TOCSY or NOESY magnetisation transfer were utilised to create the homonuclear antiphase coherence between protons within the same spin-system. This coherence is detected only if the non-vanishing J(H,H) coupling exists, generating cross-peaks with antiphase splitting in the acquisition dimension. The pattern of the positive and negative components in the doublet is correlated with the sign of the coupling relative to the direct 'J(C,H) coupling. The experiment can be applied to CHZ-CH2 moieties. Similar principle was implemented by Peti and G r i e ~ i n g e rto ~ ~measure magnitude and sign of residual homonuclear dipolar couplings involving 'HN proton. The 'J(HN,N) coupling was used to separated a and p proton states and the NOESY sequence to transfer magnetisation between protons. The proposed JHHNOESY spectra have E.COSY multiplet structure with components separated

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32 1

by 'J(HN,N) coupling in one dimension and J(HN,H) dipolar coupling in the other. The 2J couplings in NHz and CH2 groups can be evaluated in spin-state selective experiments of Permi et a[.54 Pellecchia et al." used coherence transfer through residual homonuclear dipolar couplings involving amide protons to evaluate the value of the couplings. In the proposed experiment the constant-time proton evolution was combined with the homonuclear coherence transfer. The coupling constants were calculated from the ratio between cross- and diagonal peaks in the 3D experiment. Single-bond 'J(' 3Cu,1Hu)coupling can be measured in J-modulated 2D (HACAC0)NH e ~ p e r i m e n t .The ~ ~ coherence transfer is initiated on 'Ha proton. J-modulation is generated by varying the second refocusing delay of I a 1 3 u H - C INEPT transfer. The transfer is combined with the 13Ca-13C'transfer allowing introducing J-modulation in a constant-time manner. As the transverse relaxation of the in-phase and antiphase 13Cucoherence is close, the cross-peak intensities in the spectrum change as a simple sine function of the coupling constant and the delay time. The coupling is extracted by the leastsquare curve fitting. The experiment has a benefit of 'H-15N HSQC resolution and the interpretation does not require the assignments of 13Ca and 'Ha resonances. Mittermaier and Kay57 tested the commonly used assumption of the constant value of the quadrupolar coupling constant in methyl groups of proteins. They modified 'H-13C HSQC-based sequences used for measurement of 2H relaxation rates to obtain the values of the methyl 2H quadrupolar coupling and the one bond 13Cmethy1-'3C dipolar couplings. The quadropolar coupling constant can be deduced from the above parameters assuming the standard geometry of the methyl group. The correlation between the quadropolar and dipolar couplings for the weakly aligned fractionally 2H labelled SH3 domain agrees with the uniform value of 167 kHz for the quadrupolar coupling constant.

5

Inverse Proton Detected Correlation Spectroscopy

5.1 General. - Cryogenic probes gradually become available technology. They offer 3 4 fold increase in sensitivity relative to the ordinary probes. The sensitivity increase offered by cryoprobes makes carbon-detected spectroscopy a viable option. Serber et al. 58 presented carbonyl-detected HCACO pulse sequence and the experimental data. The spectra have advantage of narrow line-width in the acquisition dimension and the absence of artefacts caused by water suppression. One of the main drawbacks of the probes is the sensitivity loss with the increase of salt concentration in the buffer. This loss can be greatly reduced if protein is encapsulated in AOT reverse m i c e l l e ~and ~~ dissolved in a solvent with low conductivity. The recovery of 80% cryoprobe sensitivity in 2D 'H-"N HSQC experiment for a protein dissolved in a buffer containing 300 mM salt was demonstrated.

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5.2 Isotope Filtered Experiments. - The efficiency of the removal of the signals from carbon-bound protons in '3C-labelled proteins is reduced due to the variation of coupling constant values. A new filtering element with high suppression level for couplings in the range 115-165 Hz was proposed by Stuart et al. 6o The method utilises composite pulse rotation refocusing scheme that produce the attenuation proportional to cos3(nJcH~)thus having a broader minimum than the INEPT-based refocusing. The suppression factor of 80-40 was demonstrated experimentally for the full range of I3C resonances in F l 13C-filtered,F2 I3C-edited NOESY-HSQC J compensated spectrum. The main disadvantage of the method is the additional delay of 1/2J reducing the sensitivity. The filter can be implemented in a semi-constant manner which partially regain the sensitivity loses. The combination of the filtering element with proton chemical shift evolution into a semi-constant-time period was also described for a doubly-tuned X-filter.6' 5.3 Isotope Edited Experiments. - The MUSIC theme was developed further by Schubert et al. 6 2 The double and triple-quantum flitters were incorporated into triple resonance experiments correlating backbone and side-chain resonances to produce amino-acid specific HSQC spectra. Pulse sequences for Gly, Ala, ThrNal/IlelAla, Thr/Ala and Asn/Gly selection were presented. The experiments have high level of selectivity, but due to high relaxation losses can only be applied to small proteins with sufficiently slow relaxation rates. The optimised INEPT transfer delays were used by Uhrin et al.63 to select resonances of methyl-containing amino acids in the HCCH-TOCSY spectra. The proposed (H)CCH,-TOCSY and H(C)CH3-TOCSY experiments have good resolution in the methyl carbon dimension due to the use of long constant time delay. Application of proton CPD decoupling in the fraction of the delay provided sensitivity increase. 5.4 Heteronuclear Double Resonance Experiments. -5.4. I HSQC/HMQC. The main source of transverse carbon relaxation is dipole interaction with directly bound protons. This source is effectively removed when protons are involved in heteronuclear MQ coherence. For that reason the relaxation rate of the MQ coherence of methine groups of macromolecules can be slower than that of the SQ one. Then the replacement of an HSQC step with the HMQC one improves the sensitivity of the experiment. The effect is particularly noticeable in constant-time (CT) experiments. Kong et al.64 described a gradient and sensitivity enhanced CT-HMQC experiment and its use in NOESY-HMQC and TOCSY-HMQC sequences. The effect of homonuclear proton spin-spin couplings is removed by the use of spin-lock method. The sensitivity gain of 33% over the HSQC version of the experiment was demonstrated for C'H groups of calmodulin. The gain for larger proteins is expected to be smaller as the contribution of proton-proton dipolar interaction into MQ relaxation increases. This contribution can be reduced by fractional deuteration. Long-range correlation HMBC and HSQC sequences were optimised by

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Phan65for correlation of exchangeable imino protons with 13C5in nucleotides dissolved in H20. In the proposed JRHMBC and JRHSQC experiments water suppression is achieved with jump-return (JR) method. The experiments also register long-range correlation with non-exchangeable protons. The reduction of carbon sweep width in 'H-'3C-correlation experiments in order to increase the resolution in the indirect dimension can result in undesired folding patterns. In such cases band-selective HSQC and HMQC experiments of Gaillet et a1.66may offer a better alternative. The experiments are suitable for small molecules and employ the method of excitation sculpting in combination with pulsed field gradients to achieve the selected region selection. 5.4.2 TROS Y. The optimisation of TROSY experiments for aromatic residues The first of the series was described in a set of successive addressed sensitivity of the 2D TROSY spectra in case of negligible proton CSA.67In such case 13C decoupling can be used without sensitivity loss. The original TROSY sequence proposed earlier for the detection of proton 1 is considerably more complex than for S = 112 systems, due to the fact that for S> 1 zero field splitting (zfs) interactions are non-zero. For first-row transition metal ions, the electronic Zeeman and zfs terms in the electron spin hamiltonian are in many experiNuclear Magnetic Resonance, Volume 30

c>The Royal Society of Chemistry, 2001

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Nuclear Magnetic Resonance

ments comparable in magnitude. This fact has a major influence on NMR and electron spin relaxation times, especially (but not solely) in NMR relaxometry experiments, where T I is measured over a range of field strengths down to the milli-Tesla region, and at the lower end of this range, the zfs energy is always larger than the Zeeman energy. For many transition metal ions, the Zeeman and zfs energies have comparable magnitudes at field strengths of several Tesla, in which case, zfs effects need to be taken into account in high resolution NMR experiments as well. A brief description of the underlying physics is as follows. The spatial quantization of the electron spin motion differs qualitatively in the zfs- and Zeeman limits (ie., where the zfs (or Zeeman) energy is dominant). In the Zeeman-limit, the electron spin executes a Larmor precession about the laboratory magnetic field, while in the zfs-limit, the electron spin motion is quantized (or more properly, polarized) along the molecule-fixed axes of the zfs tensor. The level diagrams differ in the two limits: in the zfs-limit, the levels of I fmS> non-Kramers doublets are degenerate. The details of the spin ‘precessional’ motion depend on the spin quantum number and on the symmetry of the zfs tensor. For an axially-symmetric tensor, is a constant of the motion in the zfs-limit (as in the Zeemanlimit) but is quantized along the molecule-fixed unique axis of the zfs tensor rather than along the Zeeman field. The presence of low-symmetry terms in the zfs tensor, as occurs, for example, when the molecular x- and y-axes are chemically distinct, induces an oscillation in in the case of integer spins (S = 1, 2) but not half-integer spins (S = 312, 5 0 ) . The details of the electron spin motion and spatial quantization are reflected critically in the behavior of paramagnetic NMR relaxation rates. Thus magnetic field-dependent NMR relaxation data can be used to extract spin hamiltonian information (e.g., the zfs parameters, D and E) for solution species in situations where direct ESR observation of the electron spin system is difficult or impossible. This is especially true for integer spins which are usually extremely difficult to observe by ESR. Abernathy et al. have studied these phenomena theoretically and experimentally in S = 1 and S = 2 integer spin systems. NMR relaxation in the S = 1 spin system is predicted to be strongly suppressed in the vicinity of the zfs-limit because the orthorhombic components of the zfs tensor drive into oscillation, thereby decoupling the local magnetic dipolar field associated with from the nuclear spin. The application of a Zeeman-field damps out this oscillation and reintroduces static components into when the Zeeman energy exceeds the orthorhombic zfs splitting of the If 1> non-Kramers doublet. Thus the major expected feature of the NMR T I field dispersion profile of ligand and solvent protons is a strongly rising dispersive feature, where R1 increases several-fold above the zfs-limit value. This expectation was tested experimentally and confirmed in an orthorhombic S = 1 spin system containing high-spin Ni(I1). A related phenomenon is predicted for S = 2, although in this case it is the 4th order terms of the zfs tensor which drive into oscillation. Likewise for S = 2 , the application of a Zeeman field

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reintroduces static components into G,>,thus producing a strongly rising dispersive feature in the T I field dispersion profile from which the magnitude of the 4th-order zfs tensor component can be measured. The theoretically expected behavior was compared against experimental data for a tetragonal S = 2 spin system containing high-spin Mn(III), and the theory was confirmed by simulation of the data. A relatively simple, physically intuitive model of the zfs-limit NMR relaxation behavior of integer spin systems has been described. Nilsson et al. have likewise considered these phenomena in calculations based on the Stochastic Liouville formalism. Results of model calculations for an orthorhombic S = 1 spin system were described for different relative magnitudes of the static and dynamical parts of the zfs tensor. (The dynamical part of zfs hamiltonian causes electron spin relaxation.) From these calculations they likewise conclude that zfs rhombicity depresses the zfs-limit NMR relaxation rate, a phenomenon which can be counteracted by the application of a Zeeman field. Westlund4 has derived closed-form expressions for the zfslimit NMR R 1 of S = 1 hexaaqua cations subject to an orthorhombic zfs interaction. He again found a strong dependence on the ratio of the cylindrical (D) and orthorhombic (E) components of the zfs tensor. In this study, Westlund also derived closed-form expressions for the electron spin relaxation times for S = 1 in the zfs-limit using Redfield Theory (the electron spin relaxation times were defined relative to the molecule-fixed zfs principal axis system, rather than in the laboratory frame). Bertini et aL5 have likewise derived expressions for electron spin relaxation rates for S = 1 in the zfs-limit and have incorporated the resulting expressions into computational algorithms used to calculate paramagnetic relaxation times in biological macromolecules. The approach assumes that Brownian reorientation is slow compared to electron spin relaxation, a condition that is usually amply satisfied for SYl paramagnetic ions other than Mn(I1) when bound to paramagnetic macromolecules. The slow-reorientation Redfield approach gave results in good agreement with more general calculations based on the Stochastic Liouville Equation that are valid outside the slow-reorientation limit. In polynuclear paramagnetic metal centers, Heisenberg exchange coupling between metal ions, when larger than the Zeeman and zfs energies, creates a ladder of spin states. Electron spin relaxation within different spin states of the ladder can be very different from that of isolated ions. Clementi and Luchinat6 have discussed in general terms nuclear and electron spin relaxation in systems containing paramagnetic metal dimers and have given closed-form expressions for dimer-induced NMR relaxation due to (1) nuclear-electron magnetic dipole-dipole coupling, (2) Curie spin-nuclear spin coupling, and (3) nuclearelectron scalar relaxation. A qualitative discussion is given of various aspects of the relaxation mechanism, such as the ‘fast relaxes slow’ phenomenon, whereby a fast relaxing electron spin (Co(I1)) that is tightly coupled to the thermal lattice accelerates the relaxation of a slow relaxing electron spin (Cu(I1)) to which it is Heisenberg exchange-coupled.

480

Nuclear Magnetic Resonance

2.2 Chemical Shifts. - McGarvey7 has analysed level-specific contributions to contact hyperfine chemical shifts in lanthanide and actinide complexes. In this study, he examined the tacit assumption that the Fermi contact shift in lanthanide and actinide compounds is proportional to the total . Calculating the individual contributions of the 5f0, 5fkl, and 5f+2 orbitals to , , and , he found that these contributions have very different temperature dependences and are anisotropic. Inclusion of these level-specific phenomena into the analysis gave good agreement with the observed temperature dependence of hyperfine shifts of 'H, "B, and 13C resonances in tetrahedral compounds of the formula, M-(BH3CH& (M = Pa(IV), U(IV), NP(IV))* McGarvey et al. * have further investigated hyperfine chemical shifts in strong-field d5 transition metal complexes. An improved equation for the contact shift was developed which separates the spin contribution into the d,,, dyz, and d,, portions. The relationship between the Fermi contact chemical shift and the molecular orbital structure of the complexes was considered in detail. The theory was applied to the analysis of experimental results of 'H hyperfine chemical shifts in [M(NH3)5(H20)](CF3S02)3](M = Ru(III), Os(II1)) in the solid state and in frozen solutions. In an overview article in Coordination Chemistry Reviews, Kazansky and McGarvey9 have discussed the use of NMR and ESR to probe electron spin densities in polyoxoanions.

2.3 Hyperpolarized Noble Gases. - Hyperpolarized noble gases can be produced in the laboratory by interaction of the gas with optically-pumped rubidium vapor. Polarized 129Xeis currently of interest as a potential MRI nucleus, and in physico-chemical applications, polarized xenon has been employed as a spin probe of atomic interactions at surfaces and in porous media. The maintenance of nuclear spin polarization in these experiments is limited ultimately by 129XeNMR relaxation processes, an understanding of which is needed to optimize the production of steady-state nuclear polarization. With this objective in view, Walter et al.1° have presented a detailed theoretical analysis of noble gas NMR relaxation due to magnetic hyperfine interactions between 129Xeand excited alkali metal atoms in the gas phase. 3

Lanthanide Ions

The lanthanide ions, which possess partially filled f sub-shells, have unique magnetic properties which make them particularly attractive as chemical shift and relaxation agents in NMR. The ions are relatively large, typically exhibiting coordination numbers of 8 or 9, and thus complexes with octadentate chelates such as DTPA (1) and DOTA (2) are of special importance and commonly studied. A number of recent reports involving derivativatives of these ligands are described below. Lanthanides are commonly found only in the ( + 3 ) oxidation state, except under strongly reducing conditions, and their chemical bonding to the donor atoms is largely electrostatic, since the 4f

48 1

14: Parumugnetic N M R

orbitals are effectively buried within the electronic structure. For this reason, the Fermi contact interaction is small, and hyperfine couplings with nuclear spins are due largely to the electron-nuclear dipolar interaction. For the lanthanides, J (but not S) is a good quantum number, its values ranging from 1/2 to 1Y2.

1 DTPA~

2 DOTA~

3.1 Chemical Shift Reagents. - Excluding for the moment the anomalous case of Gd(III), the lanthanide ions possess relatively large magnetic moments (proportional to J(J+ 1)). The chemical shifts they induce in NMR resonances tend to be relatively large and spatially anisotropic, described by a dipolar shielding tensor. Electron spin relaxation times are quite short (TS=0.1- 1 ps), and thus the lanthanides (other than Gd(II1)) do not ordinarily cause marked line broadening in NMR. For this reason, lanthanide ions, particularly Eu(III), have frequently been employed as chemical shift reagents. In recent studies, Sat0 et al. I and Ogasawara et al. l 2 have proposed the use of Ce(II1)based chiral shift reagents which produce less line broadening than the corresponding Eu(II1) analogs. The use of the rhodium ion has been suggested by Ogo et al. ,13 who studied the rhodium-containing supramolecular host [Cp*Rh(2' deoxyadenosine)]3(0Tf)3 (Cp = q5-C5Me5, OTf = CF3SO3-) as a new, aqueous 'H NMR shift reagent. This complex binds a wide variety of H20-soluble organic substrates via a host-guest molecular recognition process involving non-covalent 7c-7c and hydrophobic interactions. Chiral Eu(II1) shift reagents are useful for resolving NMR spectra of racemic mixtures.15y16Axt et all4 have studied the effectiveness of two commercially-available chiral shift reagents, of which the more effective was Eu(II1) tris-[3-(trifluoromethylhydroxymethylene)-(+)-camphorate]. This complex compared favorably with the widely-used achiral shift reagent Eu(fod)3 in an analysis of NMR spectra of racemic mixtures of polycyclic organic compounds, and optimal concentrations of both shift reagents are reported. Rothchild and Shariff17 have described the use of a related camphorated chiral shift reagent in conjunction with an achiral Eu(II1) shift reagent to facilitate the analysis of H homonuclear COSY NMR spectra of glutethimide (an inhibitor of aromatase). The spatially anisotropic dipolar chemical shifts that are produced by lanthanide shift reagents (LSR) can be used for molecular structure analysis. Goncalves et al. * describe a computer program which quantitatively analyses lanthanide-induced pseudocontact (dipolar) shifts, facilitating the determination of the locations of lanthanide binding sites on the substrate. Lanthanideinduced shifts have also been used in conjunction with molecular mechanics

482

Nuclear Magnetic Resonance

studies and ab initio calculations of molecular geometries for the purpose of conformational analysis. As part of an extensive on-going research program, Abraham e? aZ. l 9 have used these combined approaches in the confomational analyses of seven epoxides, for which they measured paramagnetic chemical shifts induced by shift reagent, Yb(fod)3. 3.2 Use of Gadolinium Complexes as NMR Relaxation and Contrast Agents. Among lanthanide ions, the behavior of Gd(II1) is anomalous in that its electron spin relaxation time tends to be very long (1- 10 ns), a property that makes this ion an extremely potent relaxation agent in NMR and a highly effective contrast agent in MRI. The anomalous relaxation behavior results from the half-filled f-shell of Gd(II1) and the consequent absence of low-lying electronic states with non-zero orbital angular momentum. A great deal of experimental interest over the past decade has centered on optimizing Gd(II1) complexes with respect to function as a relaxation agents in NMR and/or as contrast agent in MRI. An effective contrast agent should be tight-binding toward the highly-toxic Gd(II1) ion, slowly reorienting, and it should contain water in at least one coordination site that is in rapid chemical exchange with the bulk solvent.20 The 7d Eu(I1) ion is isoelectronic with Gd(II1) and thus is likewise expected to be a very potent relaxer of nuclear spins. This ion is, however, readily oxidized to the much more common Eu(II1) oxidation state and has for this reason been studied very little. Caravan et aZ.21have studied the proton and 7O NMR relaxation times and the the pressure-dependent 1 7 0 paramagnetic chemical shifts by the hexaaqua Eu(I1) ion, for which they also reported the ESR spectra. Eu(I1) has exceptional properties. The chemical exchange rate of bound water was found to be exceedingly fast, faster than for any other aqueous cation. The ESR spectra demonstrated that the electron spin relaxation times are extremely long, longer even than for hexaqua-Gd(III), confirming that Eu(I1) is indeed a potent relaxer of nuclear spins, although presumably too strong a reducing species to be used in medicine. Several laboratories, notably those of Merbach and Aime and their coworkers, have conducted extensive and systematic investigations of Gd(II1) complexes with ligands of high dentisity with the objective of understanding solution structures, solution dynamics, chemical exchange kinetics, and other physicochemical aspects relevant to the NMR relaxation mechanism. Work of Aime and co-workers has recently been reviewed.22In the Merbach laboratory, an array of NMR techniques are brought to bear on a single system, including, for example, variable temperature and variable pressure measurements of 'H and I7O NMR relaxation times and chemical shifts, and proton relaxometry (measurements of the magnetic field dependence of the solvent proton T I over a range of several orders of magnitude using field-cycling techniques). Variable pressure T1 measurements are valuable in that they provide an experimental criterion for distinguishing competing mechanisms of chemical exchange of water ligands (associative versus dissociative mechanisms): associative mechanisms exhibit positive and dissociative mechanisms negative, activation

'

483

14: Paramagnetic N M R

volumes. Data analyses in these studies frequently involve a simulaneous fit of theory to the results of several different experiments, in this way providing a comprehensive picture of the relaxation mechanism. The focus of published studies involves, on the one hand, fundamental physical chemistry, and on the other, the optimization of lanthanide complexes as practical MRI contrast agents. The tripodal ligand TTAHA6- (3) forms a Gd(II1) complex, the properties of which have been investigated by ‘H relaxometry and by 1 7 0 relaxation and chemical shift studies.23This ligand possesses 10 potential N/O donor atoms, and the Gd-TTAHA3- complex is 10-coordinate in the solid state; but potentiometric studies of the indicate that the Gd coordination number is seven (N304 coordination) in solution, so that two coordination sites are available to the solvent. It appears that one of the tripodal arms is uncoordinated in the solution complex. The presence of two coordinated waters provides a significant advantage in proton relaxivity with respect to commercially-availableGd contrast agents, which possess only one inner-sphere water. Based on this fact and on the high rate of solvent exchange, the authors conclude that Gd-TTAHA3- is an interesting candidate as a potential contrast agent.

N

-OOc’

‘coo-

3 TTAHA~

Gd-DTPA (1) is a widely used contrast agent. Toth et al.24have attempted to increase the proton relaxivity by incorporating the Gd-(bis-amide)-DTPA derivative (4) into a poly(ethy1ene glycol)-based polymer. They report the magnetic field- and temperature-dependent 7‘1 relaxation produced in solvent ‘H and 170resonances by this conjugate with the objective of understanding how the kinetics and mechanism of the water exchange reaction are affected by incorporation into the polymer. Due to high internal flexibility, the increase in the reorientational correlation time, zR,relative to the parent DTPA-bisamide complex was small. A fortuitous competition was observed in the inter- and intra-molecular T I mechanisms that resulted in approximately temperatureindependent behavior in the water proton relaxivity. They propose that this complex might be useful for calibrating MRI signal intensities and TI values.

\coo4 DTPA-BMA~

484

Nuclear Magnetic Resonance

Gd-DOTA (2) is a commercially used MRI contrast agent derived from the parent tetraazamacrocycle structure, cyclen. Water chemical exchange into and out of the coordination sphere of Gd-DOTA is relatively slow, about 70 times slower than for the aqueous Gd(II1) ion. This situation acts to suppress the water proton relaxivity and is therefore undesirable with respect to function as a contrast agent. For Gd(H20)g3+, water chemical exchange is associatively activated, i. e., it involves an increase in Gd(II1) coordination number from 8 to 9, and the activation energy for this process is relatively low. As part of a search for fast-exchanging Gd(II1) complexes, Toth et aZ.25 have studied the hydration equilibria, i. e., chemical reactions between mono- and bisaqua species, in the octa-coordinate complex Gd-D03a (5). This study combined 1 7 0 NMR with optical spectroscopy of the homologous Eu(II1)D03a complex. The Eu(II1) ion has an absorption band at 580 nm which is very sensitive to changes in the coordination environment and provides an experimental probe of the state of hydration. Water exchange in Gd-D03a, as in Gd-DOTA, is much slower than in Gd(H20)g3+, indicating that the rigid structure of the macrocycle resists an increase in coordination number and thus increases the activation energy of the associative mechanism. Szilagyi et a1.26 have studied studied chemical exchange kinetics in the Eu(111)-DOTA chelate, which likewise forms major and minor isomers. They used 'H and 170NMR in conjunction with UV-vis spectroscopy to investigate the role of protonation in the chemical exchange reactions which transfer hydrogen ions and whole water molecules between the bulk solvent and sites in the coordination sphere. Proton chemical exchange was found to be acidcatalysed in strongly acidic media, this reaction accounting for a major portion of the total proton relaxivity. The role of protonation in the intramolecular processes which interconvert the major and minor conformational isomers wds also studied, but in these reactions protonation had little effect. Noting that squaric acid reacts with amino groups under mild conditions, Aime et al.27 have used the ligand D03ASQ (6), to conjugate the chelate D03A (5) to polyamino acids. Conjugates of Gd-D03ASQ-lysI6 and GdD03ASQ-ornl14 were prepared and characterized by 'H relaxometry and 170 T2 relaxation. The proton relaxivity of the conjugated complexes, especially of the polyornithine conjugate, was quite high (R1z35s-l at 20 MHz). 170 relaxation measurements showed that water chemical exchange is fairly rapid in these conjugates, comparable to that in the unconjugated Gd-D03A and Gd-D03ASQ chelates. The authors conclude that squaric acid (somewhat surprisingly) participates in the lanthanide coordination, and that a single water is present in the Gd(II1) coordination sphere. Other factors being equal, proton relaxivity scales with the number of coordinated water molecules. D03A (5) is heptacoordinate, and thus is expected to have two bound waters. Gd(II1) chelates of this and two other heptadentate ligands, PCTP (7)and PCTA (S), were studied by Aime et a1.28 with respect to proton relaxometry, the kinetics of water exchange, and the water coordination number. The Gd-PCTP chelate is particularly interesting because the phosphonic acid groups of the side-chains, due to their high

485

14: Paramagnetic N M R

=rlH 0

6 D03ASQ: R

k

7 PCTP: R = PO3H2 8 PCTA: R = COOH

5 D03A: R = H

charge, were believed to produce a significant outer sphere R1contribution. To the degree that this is true, relaxation of this complex results not only from the inner-sphere bound water molecule but also from weakly bound water in the second coordination sphere. Experimentally, Gd-D03A and Gd-PCTA exhibit higher molar relaxivities at 20 MHz than Gd-DOTA, apparently due to the higher water coordination numbers than occurs in the DOTA complex ( q = 2 vs. 1). Gd-PCTP possesses a only a single bound water, but its relaxivity is substantially higher than that of Gd-DOTA, a finding that was interpreted as due to an outer sphere R1 contribution of Gd-PCTP. The kinetics and mechanism of water chemical exchange was further studied by means of temperature- and pH-dependent measurements of the water 'H Tiand 170T2. Aime et aL29 have worked with the Eu(II1) complex of the DOTA-derived ligand DOTAM (9). (Eu(III), unlike Eu(II), is much weaker as an NMR relaxation agent than Gd(III).) Eu-DOTAM exists as a pair of isomers which differ in the conformations of the side-arms. Using 2-D EXSY NMR, the 'H spectra of both isomers were assigned and the equilibration kinetics was measured. From a lineshape analysis of the * H 2 0peak, measured in acetonitrile solution, the water exchange rates of the two isomers were determined, and that of the minor isomer was found to be much faster than of the major isomer, a result that was interpreted in terms of steric effects. The authors proposed that stabilization of the minor isomer in DOTA-derived complexes might provide a route to increasing the proton relaxivity. The replacement of the four carboxylate sidechains of DOTA (1) with amides gives the octadentate ligand DTMA (ll), the Gd(II1) complexes of . ~ which are considerably less stable than those of DOTA. Alderighi et ~ 1 have studied the stability and proton relaxivity of this complex, concluding that it is not a suitable MRI contrast agent. Similar results were obtained by Wang et al. ,31 who studied bis-amide derivatives of DTPA. A chiral DOTA-derived chelate, DOTMA (12), forms a Gd3+ complex R7~n

(

/-R

">

R ~ N - N ~ R

2 DOTA:R=COO9 DOTAM:R=

-c=o

11 DTMAR=

-c=o

I

NHZ 10 DOTP: R = -POaH*

12 DOTMA:R=

I NHCH3

-f-CH3

~

486

Nuclear Magnetic Resonance

which exhibits high proton relaxivity and is considered a potential MRI contrast agent. Di Bari et al.32 have studied the homologous Yb-DOTMA3+ complex in regard to its conformational equilibria in solution. They used 2-D H EXSY spectra to measure the rates of conformational exchange reactions and steady-state NOE's to assign the predominant conformer. Other interesting DOTA-derived chelates have been described. Zhang et a1.33 have synthesized a DOTA-derived complex in which parent DOTA macrocycle bears amidophosphonate substituents ((2), but with R = -CO-NHCH2-P03H2). The proton relaxivity is strongly pH-dependent in the physiological range, a property that the authors propose can be used to monitor in vivo pH-changes in MRI. In subsequent work from the same l a b ~ r a t o r ythe , ~ ~use of Tm-DOTP (10) was investigated as a potential in vivo MRI probe of temperature and pH. Proton and 31P chemical shifts were measured as a function of temperature, pH, and [Ca2+]concentration over the physiological range. Because different protons on DTPA exhibited qualitatively different temperature dependence, the authors proposed that temperature measurements should be based on chemical shift differences. The chemical shifts of 31P and 'H were found to vary linearly with pH over the range, pH = 6-8. Using chemical shifts of both resonances, a method is proposed for the simultaneous measurement of temperature and pH in MRI. Polyester35 and dendrimer complexes of Gd(II1) have been studied with respect to their properties as relaxation/contrast agents. High molecular complexes potentially offer long reorientational correlation times (zR)and an accompanying increase in solvent proton relaxivity, other factors such as the water chemical exchange rate and electron spin relaxation rate being constant. However, internal flexibility acts to counter the desired effect of increased molecular weight on TR. Bulte et al.36 have prepared Gd-DOTA complexes which are conjugated to a generation-5 ammonia-core polyamidoamine dendrimer. They proposed the use of these conjugates as T2 contrast agents in some circumstances, since R2 (but not R1)was found to be a strongly increasing (quadratic) function of field strength. They interpreted this behavior as the result of the Curie spin relaxation mechanism. Kellar et ~ 1 have . ~ likewise concluded that Curie spin relaxation provides an important 7'2 mechanism for Dy3+-containingcontrast agents. As an alternative approach to increasing molecular weight and thus to lengthening zR, Merbach and c o - w ~ r k e rhave s ~ ~studied ~ ~ ~ a complex in which Gd-DTPA (1) is Conjugated to a modified aspartyl-lysyl dipeptide which binds via hydrophobic interactions to serum albumin, a 70 kD blood protein. In this conjugate, which is a potential blood-pool contrast agent, water chemical exchange kinetics and electron spin relaxation were scarcely altered relative to Gd-DTPA, while Brownian reorientation, though slowed, remained an order of magnitude faster than than of the protein, indicating considerable internal flexibility. Thus the enhancement of proton relaxivity was less than expected. In a search for potential immunocontrast agents, Curtet et aL40 have studied related polylysine-Gd-DTPA and polylysine-Gd-DOTA complexes with antiCEA F(ab')2 fragments.

~

14: Paramagnetic N M R

487

Cyclodextrins are cyclic oligosaccharides consisting of several (6-8) 1,4linked d-glucopyranoside units which form a cone-shaped hydrophobic cavity, the size of which depends on the number of glucopyranoside units. The acyclodextrin moiety (6 subunits) forms inclusion complexes directly with bare lanthanide ions with the glucopyranoside anomeric oxygens acting as the donor atoms4' to the metal. The y-cyclodextrin (8 subunits) forms a weak inclusion complex with Tm-DOTA- , which has been investigated with regard to its proton relaxation properties in the hope that it might exhibit long zR values and enhanced proton relaxivity . Zitha-Bovens et ~ 1 have . prepared ~ ~ this complex and studied its stability and NMR properties ('H and 13C chemical shifts and 'H Ti's). The Tm(II1) ion was chosed for its capacity to produce both large chemical shifts and large relaxation enhancements. The stability of the inclusion complex was judged to be inadequate for use as an MRI contrast agent. In yet another approach to lengthening the reorientational correlation time, Andre et u Z . ~ have ~ conjugated Gd-DOTA (2) to an amphiphilic surfactant which forms micelles in solution. In this system, the water exchange rate remains rapid, similar to that of Gd-DOTA, but the reorientational correlation time lengthened to values an order of magnitude longer than for Gd-DOTA, and high proton relaxivity, characteristic of macromolecular systems, resulted. A number of Gd-DTPA derived complexes have been studied. Muller et a1.44 have studied the 'H, 13C, and 31P relaxation properties of Gd(II1) complexes of the DTPA-derived ligand (13). The proton relaxivity and the complex stability were both found to be higher than for the parent Gd-DTPA complex. The relaxivity increase apparently arises both from an increase in zR and from a shorter Gd-H distance from Gd(II1) to the bound water protons. The hydrophobic side-chain of the ligand (13) promoted non-covalent complex formation with serum albumin, which resulted in an increase (about five-fold)

-mJ \-coo13

in proton relaxivity over that of the unconjugated complex. Aime et ~ 1have . ~ likewise studied conjugates of Gd-DTPA to serum albumin, finding enhanced proton relaxivity in spite of a slowing of the water exchange rate. These authors also studied45 the binding of malonate-lanthanide complexes of Eu, Gd, Tb to serum albumin. Aime46and Adzamli et ~ 1have . studied ~ ~ the proton relaxometry of Gd-DTPA-BMEA (BMEA = bis(methoxyethylamide), OptiMARK), a commercial contrast agent awaiting FDA approval. In studies of other Gd-DTPA-derived contrast agents, Bligh et u Z . ~ * measured the proton relaxivity of Gd-DPTA-derived chelates containing bulky alkyl and

~

Nuclear Magnetic Resonance

488

aryl side-chains; one of these chelates was observed to bind serum albumin, causing a significant relaxation enhancement. Lammers et ~ 1 have . synthe~ ~ sized two DTPA-derived ligands in which two of the carboxyl ligands have been converted to butyl-amide (DTPA-BuA2, (14)) and glucosamide (DTPAGlucA2, (15)) linkages. The pH-dependence of the proton relaxivities and the amide chemical exchange kinetics of the Gd(II1) complexes are reported. -OOc7 ,NeN-N/-

o=cI R

rcoo14 DTPA-BuA~:R = NHCH2CH2CH2CH3

yZo

15 DTPA-GIuc~: R=

w

-NH

OH

Wang et aZ.50 have synthesized the bis(N,N-dimethylamide) and bis(N,Ndiethylamide) derivatives of DTPA (i.e., (1) in which R = M e , Et) and characterized their Gd(II1) complexes with respect to proton relaxivity. A single inner-sphere water molecule was shown, based on I7O NMR chemical shifts, to be present in these complexes. Another modified Gd-DTPA chelate was reported by Wei et aZ.,51 who introduced two aromatic 01- and o-aminobenzoic acid) side-chalns into the ligand, a modification which enhanced the proton relaxivity. Three other modified DTPA chelates were prepared by Wang et al.,52,53who report the synthesis of novel bis(amide) derivatives of DTPA as well as of two bis(amide) derivatives of EDTA. However, the solution stabilities of the corresponding Gd-complexes was too low for their use as contrast agents. Isopropyl and t-butyl bis(amide) derivatives of EDTA (16) were studied as Dy3+, Gd3+, and Cu2+ chelates by 170 chemical shift and solvent proton relaxation measurements. These species exhibited hexacoordinate chelation to the metal, and the lanthanide complexes contained 2-3 inner sphere water molecules. Solution stabilities of these complexes were measured by potentiometry but were much lower than that of the DTPA chelate. R = i-propyl R = Fbutyl

The ligand taci (= 1,3,5-triamino-l,3,5-trideoxy-cis-inositol) forms interesting trinuclear complexes with Gd(II1) and Dy(II1) which possess two bound waters on each of the three metal ions.54 The solid state structure (17) is maintained in solution. Variable pressure T1 studies of 170of the solvent indicated that water chemical exchange reaction is associative in mechanism and is relatively slow (70-fold slower than for aqueous Gd3+). Molecular reorientation of the complex was also found to be unexpectedly rapid, probably because of the roughly spherical molecular shape. Based on its relatively low proton relaxivity as well as its poor solution stability, this complex is considered unsuitable as an MRI contrast agent.

14: Paramagnetic N M R

489

17

18

Bligh et al. 55 report the solvent proton relaxivity of Gd-cryptate complexes, in which the lanthanide is encapsulated within the hexaimino cryptand (18). This complex combines the high stability of cryptates with high proton relaxivity resulting from two coordinated solvent molecules and a rapid water chemical exchange rate. Inoue et al. 56 have synthesized a 34-membered macrocyclic chelate with six pendant carboxymethyl groups. This chelate binds two Gd3' ions. The proton relaxivities, measured at 250 MHz, were similar to values for other Gd-chelates. Asato et a1.57 have reported the synthesis of a 36membered macrocyclic chelate which binds four divalent metal cations with mixed (N,O) ligation. Spyroulias et a1.5g have report the preparation of four new water-soluble, double-decker Ln(II1) bis-porphyrin complexes with possible application as contrast agents. Wetzel and R ~ e s k yhave ~ ~ described an X-ray and NMR characterization of functionalized cylooctatetraene sandwich complexes of lanthanide ions. Macrocyclic ligands encapsulating two lanthanide ions have also been studied.60 Howell et a1.61 have studied lanthanide (Gd, Eu, Tb) complexes with a 14-dentate ligand which sequesters two lanthanide ions in a biprismatic structure where the coordination sphere contains no bound solvent. These complexes are quite stable and the proton relaxivity is relatively low. Arts et a1.62have described a 'H and 13Cchemical shift and relaxation time study of Gd(II1) and Dy(II1) complexes with the tetracarboxylate ligand (19), which was prepared by oxidative cleavage of the carbohydrate trehalose. Schmitt-Willig et al. 63 have synthesized and characterized a new water-soluble Gd complex based on the ligand (20) that is now undergoing Phase (111) trials as a contrast agent for liver. Wang et al,@ have studied complexes between Gd and a DTPA (1) ligand that was modified by the introduction of an additional methylene group into a side-arm. They find that this complex has one coordination site occupied by water and a water proton relaxivity that is comparable to other Gd complexes used as commercial contrast agents.

OH

-0OC

COO-

19

\-0

20

490

Nuclear Magnetic Resonance

In addition to the use of Gd(1II) complexes as contrast agents in proton MRI, the Gd(III)-DOTP5- complex (lo), which is highly anionic, has successfully been as a chemical shift agent in 23Na and 7Li MRI. This complex binds alkali metal cations in the extracellular aqueous pool, thus differentiating the intra and extra-cellular 23Naand 7Li resonances. 19F MRI is an emerging area of interest because very high contrast images can be obtained using suitable fluorine-containing contrast agents which are localized to specific pools in the body. Kasai6' has described water-soluble metal-porphyrinates that are substituted at the meso positions with ionic have prepared Gdgroups that contain 24 or 48 fluorine atoms. Platzek et DOTA contrast agents that contain perfluoroalkyl side-chains. Somewhat outside the range of this review is a competing approach to MRI contrast enhancement based on superparamagnetic nanoparticles composed of i r ~ n - o x i d e ~ Oor- ~Mn(I1)-containing ~ h y d r ~ x y a p a t i t eThe . ~ ~ interested reader is directed to recent reports. 3.3 Novel Lanthanide-containing Complexes. - A number of lanthanidecontaining complexes, some highly novel, have been described in studies where paramagnetic NMR was not the primary focus, although in most cases 'H and/or I3C NMR, usually in a combination of 2D techniques, was used to ~ ~ . ~these ~ are elucidate aspects of the solution structure and ~ h e m i s t r y . Since may be of interest to scientists concerned with NMR effects of the lanthanides, the more unusual structures will be described. Triple-stranded dimetallic helicates have been prepared77 which bind Eu3+ and Tb3+ ions through carboxylate ligands. These species are water soluble in the pH range 4-1 3 and exhibit a stability comparable to that of Eu-DOTA. Lisowski and Starynowicz have synthesized m o n o n ~ c l e a rand ~~ din~clear~~ La(III), Ce(III), and Eu(II1) complexes of a novel pentacoordinate (N203) Schiff base macrocyclic ligand. Detailed assignments of the 'H and I3C NMR spectra were obtained using COSY, NOESY, and HMQC techniques. Lisowskiso has also prepared the interesting N6 macrocycle (21), which forms hexadentate complexes with lanthanides. La(III), Ce(III), Eu(II1) and Yb(II1) complexes, studied by COSY, NOESY, and HMQC 2D NMR in chloroform/ methanol solution, were found to exist in a helical (D2),chiral structure. These

21

49 1

14: Paramagnetic NMR

complexes formed diastereoisomeric adducts with were distinguished by 'HNMR.

D-

and L-amino acids that

3.4 Solution Equilibria of Lanthanide Complexes. - Daunomycin and adriamycin are prototypical members of the anthracycline anti-tumor antibiotic family that are used in the treatment of human cancers such as leukemia. These compounds have side-effects including severe cardiotoxicity, probably due to the 1-electron reduced semiquinone species. But they can be stabilized with respect to reduction by complexation with metal cations. Wei and Ming8' have conducted comprehensive 2-D NMR analyses of the lanthanide complexes of these compounds, determining the binding sites, binding stoichiometries and chemical exchange properties of the metal-bound forms. NMR studies of ion-pairing and selective solvation have also appeared. Fratiello et al. 82 have studied Sm(II1)-isothiocyanate complexation in wateracetone-Freon mixtures. Using 13Cand 15NNMR, they observed, at temperatures below - 100 "C, separate signals for coordinated and free NCS- ions. Formation constants and chemical shifts were determined for Sm(NCS), + 3 - n (n = 1-4) species. Experimental 'H and I3C NMR solvation of Mn(II), Cu(II), Co(I1) and Ni(I1) s t ~ d i e s ~of~ , preferential *~ cations in various mixed solvent systems have also been reported. Panyushkin and co-workers have studied the equilibria of ternary complexes of lanthanide ions with mono- and bidentate carboxylic acids in acetone solution.85They have d i s c u ~ s e dsystematic ~ ~ , ~ ~ approaches to the analysis of NMR data in rapidly equilibrating solution complexes.

4

D-Block Ions

4.1 Polynuclear Metal Complexes. - Asokan and Manoharan88 have studied the 'H NMR spectroscopy of the dinuclear Cu(I1) complexes (22) and (23) with a view to understanding the electronic and spin state structure. Due to Heisenberg exchange coupling, the spins of the Cu(I1) ions, which are individually S = 1/2 species, couples to form a spin ladder with S = 0 and S = 1 spin states. The former is diamagnetic. The latter is paramagnetic but has spin properties which are very different from those of the Cu(I1) monomers. Unlike S = 112, an S = 1 spin state is subject to zero-field splitting interactions which act to shorten the electron spin relaxation times and consequently sharpen the NMR spectra. The proton NMR spectra in acetonitrile contained a set relatively narrow hyperfine-shifted resonances which exhibited antiCurie temperature behavior. Magnetic moments of the complexes were measured in solution by the NMR (Evans) method, and the solid state magnetic susceptibilities were measured using SQUID at variable temperatures down to cryogenic. The temperature dependence of the hyperfine chemical shifts was used to compute the Heisenberg exchange coupling constant (4 in solution. The measured value of J was consistent with the observation of anti-Curie temperature dependence of the 'H chemical shift

Nuclear Magnetic Resonance

492

22

23

data. The hyperfine chemical shifts were assigned and the ‘H relaxation mechanism discussed. Corsi et al. 89 have analysed hetero-binuclear and -trinuclear complexes in which Fe(II1) is linked to one or two Cu(I1) ions by bridging cyanide ligands. The ‘H, 2H and 19F spectra are reported along with solution susceptibilities (NMR Evans’ method). The NMR spectra indicate that the iron is low-spin, and that the Fe(II1) and Cu(I1) ion(s) are ferromagnetically coupled. The ferromagnetic coupling has the effect of shortening the electron spin relaxation times, which leads to relatively highly resolved NMR spectra. Abbati et al. have prepared two novel polynuclear manganese complexes, of which one is an S = 17/2 heptanuclear species consisting of a six-membered manganese crown ring comprised of two Mn(I1) and four Mn(I1I) ions, plus a central Mn(I1). Proton NMR indicated that the valences are localized solution. Wright et al. 91 have prepared several dinuclear Mn(II1) complexes, for which they reported ‘H NMR spectra. The acetate resonances which occurred downfield in the region 58-80 ppm, was found to correlate strongly with the Heisenberg exchange coupling constant J. Cotton et al. 92 have prepared paramagnetic linear tricobalt compounds (24) for which the ‘H (1-D and COSY) and 13C NMR solution are reported. The solution magnetic susceptibility was measured by NMR as a function of temperature between 193 and 308 K. The magnetic susceptibility increased with increasing temperature, an observation which was modeled as an S = 112 to S = 5/2 spin cross-over transition.

CI-c o - b -

co- CI

24

25

The chiragen ligand family is based on structure (25). Murner et have shown that in the presence of divalent metal ions, dinuclear helical metal complexes with predetermined chirality form spontaneously in solution. The

493

14: Paramagnetic N M R

complex structure is a triple helix with a meta1:ligand stoichiometry of 2:3. CD and NMR showed the complex is homochiral. From the temperature dependent hyperfine chemical shifts and the solution magnetic susceptibilities, Charbonniere et al,94 found that a di-iron(II1) complex in this family exhibits spin-cross-over behavior, while the Fe(I1)-Co(I1) complex and a mono-iron(II1) complex exhibited simple Curie behavior in the H hyperfine shifts. NMR has been used to study the spin state behavior of other di- and trinuclear iron-containing complexes. Weyland et al. 95 have characterized spin states of unusual bi- and trinuclear Fe(II1) complexes in which the metal ions were bridged by 1,3-diethynylbenzene and 1,3,5triethynylbenzene as spacers. These complexes are of potential interest organic ferromagnets. Their spin state structures in both ground and excited electronic states were studied using NMR, ESR, and SQUID techniques. Janiak et al.96 and Kersting et al.97 have studied di-iron(I1) and di-iron(II1)-containing species with sulfur-bridged metal centers with respect to their spin-cross-over behavior. Magnetic susceptibility measurements and proton TI measurements were used in conjunction with Mossbauer and IR spectroscopy to demonstrate, in the case of the diiron(I1) species, the existence of a solution equilibrium between the low-spin and high-spin forms that shifts toward high-spin with increasing temperature. At room temperature, the concentrations of the high- and low-spin states were approximately equal. 4.2 Mononuclear Complexes. - Spin cross-over behavior has also been studied9*in solutions of mononuclear Fe(I1) complexes through measurements of the paramagnetic susceptibility using the NMR (Evans) method. Sour et a1.99 have reported a novel photoinduced spin change in the mono-iron(II1) complex (26). Trans to cis isomerization in one of the photosensitive ligands induced a partial spin state change from high- to low-spine in the Fe(II1) ion. Koch et a1.lo0 have reported NMR chemical shifts and other spectral properties of low-spin Fe(111) semiquinonate complexes. Chmielewski and Latos-Grazynski'O' have prepared two highly novel 0organo-Ni(I1) species for which 'H and 2H NMR spectra are reported. One is

26

27

494

Nuclear Magnetic Resonance

a mono-o-phenyl compound, (02TPP)Ni1*(Ph)Br, the other a bis-o-phenyl compound (27), (02TPP)Ni11(Ph)2, (02TPP = dioxatetraphenylporphyrin). Both of these compounds exhibited upfield-shifted 'H NMR patterns with no parallel to any nickel(I1) porphyrin or heteroporphyrin, with hyperfine shifts in the range - 165 to - 31 ppm. These shifts indicated a change in the nickel(I1) -y~)2ground state from (dxy)2(dxz)2(dyz)2(dz~) l(d,z - y ~ ) I to the (dx,>2(dx2 (dxz)2(dz2)'(dxz)1 and (d,y)2(dx2-y2)2 (dz~)2(dyz)1(dxz)1 configurations for the mono- and bis-phenyl adducts, respectively. Fettoruso et aZ.lo2 have reported * H NMR spectra of a paramagnetic porphyrin peptide compound, Fe(II1)-Mimochrome I, based on a helixporphyrin-helix sandwich, prototype of a new class of heme protein models. Proton NMR demonstrated the existence in DMF solution of two hexacoordinated isomers, A and A, which were characterized. Shih et aZ.'O3 used 31Pand lH NMR to study solution complexes between a macrocyclic Ni(I1) chelate and the N-7 position of a single, isolated extra-helical guanine resonance of DNA. 4.3 Compexes of Pharmacological Interest. - Several NMR studies of the binding or paramagnetic metal ions to pharmacological agents have been reported. Gentamicin, an aminoglycoside antibiotic in the same family as Streptomycin, forms complexes with both Fe(I1) and Fe(II1). These complexes have been studied by Priuska et a1.1°4 with the objective of characterizing the spin states of iron. Solution magnetic susceptibilities, determined by the Evans method, suggested that high spin S = 512 and low spin S = 1/2 Fe(II1) species coexist in solution. Proton TI'S were also consistent with a mixture of spin states in solution. Streptonigrin is a quinone-containing antibiotic produced by Streptomyces fEoccuZus that requires bound divalent metal cofactors for full activity as an anti-tumor agent. Wei and Minglo5 have studied the Fe2+, Co2+,and Yb3+ complexes of streptonigrin by 2-D 'H NMR and by 'H relaxation. These studies showed the site of metal binding to be a quinolinequinone picolinate moiety. Experiments which monitored the hyperfine-shifted 'H resonances of the Co(I1) upon addition of calf thymus DNA revealed Co2+-Streptonigrin binding to DNA. Gaggelli et al. lo6 have studied the solution structure of the Cu(I1) complex of amikacin, an aminoglycoside antibiotic effective against gram-negative bacteria. 13Crelaxation was used to probe the Cu(I1) binding site. Comparison of the metal-free and Cu(I1) complexes demonstrated a gross conformational change upon metal binding. Several reports of solution NMR studies of paramagnetic metal-nucleotide complexes have appeared. Shtyrlin et al. Io7 used solvent proton NMR relaxation in conjunction with 'Hand 31Phigh resolution NMR and with spectrophotometry to probe the structures of solution complexes of ATP with Co(II), Ni(II), and Cu(I1). Stepanek et a1.'08 have used 13C and 31PNMR to study Ni(I1) complexes with AMP, and Du et a1.Io9have used 'H and 31PNMR to study Fe(II1)-ATP binding.

14: Paramagnetic N M R

495

Kingry et aZ.’*O have used ‘H and 2H NMR to study metal binding and solution equilibria in Cr(II1) pyridinecarboxylate complexes in solution. have used 13C NMR in conjunction with UV-vis spectroLedesma et al. scopy to map the binding sites of Cu(I1) to two aromatic a-hydroxy hydrazones.

5

Porphyrins

Chiroporphyrins (28) possess asymmetric centers bound to the meso carbons and close to the porphyrin cores. They are currently of interest as potential asymmetric catalysts in atom-transfer reactions, enantioselective receptors of chiral axial ligands, or chiral NMR shift reagents. Mazzanti et aZ.l12 have synthesized and determined the structure of the Fe(II1) tetramethylchiroporphyrin (28), which is strongly ruffled and domed with a five-coordinate Fe(II1) bearing a chloro ligand. The ‘H NMR spectrum is consistent with C2 symmetry and S = 5 /2 spin state. The pyrrole proton resonances are shifted downfield to 80-100 ppm at 293 K, more than in the corresponding tetraaryl derivatives. The cyclopropyl protons H1 appear at 160-200 ppm, in keeping with the nearly perpendicular orientation of the CI-HI bond with respect to the porphyrin mean plane. The temperature dependence of the ‘H NMR spectrum suggests substantial zero-field splitting.

28 R =

AMe

29 X = O , S e

Me

Me

Pacholska et al. l 3 have synthesized and studied the ‘H NMR and ESR properties of Ni( 11) complexes with the partially-saturated porphyrin (29) (a hydrocorphin), in which Ni(I1) has a high-spin d8 configuration (S= 1). The 300 MHz proton NMR spectrum was assigned based on intensities, linewidths, methyl substituent effects, and selective deuteration at the P-pyrrole positions. From the paramagnetic shifts is was concluded that both (X=O,Se) derivatives were in a (dxy)2(dxz)2(dyz)2(dxz_y’)l(dz~)* configuration. Curie plots of the proton shifts did not extrapolate to the resonance positions in the diamagnetic reference compound, an observation that was interpreted as due to a T - * dipolar contribution arising from zfs anisotropy. Mnr11-tetra(4-sulfonato-phenyl)porphyrin (Mn-TPPS) is a water-soluble porphyrinate which exhibits unusually high proton relaxivity and has conse-

496

Nuclear Magnetic Resonance

quently been studied as a potential MRI contrast agent. Bradshaw et ~ 1 . " ~ have prepared two new water-soluble Mn"' porphyrinates, TPPAS (30) and TPPIS (31), which they studied by proton relaxometry. The proton relaxivity of TPPAS at pH 7 was only about half as large as for Mn-TPPS, and in vivu studies on rats showed too high a toxicity for clinical use. Variable temperature high resolution proton NMR exhibited proton tautomerism in the two HPpyrrole proton peaks. Also, Shi et al. 1 1 5 have published NMR spectra of a MnIII porphyrinate in which the mesu substituent is p-hydroxyphenyl.

In an analysis of the proton relaxivity due to Mn-TPPS, Bryant et ~ 2 . " ~ have noted that an explanation of the large observed proton relaxivity of this complex requires the assumption of an unphysically short distance between the paramagnetic spin and the protons of bound water molecules. They therefore explored the possibility that the short apparent electron-proton distance may be due to the effects of electron spin delocalization into the porphyrin orbitals. A modified Mn-TPPS complex was synthesized in which six bromine atoms were attached to the P-pyrrole positions of the porphyrin. The low-field relaxivity of Mn-TPPS-Br6was found to exceed that of Mn-TPPS, a result that was attributed to effects of electron spin delocalization.

5.1 Effect of Axial Ligation on Spin State. - The high-spin (S = 5/2) and lowspin (S = 1/2) configurations of heme moieties are close in energy and spin-state transitions can in appropriate cases be induced by changes in the axial ligand or the peripheral substituents.' l 7 When the spin state spacing is comparable to kT, an equilibrium mixture of spin states can occur. Distinct from this latter possibility, heme groups can, when liganded by weak field counterions, exhibit an intermediate, quantum mechanically-mixed, S = (3/2, 5/2} spin state. The simultaneous presence of multiple spin states in a thermal equilibrium sample leads to non-linearity in Curie-Law plots of isotropic 'H chemical shifts. Nesset et al. have studied the temperature dependence of proton chemical shifts in a variety of phenyl-substituted five-coordinate Fe(II1) tetraphenylporphyrinates, in which the axial ligand was perchlorate. The temperaturedependence of the 'H chemical shifts was interpreted in terms of two thermally-populated spin states, the energy separation of which was deter-

14: Paramagnetic N M R

497

mined in the analysis. The authors advise that conclusions concerning spins states should not be based on measurements at a single temperature. Munro et a1.119 have studied a proteolysis fragment of cytochrome c, in which the spin state is a thermal equilibrium mixture of a ground S = 1/2 state and a slightly higher energy quantum-mechanically admixed S = (3/2, 5/2} state. pH-dependent changes in paramagnetic susceptibility were measured using 'H NMR (the Evans Method), in which the paramagnetic sample and diamagnetic reference solution are contained in coaxial quartz sample tubes. Changes in the effective magnetic moment were monitored using a combination of Mossbauer, ESR, SQUID, and NMR susceptometry methods. Smyrnov et have used 19F NMR to characterize the spin-state modulation of Co(I1) in the Co"-porphyrin (32), which is fluorinated on the pyrrole (or pyrrole and phenyl) positions. Two axial ligands of this complex were studied, one containing the o-donor tetrahydrofuran, another containing 1-methylimidazole as an axial ligand. The imidazole ligand occurs in a complex where Co" is six-coordinate and high-spin in the F2g-TPP complex. A high spin state is indicated by the large down-field chemical shift (280 ppm) of the P-19F resonance. This is the first example of high-spin Co" in a porphyrin complex. Proton NMR, used in conjunction with ESR, provides a useful method for identifying spin state changes in iron porphyrins as well. Working with a five-coordinate high-spin Fe(II1) porphyrinate, Song et al. 121 observed a high-spin to low spin transition upon the addition of a thioacetate ligand. Proton NMR and ESR were also used122to demonstrate the S = 5/2 spin state of iron in a pentacoordinate Fe(II1) isocyanate 'picket fence' porphyrin complex.

32 R = CgH5or &F5

have studied nine new high spin Fe(II1) porphyrinates in Ueyama et which the axial ligand is a thiolate, hydrogen-bonded to an amide group (-S-- .H-N). These compounds were synthesized as models of the active site of cytochrome P-450, as well as of thiolate ligands of the iron-sulfur clusters in Fe-S proteins. The 'H NMR spectra were characteristic of five-coordinate high-spin Fe(1II) porphyrinates are reported. The peak of the amide proton was unobservable in the 270 MHz proton NMR spectrum, but the corresponding 2H NMR peak was observed in partially deuterated samples. have studied 'strapped' porphyrins (33), in which the highPrevot et spin Fe"' ion is ligated axially by I3C=O. Their objective was an under-

498

Nuclear Magnetic Resonance

standing of why the ratio of dioxygen binding affinity to CO binding affinity is less in hemoglobins and myoglobins than in model compounds. Evidently, the 0 2 ligand is stabilized, or the CO ligand is destabilized in the proteins. It has been suggested that hydrogen bonding to a near-by amino acid residue stabilizes the relatively polar Fe-02 bond relative to the relatively unpolar Fe-CO bond. The substituents X and Y were varied to provide (or otherwise) the requisite hydrogen bonding capacity. 3 C = 0 chemical shifts were measured and found to correlate with C=O stretching frequencies. It was shown that an OH substituent in the strap was able to form a strong hydrogen bond with a methoxy axial ligand of Fe(II1). The authors conclude that hydrogen bonds are responsible for the large observed range of 13C chemical shifts in the ligand. have reported 'H NMR spectra of a paramagnetic Fettoruso et porphyrin peptide compound, Fe(II1)-Mimochrome I, based on a helixporphyrin-helix sandwich, prototype of a new class of heme protein models. Proton NMR demonstrated the existence in DMF solution of two hexacoordinated isomers, A and A, which were characterized. Fe(II1) porphyrins with axial imidazole ligands exhibit a large range of chemical shifts of the heme methyl protons. This phenomenon is thought to reflect the influence of the orientation of the plane of the imidazole ligand in stabilizing different electronic configurations of Fe(III), a phenomenon associated with large asymmetric spin distributions on the peripheral carbons. Internal rotation of the axial imidazole ligand can usually be frozen out at low temperature, or, alternatively, hindered by bulky substituents at the meso positions of the porphyrin. Nakamura et a1.126have reported a detailed 'H NMR and ESR study of several low-spin Fe(II1) porphyrinates that possessed bis( 1-methyl) imidizole axial ligands and were substituted at the meso positions with a series of increasingly bulky alkyl moieties ( R = H , Me, Et, i-Pr). The effect of imidazole orientation on Fe(II1) electron configuration, manifested in the 'H chemical shifts and in the ESR spectra, were studied. The isopropyl meso-derivative exhibited the unusual electronic configuration, (dxz,dyz)4-dxy' on Fe(III), while the H-, Me-, and Et- derivatives exhibited the (dxy)2-(dxz,dyz)3 configuration. Further investigating these phenomena, Nakamura et al. 127 observed the (dxz,dyz)4-dxy1 configuration in a low-spin Fe(II1) porphyrinate which contained aryl meso substituents and dicyano axial ligands. The cyano ligands are believed to stabilize the (dxz,dyz)4-dxy1 electron configuration by an interaction between the iron(dn) and cyano(pn*) orbitals. The authors proposed that the electron donating ability of the meso-aryl groups also stabilizes this configuration. Proton chemical shifts provide information about the electronic configurations of Fe(111) porphyrins. In heme proteins, the dipolar magnetic susceptibility tensor of the paramagnetic center provides information that can be used to determine the orientation of the heme moiety within the heme cavity. Chemical shifts of the protons attached to the heme itself contain large contributions arising both from the isotropic Fermi contact interaction and from the dipolar shielding tensor. Since the former are difficult to calculate

14: Paramagnetic N M R

499

from Frontier molecular orbitals, proton chemical shifts do not provide useful information concerning the dipolar shielding tensor. C-13 chemical shifts, on the other hand, have much larger contact than dipolar contributions, and thus they are useful in developing a quantitative picture of spin distribution on the heme that is useful in orienting the principal axes of the dipolar tensor. Louro et aZ.12* have used lH-13C multiple quantum NMR to measure the 13C chemical shifts of a substituents in five different tetraheme ferricytochromes c3. The chemical shifts were assigned and aiialysed in terms of a model of the n molecular orbitals of the heme under perturbed D4 symmetry, which yields the orientation of the rhombic perturbation and a splitting parameter AE. The analysis can be used to locate the principal axes of the paramagnetic susceptibility tensor, and it also provides orientational constraints for the axial imidazole ligands.

6

Spin Labels and Spin Probes

6.1 Probes of Surface Exposure. - Water-soluble spin-labels can selectively broaden the NMR resonances of nuclei in prote!ns or on lipid bilayers to which they approach reasonably closely ( ~ 2 A). 0 Scarselli et aZ. 129 have described a very careful study in which they use TEMPOL as a probe of surface accessibility of the small protein Temdamistat, a 73 residue protein which for which the structure and assignment of the proton NMR spectrum were known. The analysis utilized the TOCSY NMR spectrum, for which a data set consisting of 90 attenuations of cross-peak intensities by the spin-label was constructed. The authors expected that paramagnetic effects at different surface sites might be affected by steric hindrance of the approach of the label and/or the mobility of amino acid side-chains at the surface. Two observed quantities were defined and measured at each residue: (1) the cross-peak attenuation, and (2) the exposed surface area. Based on this analysis, a surface map of the protein was drawn showing the patch that is principally affected by the spin-label. Arumugam et al. 130 have studied the exposed surfaces of the 173 residue catalytic domain of human matrix metalloproteinase 3 in the presence and absence of a 126 residue inhibitory peptide (human TIMP-1). As an amidedirected surface probe, they used Gd-EDTA, and monitored the effects of this probe on amide proton linewidths. The derived map of surface coverage in the complex was consistent with an X-ray structure of the same complex. Beswick et aZ.l3I have developed an improved method for locating resonances of membrane-embedded proteins and peptides within a bilayer membrane, Their method uses paramagnetic relaxation enhancements produced by the commercially-available spin label, 1-palmitoyl-2-stearoyl-(10-doxy1)-snglycero-3-phosphocholine in two peptides embedded in dodecylphosphocholine micelles. The use of this spin label avoids problems inherent in the use of spin-labelled fatty acids at acid pH where the carboxyl is protonated. The paramagnetic contribution to each intraresidue (H-N-C,-H) cross-peak in the

500

Nuclear Magnetic Resonance

NOESY spectrum was determined. An analysis is described for extracting the Paramagnetic relaxation contribution to cross-peak volumes in NOESY spectra. The experimental results are expressed in plots of paramagnetic relaxation enhancement as a function of residue number in the peptide. Spooner et al.132have used a nitroxyl spin label conjugated to a specific cysteine mutant of the membrane-embedded lactose transport protein (Lacs) from Streptococcus thermophilus. They used I3C CP-MAS NMR to monitor the effect of the spin label on the I3C resonance which binds to an interhelix loop of the protein. The resonance was not observed, which lead the authors to conclude that the I3C nucleus is close to the spin labels and broadened beyond detection. Chen and Marshall133used paramagnetic spin labels to study the structure of one helical segment of the G-protein receptor neurokinin- 1 embedded in phospholipid vesicles. For this purpose, they synthesized a 34residue peptide fragment which corresponds to the sixth transmembrane helix of neuokinin, labelling the aromatic side-chains with fluorine. They studied the effects of three spin labels (Gd3+, ferricyanide, and TEMPO) on the 19F resonance of the peptide embedded in vesicles. Almeida et ~ 1 . have l ~ ~used 'H NMR in conjunction with ESR and fluorescence quenching to study the partition of the two spin radicals TEMPO and TEMPOL between the aqueous and lipid phases in micellar dispersions prepared from sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium chloride (CTAC). In these systems, the Paramagnetic fragments were localized at the interface between the polar headgroup region and the hydrophobic interior of the micelle. In a similar vein, Vasil'tova et used 'H N M R to demonstrate that the paramagnetic probe, cetylviologen cationradical, is localized at vesicle surfaces in suspensions of dipalmitoyl lecithincetylviologen vesicles. Paramagnetic electron-nuclear interactions have a much longer range than nuclear spin-nuclear spin interactions and thus have potential for studying long-range structural aspects of multi-domain proteins. Biekovski et al. 136 have exploited this property to provide distance constraints which assist the locating the relative positions of different domains in a multi-domain protein. They studied calmodulin, a 148-residue calcium-binding protein that can bind up to four Ca2+ ions and is organised into two structurally similar globular domains, separated by a flexible linker. The paramagnetic lanthanide ion Tb3+, which binds at the Ca2+ binding sites, has been use to provide long-range distance constraints of the structure. Tb3+ bound to one domain produced pseudocontact shifts, as well as residual dipolar couplings of 5N resonances due to a slightly non-isotropic orientation of the paramagnetic protein in the static magnetic field. A titration of I5N-labeled (Ca2+)4-calmodulin (the holoenzyme) was titrated with Tb3+ and changes were monitored in 2-D 'H-15N HSQC NMR spectra. The results showed great promise in determining the relative orientations of the domains. 6.2 Distance Constraints. - The use of relaxation enhancements produced by spin labels to measure distances from nuclear spins to the paramagnetic center

14: Purumagnetic N M R

50 1

and as a probe of internal motions of proteins was studied by Jacob et aZ.137 They synthesized model poly-L-proline peptides of varying length with a tryptophan at one end and a nitroxyl spin radical at the other. The paramagnetic relaxation enhancement of the tryptophan protons was found to decrease, as expected, with increasing number of proline spacer units, but the distances inferred from these measurements was less, in every case, than expected for a rigid linear polypeptide structure. Molecular dynamics calculations indicated that this difference arises from molecular motions, which cause the weighted average of the dipolar distance factor, 2, to be smaller than the corresponding factor calculated from the linear distances within the rigid peptide. The insertion of a central glycine hinge into the poly-proline peptide resulted in a dramatic increase in the paramagnetic relaxation enhancements at the tryptophan. Thus internal motions can have a large influence on the electron-nuclear dipolar relaxation mechanism. G ~ c h i n has ' ~ ~ studied a ternary complex between the octamer duplex d(TTGGCCAA)2 with two molecules of the drug chromomycin-A3 and a Co(1I) cation that functions as a paramagnetic spin label. High-spin Co(I1) is a d7, S = 2 ion which possesses a highly anisotropic paramagnetic susceptibility. This produces large dipolar hyperfine chemical shifts in proximal NMR resonances, and when the magnitude and principal values of the paramagnetic susceptibility tensor are know, hyperfine chemical shifts can provide detailed molecular structure information. Lohr et al. 39 have reported detailed quantum mechanical calculations of the relations between magnetic susceptibility anisotropy, g-anisotropy, and pseudocontact chemical shifts for Co(I1). These results are applied to an interpretation of experimental data for Co(II)(acac)2(H20)2. The paramagnetic broadening due to Co(1I) is small due to the extremely short characteristic electron spin relaxation time, which permits its use in high resolution NMR experiments. When the orientation and principal values of the paramagnetic susceptibility tensor are known, the dipolar hyperfine shifts can be calculated as a function of position of the nuclear spin position with respect to the dipolar principal axis system. The full susceptibility tensor was measured from the solution magnetic moment of the cobalt complex, which in turn was measured from the isotropic shift of an inert reference relative to the shift in the diamagnetic zinc complex. A data set of 138 pseudo-contact shifts were measured in 2D DQF COSY spectra and analysed in terms of the known Co(I1) dipolar shielding cone. The structural results were used to construct the binding site of Co(I1) and chromomycin-A3 to the octamer DNA duplex. In subsequent work,140 a protocol was developed and critically evaluated for using the pseudocontact shifts in structure determination. In this procedure, the pseudocontact shifts were input as experimental restraints in molecular dynamics simulations both with and without NOE constraints. The structure determined from pseudocontact shifts had much higher precision and accuracy than the corresponding NOE-determined structure. Ramos and VaraniI4l have used proxy1 spin labels attached to doublestranded RNA to investigate the structure of a complex between the third

502

Nuclear Magnetic Resonance

double-stranded RNA-binding domain of the Staufen protein from Drosophila (dsRBD3)8 and double-stranded RNA. Double-stranded RNA-binding domains are ubiquitous modules that recognize double-stranded RNA of any sequence. The spin label was attached to chemically synthesized RNA containing single 4-thio uracyl bases at the desired positions. Line broadening in the spin-labeled complex, relative to a reduced control, was monitored in H-I 5N HSQC spectra of lSN-labeled Staufen protein. The authors estimate that resonances within ~ 1 2of the label are broadened beyond detection, while resonances within z 16 are broadened significantly. Dinesen et al. 142 have studied the dynamics of translational encounters between the PF6- anion and four water-soluble spin labels, 4-OH-TEMPO, 4NH2-TEMP0, and 3-carboxy-PROXYL. The paramagnetic spin label is the agent of intermolecular 19F relaxation in PF6-. The physical situation is related to intermolecular solvent NMR relaxation, but the theoretical treatment differs in that integration over the nuclear spins of the bulk solvent is not involved; rather, relaxation results from short range encounters of the paramagnetic center and the nuclear spin. The dynamics of this process is probed by I9F relaxometry. The results were successfully described in terms of the translational diffusion of hard spheres, with a treatment that accounts for electrostatic interactions. Micha-Screttas et al. 143 have studied the solution dynamics in interactions between between dissolved Li+ ions and several dissolved radical anions (those derived from biphenyl, naphthalene, phenanthrene, anthracene and trans-stilbene). The paramagnetic chemical shifts of 6,7Liwere measured in tetrahydrofuran solutions as a function of radical anion concentration. The results are interpreted in terms of the strength of ionpairing, which the authors describe as weak. In an NMR study of the structure of a cisplatin complex with a double stranded DNA undecamer, Dunham et a1.lM used a paramagnetic label attached to the cisplatin moiety to provide distance constraints in NMR spectra. The DNA duplex was cross-linked at a specific site (G6-C7) to a cisplatin analog which contained a nitroxyl spin-label attached to one of its amino ligands (structure 34). The paramagnetic center acts to suppress crosspeaks in NOESY spectra; this effect varies as the product of the electron and nuclear magnetic moments times Y - ~ . Because of the relatively large magnetic moment of the electron (658 times that of the proton), the electron-nuclear magnetic coupling has relatively long range ( x 2 5 compared to z 5 for 'H-'H NOE's) and thus gives rise to observable couplings in nucleic acid structures where H-'H NOE's uninformative about global structure. Addition of the long-range electron-proton restraints allowed for refinement of a duplex structure in excellent agreement with the diamagnetic NMR data but exhibiting different positioning of the duplex ends. Moldrheim et have studied the binding of divalent cations to deoxyribonucleotide oligomers. Paramagnetic perturbations of IH proton line-widths and relaxation times by Mn(I1) and Co(I1) were studied in three short deoxyribonucleotide duplexes (2 decamers and a dodecamer). Decreases of cross-peak intensities were recorded in 2D NOESY spectra, and line-broad-

A A

A

A

14: Paramagnetic N MR

503

34

35

ening was monitored inl-D protoii spectra to probe the sites of metal binding. The results are interpreted in terms of a proposed rule for sequence-selective cation binding to G-N7 in oligodeoxy-ribonucleotides, whereby binding affinity decreases in the order: 5’-GG 2G A 2GC 2 GT. Epperson et a1.146have used Co(I1) as a spin probe of the internal solution structures of dendrimers which contain specific metal recognition sites. The proton NMR spectra of two 2,6-amido pyridine dendrimers were fully assigned by a variety of 2-D NMR techniques, including NOE difference, EXSY, COSY, and TOCSY. Then the relaxation times of hyperfine-shifted resonances by the Co(I1) were used to calculate metal-proton distances and in this way to construct a molecular model of the dendrimer interior. The presence of sizable internal cavities was inferred.

6.3 Solid-state NMR. - Heise et a1.147report the NMR spectra of ‘ H , *H, 13C nuclei directly attached to nitronylnitroxyl spin labels. These experiments were conducted in the solid state using MAS-NMR, as well as in solution. The spectra were used in conjunction with ab initio calculation to map spin density in the labels. The 13C chemical shifts were spread over 1900 ppm and were well resolved, while the ‘H spectra were spread over about 60 ppm and were much less well resolved. The authors conclude that the spin distribution is determined by a polarization mechanism modulated by effects of hyperconjugation and competing polarization paths. Maruta et al. 148 have mapped spin density in the 4-hydroxy-TEMPO spin label, which is of particular interest because it has been shown to act as a molecular ferromagnet at low temperature. Spin density was determined from 2H MAS solid-state NMR spectra. The mechanisms of spin density distribution were considered, and it was likewise concluded that spin polarization, modulated by hyperconjugation, was involved. In addition, an intermolecular pathway for spin polarization, mediated by hydrogen bonds, was proposed. This proposal was supported by solution-state ‘H and 2D NMR spectra, which exhibited a spin density distribution that was radically different from that in the crystal. 6.4 Novel Spin Labels. - New spin labels have been prepared. Belton et al. 149 report the synthesis of a water-soluble nitroxyl spin label 1,1,3,3-tetrakis(trideuteromethyl)isoindolin-2-yloxyl-4-sulfonate(35) and report its ESR spectrum. Katoch et a1.150 have synthesized a novel steroidal spin label and have surveyed its properties with respect to ‘H and l3C NMR, ESR, and differential scanning calorimetry measurements in studies of biomembranes.

Nuclear Magnetic Resonance

504

6.5 Correlated NMRESR Spectroscopy. - Chemical systems containing free radical spin labels can be studied both by NMR and ESR. ESR experiments traditionally have been restricted to a relatively low range of Zeeman field strengths, but in recent years great strides have been made in the development of high-field ESR through the use either of high frequency microwaves or farinfrared radiation. These experimental advances bring the possibility of combining ESR with high-resolution NMR techniques. Heschke and Spiess' 5 1 have developed a high field ESR method for determining the relative orientation of the g and hyperfine tensors in disordered systems by correlation of NMR frequencies to the ESR fields. Simple and characteristic correlation patterns were obtained in experiments on the nitroxide spin probe TEMPOL in a polymer matrix. The pulse sequence used was based on electron-electron double resonance and spectral hole-burning techniques. This method of irradiation overcame limitations due to excitation bandwidth and deadtime.

7

Reaction Kinetics

NMR is a powerful tool for studying the kinetics and mechanisms of fast chemical exchange reactions in samples at thermal equilibrium. Paramagnetic systems, because of their large characteristic chemical shifts and, in the cases of Mn(II), Cu(II), and Gd(III), highly efficient NMR relaxation mechanisms, often exhibit dramatic chemical exchange effects in NMR. Limiting cases correspond to slow exchange, where two exchange-broadened peaks are observed, and fast exchange, where a single exchange-broadened peak is present. The sensitivity of NMR spectra to chemical exchange phenomena in paramagnetic systems can be extraordinary. For example, Gd(II1) is an extremely efficient relaxation agent toward nuclear spins, so that 'H and 1 7 0 nuclei on coordinated water molecules are relaxed several orders of magnitude more rapidly than in the bulk solvent. When the chemical exchange reaction which equilibrates bound and free waters is rapid, the free water peak exhibits a line-width and T1,2relaxation times which are largely determined by the very of water in the bound site. Also, the large characteristic small fraction ( z hyperfine shifts of paramagnetic molecules often cause, in the presence of rapid chemical exchange reactions, correspondingly large enhancements in linewidths and T2 -- In the fast-exchange region, chemical-exchange induced line broadening is proportional to the square of the chemical shift in rad s - l between the (non-exchanging) sites, and thus scales with the square of the Zeeman field strength.

'.

7.1 Chemical Exchange of Solvent Coordinated to Actinide Ions. - The actinide ions, U(IV) and Th(IV), form tetravalent aqueous cations, U(H20)104+and Th(H20)1O4+,the former paramagnetic and the latter diamagnetic. Farkas et have used 170TI and T2 relaxation measurements to study the water chemical exchange reactions of these species, and of UF(H20)93+.Water exchange on the diamagnetic Th4+(aq) cation was mea-

505

14: Paramagnetic N M R

sured by using the aqueous lanthanide ion, Tb3+(aq), as a chemical shift reagent and monitoring H2I7O line broadening. Water exchange was surprisingly rapid: k,,%5 x lo6 s-l for U(H20)104+,and ke,>5 x lo7 s-l for Th(H20)Io4+.This study is apparently the first of its kind for M4+(aq)cations. The aqueous uranyl ion, U02(H20)s2+,is diamagnetic. To measure the rate of chemical exchange between bound and free water, Farkas et al. ls3 used the lanthanide shift reagent, Tb3', as above to shift the 170resonance of bulk water with respect to that in the bound site, and measured chemical exchange rate constant from the 1 7 0 line broadening. Measurements of the temperature dependence of the rate constant were used in conjunction with ab initio calculations to probe the chemical exchange mechanism, which the authors conclude proceeds via dissociative exchange. A ternary U02(oxalate)F(H20)2complex was also studied. 7.2 Electron Self-exchange Reactions. - Redox couples often involve equilibration of a diamagnetic and a paramagnetic species, and in these cases, NMR is useful for measuring rate constants of the electron self-exchange reactions. Krylova et al. 154 have studied the rate and mechanism of electron self exchange in the CUIWCU~~L couple, using the ligand oxathiane[ 12laneS4 (36) Selfexchange rates were measured at 500 MHz using a proton resonance assigned to the protons attached to the bridgehead carbons. Measured rate constants were of the order of 106 M-' s-', which is greater than previously reported values for low molecular weight CU'-~Isystems. The authors conclude that the high self-exchange rate results from a steric factor, namely, that inversion of the sulfur donor atoms is avoided during the reaction. *CdL + Cu"L @ *Cu"L + CulL: oxathiane[12]aneS4 (L) =

cs?7 s

36

Proton NMR studies of electron self-exchange of the Cul/" couple has also been reported by Lange et al.,155using complexes of the ligand 2,2'-bis(3-(2pyridy1)-1-methyl triazoly1)biphenyl. Jameson and Anand'56 have developed an undergraduate laboratory experiment using NMR to illustrate electron selfexchange. Metelski and Swaddle157 have studied the mechanism of electron selfand exchange in the couples O S ( C N ) ~ ~ - ' ~M- ,o ( C N ) ~ ~ - ,/ ~ - w(cN)83-/4- by measuring 13Cline broadening in the diamagnetic species upon addition of the paramagnetic partner. This study builds upon previous research which indicated that self-exchange of the Fe(CN)b3-I4- couple is catalysed by cations, and that in the case of K+, three cations appear to be involved. A characteristic of this mechanism is a large positive activation volume. Whether similar behavior occurs in the self-exchange reactions of Os(CN)b3-I4-, Mo(CN)g3--/ 4 - , and W(CN)g3-14- was studied through variable pressure I3C NMR measurements in which the activation volumes were directly measured. Large,

506

Nuclear Magnetic Resonance

positive activation volumes were indeed observed. The proposed reaction mechanism involves a partially-deaquated alkali metal cation acting as a bridge between cyanometallate anions. The positive charge of the bridging cation provides a low energy pathway for the electron transfer step. In contrast, cation-cation self-exchange reactions appear not to involve a bridging anion, since in that case the negative charge of the bridge inhibits, rather than facilitates, electron transfer. Sowrey et a1.15* have studied the kinetics of the self-exchange reaction [Fe30(02CCMe3)6(py)3]f” (py = pyridine), by ‘H NMR at 270 MHz. An interesting aspect of this study is that both the oxidized and reduced species in the couple are paramagnetic. Often the paramagnetic species is not resolved in the NMR spectrum, but in this system, the protons of both py and Me are far from the paramagnetic center, and good quality NMR spectra were obtained. The redox reaction involves a reduction of one the iron atoms of the trinuclear complex. The three iron atoms are strongly antiferromagnetically coupled in both the oxidized and reduced forms of the complex. 7.3 Proton Transfer Reactions. - Momot and Walker159have used variable temperature proton NMR to study the chemical exchange kinetics of axial methylimidazole ligands on the Fe(II1) porphyrin (tetramesitylporphyrinatoiron(III)bis(2-methylimidazole). In the low energy structure, which is frozen on the NMR time-scale below 200 K, the planes of the rings of the two methyl imidazole ligands are at right angles to each other and at right angles to the plane of the porphyrin. Frozen unsymmetrical axial ligands, along with the ruffling of the porphyrin core, introduce asymmetry into this otherwise symmetric complex, and the four pyrrole protons have different chemical shifts. Chemical exchange broadening due to rotation of the ligands is observed between 200 and 270 K, and at higher temperature, the reaction is fast on the NMR time-scale. Rate constants and the activation enthalpy and entropy were determined by analyses of the chemical exchange-induced R2 enhancements in Hahn spin-echo experiments. The data were analysed using Mathematica code, written by the authors, which simulates, based on the Bloch equations, the evolution of the proton magnetisation in spin-echo experiments. The Preyssler anion, [NaP5W30O1 (37), is a remarkable heteropolytungstophosphate anion which is very stable in aqueous media. The cryptate structure was first crystallized with an internal Na+ ion on the unique axis, and later work showed that this ion could be replaced by alkaline earth or lanthanide cations. The encapsulated metal ion is coordinated by a single water molecule and lies on the five-fold axis at a position that is displaced from the center of the cryptate. Working with the Eu(II1) cryptate species, Kim et al. I6O observed 31Pchemical shifts arising from internal phosphate resonances in two different states of protonation. A protonated phosphate resonance was observed in strong acid, and an unprotonated phosphate peak was seen in less acidic solution; both peaks were observed simultaneously at intermediate pH, demonstrating that the proton transfer reaction between them is slow. This is a

B W 00

OP

0 Na'

37 The Preyssler anion

highly unusual if not unique example of slow proton transfer between two internal sites within a cryptate. That proton involved in this proton tranfer reaction also exchanges slowly with the bulk solvent phase was shown by progressive deuteration experiments. The reaction kinetics were studied using multinuclear (31P,'H and 183W)NMR on the Eu(II1) cryptate, the chemical shifts of which are very large.

7.4 Ligand Dynamics. - Slow chemical exchange where ,z, is the order of seconds can be studied by 1-D saturation transfer or 2-D EXSY experiments. Morice et al. I 6 l have used saturation transfer techniques to study the kinetics of ligand chemical exchange in low-spin bis(amino ester) Fe(II1) complexes. Separate 'H peaks were observed due to bound and free ligand. Saturation of the methyl 'H peak of the free ligand caused an intensity decrease of the corresponding peak in the bound ligand. This observation established slow chemical exchange equilibrium and provided a basis for assignment. The naturally occurring hydroxamic acid desferrioxamine B is a siderophore have studied the which is involved in microbial iron transport. Birus et kinetics and mechanism of the complexation of this compound by Cu2+ and La3+using NMR and stopped-flow techniques.

8

Biological Systems

Determination of solution structures of proteins with paramagnetic centers has historically presented a difficult problem because paramagnetic couplings to near-by nuclear spins broadens their resonances so that the scalar couplings are unresolved and decreases NOE's. However, great progress has been made in recent years in developing new methods suitable for assigning resonances near the paramagnetic center. Much of this work is the subject of recent review^,'^^,^^^ particularly on Fe-S165,166and heme proteins. A review of 1673168

508

Nuclear Magnetic Resonance

the magnetic spectroscopic (ESR, ESEEM, Mossbauer, MCD and NMR) studies of low-spin ferriheme centers and proteins has also appeared169.Some advances in assigning resonances near the paramagnetic centers are the following. 8.1 Copper Proteins. - Among copper proteins, an NMR solution structure has been reported for the plastocyanin from Prochlorothrix h o l l a n d i ~ a ' (a ~~ blue copper protein), The copper-containing nitrous oxide reductase from Pseudomonas stutzeri contains a dinuclear Cu(I1) center. Holz et al. I 7 l observed several sharp, well-resolved hyperfine-shifted proton resonances in the +60 to - 10 ppm range. The temperature dependence of these resonances was measured and found to exhibit anti-Curie behavior. Fits of the temperature-dependent chemical shift data provided the energy separation between the ground (2B3,) and excited (2B2u)electronic states of the Cu(I1) center. The same group 72 studied the dinuclear Cu(1I) center in the hyperactive Cu(I1)substituted aminopeptidase from Aeromonas proteolytica. In this system, the hyperfine shifted resonances were again highly resolved but exhibited Curie, rather than anti-Curie behavior. The temperature-dependent chemical shift data was fit to give the Heisenberg exchange coupling constant J. have probed the structure and spin properties of the Donaire et electron transfer protein azurin (from Pseudomonas aeruginosa) by substituting the Cu(I1) site with Co(I1) and Ni(I1). The Co(I1) ion, in particular, produces large pseudocontact shifts in H resonances near the paramagnetic center. These shifts have been used to determine the orientation and principal values of the paramagnetic susceptibility tensor, which was approximately uniaxial. Pseudocontact shifts of resonances near the z principal axis, corrected to constant distance, were large and negative, while distance-corrected shifts of resonances near the transverse plane were smaller and positive. Contact shifts of ligands resonances of the Co(I1) ligands were analysed in terms of spin polarization mechanisms. In a subsequent study,174 proton relaxation of Co(I1)-substituted azurin was used to demonstrate that the Co(11) coordination is tetrahedral in the unfolded state of the protein and to identify the Co(I1) ligands. Proton NMR spectra of the Co(I1) and Ni(I1)-substituted forms of the Type 1 blue copper protein amicyanin have likewise been collected and assigned.175 A comparison of isotropic shifts of ligand resonances showed that the Cu(I1) coordination is very similar in azurin and amicyanin. Battistuzzi et al. 176 have used hyperfine-shifted 'H and I3C resonances to probe spin delocalization on Cu( 11)-ligands in blue and purple copper proteins. Venerini et al. 177 have studied a Cu(II)/Co(II) derivative of the Cu(II)/Zn(II) superoxide dismutase from Photobacterium leiognathi. The hyperfine-shifted ' H NMR resonances demonstrated that the Cu(1I) and Co(I1) ions are linked by a bridging amidazole ligand. NMR relaxation due to Co(1I) results from both dipolar and scalar interactions. Ray e t al.l7* attempted to differentiate the dipolar and scalar T I contributions for the 3 1 P resonance in Co(I1)-ADP complexes. The scalar contribution was found to be negligible.

509

14: Paramagnetic N M R

8.2 Fe/S Proteins. - Several solution structures of Fe-S proteins have been published. These include the parsley [2Fe-2S] ferredoxin, 79 the oxidized [3Fe-4S] ferredoxin I1 from Desulfovibrio gigas, 8o the oxidized Desulfovibrio africanus ferredoxin I; 18' an artificial [8Fe-8S] protein variant of Bacillus schlegelii [7Fe-8S] ferredoxin; 82 the reduced Clostridium pasteurianum rubredoxin; 83 the [4Fe-S4] ferredoxin I from Desulfovibrio desulfuricans norway. i84 Xia et al. 185 have assigned the hyperfine-shifted resonances a Reiske ferredoxin that is a component of the toluene 4-monooxygenase system. Because the short relaxation times in this protein precluded the use of NOESY spectra, selective isotopic labeling was used.

'

'

8.3 Heme Proteins. - Paramagnetic interactions in heme proteins provide important and detailed information concerning the orientation of the heme moiety within the heme pocket, and the distribution of spin density at peripheral sites on the heme. The latter is determined by the molecular orbital which contains the unpaired spin, and thus NMR spectroscopy is useful for determining the electronic state of the heme. Detailed reviews of recent work are cited above. A number of studies have been reported on the effects of heme orientation and of ligand orientation with respect to the heme.18671*7 Nguyen et a1.'88 report the determination of the anisotropy and orientation of the paramagnetic susceptibility tensor of sperm whale metmyoglobin cyanide. This analysis was based on a comprehensive assignment of the proton resonances of the heme cavity, used in conjunction with the crystal coordinates of carbonyl myoglobin. Knowledge of the pseudocontact chemical shift contribution provided a quantitative determination of the contact hyperfine shifts, which were used to probe spin density distribution on the heme. Tsan et al.lg9 have achieved extensive 'H and 15N assignments of ferrocytochrome c'. The protein was shown to contain an S = 2 high-spin Fe(I1) center. The orientation and anisotropy of the paramagnetic susceptibility tensor were determined and used to decompose the measured hyperfine chemical shifts into contact and pseudocontact contributions. The pattern of contact shifts was found to be highly unusual, exhibiting two-fold symmetry due to interactions between the molecular orbitals of the porphyrin macrocycle and the the axial imidazole ligand. In related theoretical work, Shokhirev and Walkerlg0 have reported Huckel calculations of the effect of the axial ligand nodal plane orientation on the contact and pseudocontact shifts of a symmetric octamethylferriheme center, and Bertini et al. 191 have proposed a heuristic equation to describe the same phenomenon. Boyd et al. 192 have measured 'H and 15N hyperfine chemical shifts along the backbone of cytochrome c, comparing oxidized and reduced forms of the enzyme. They conclude that 15N redox shifts provide a highly sensitive probe for monitoring small cooperative structural and energetic alterations that could play a role in interprotein electron transfer. Ohmura et al. i 9 3 also report an N M R study of oxidized and reduced "N-labeled cytochrome c. Based on their observations of the redox-dependence of the linewidths of the compo-

510

Nuclear Magnetic Resonance

nents of the l5N-lH doublet, they propose a new criterion for assigning the imido protons of coordinated imidazole ligands in heme proteins. Wang and Lu194have used 1-D and 2-D proton NMR to locate the Mn(I1) center within an engineered Mn(I1)-cytochrome c peroxidase (a protein which mimics Mn(I1)-peroxidase). A full assignment of hyperfine-shifted resonances from the heme pocket as well as of catalytically-relevant amino acids was achieved. Mn(I1)-induced line-broadening was used to locate the Mn( 11) ion within the heme pocket. Ohmura et a1.19' report a 'H NMR study which compares the properties of two forms of myoglobin, related by a 180" rotation of the heme group in the heme pocket. Water-protein interactions have been studied in native and partially-unfolded equine cytochrome c by solvent proton relaxometry. 196 Two pathways of solvent proton relaxation were demonstrated, one involving exchangeable protons, primarily of lysine side-chains, and a second from 'long-lived' water. The effect of the denaturant 3M guanidinium chloride on the exchangeable pool is described. Ferricytochrome c can bind to a wide range of ligands including pyridine and its methylated derivatives, which displace Met-80 and ligate to the iron. The kinetics and equilibria of these reactions have been studied by 2D EXSY spectroscopy,197 Solid-state 2H NMR studies of deuterium-labelled cytochromes have also been reported. Liu et aZ. 19* have measured hyperfine-shifted 2H MAS-NMR spectra of a deuterated imidazole ligand bound to the high-spin Fe3+ state of cytochrome c peroxidase. Information was obtained concerning ligand dynamics. Kim et al,199have used solid-state 2H and 31PNMR to study the association of ferrous and ferric cytochrome c with cardiolipin in model phosphatidylcholine bilayers. Cytochrome c is a highly basic water-soluble electron transfer protein which binds to sites on the inner mitochondria1 membrane, and it is proposed that cardiolipin may be involved in this phenomenon. Cytochrome c binding was monitored by 2H ss-NMR of lipids specifically deuterated at the glyceryl moiety. 8.4 Mn(I1)-containing Proteins. - NMR studies of two Mn(I1)-containing proteins have been reported. Banci et U Z . ~ * ~have used water proton relaxometry to study the CN - adduct of the Mn(I1)-depleted manganese peroxidase in experiments where Mn( 11) was progressively reintroduced into the sample, An observed T1 enhancement demonstrated that Mn(I1) binds to the enzyme. Oxalate did not affect the enhancement, demonstrating that a ternary Mn(I1)oxalate-protein complex is not formed, and that oxalate does not remove Mn(I1) from the protein. In contrast, oxalate did remove Gd3+ from the enzyme. Raghunathan et aL201 have used 13C NMR relaxation to characterize Mn(I1)-nucleotide complexes with yeast 3-phosphoglycerate kinase. 3C Ti measurements were conducted on the uniformly 3C-labeled Mn(I1)ATP and Mn(I1)ADP complexes with the protein at three 13C resonance frequencies, 75.4, 125.7, and 181 MHz. The I3C resonances were well-resolved, and the

14: Paramagnetic N M R

51 1

relaxation data yielded distances from Mn(I1) to all ten chemically-distinct carbons.

8.5 Advances in Methodology Related to Paramagnetic NMR of Proteins. Wilkens et al.202 have examined, through density function calculations, the validity of the point dipole approximation in Fe/S proteins. They studied a uniformly 5N-labelled Rubredoxin from Clostridium pasteurianum, a protein which contains a single [FeS4]center and has a molecular weight of 6400. With direct observation of the 15N resonance, Ti's of the amide nitrogens were measured by inversion/recovery. The objective of the work was to account for the break-down of the point dipole approximation in the analysis through a calculation of effective interspin distances which properly account for the delocalization of spin density onto ligand orbitals. To calculate the spatial distribution of the electron spin magnetic moment, density functional calculations of the spin-differential field gradient tensor were undertaken. The expectation values of the electric field gradient operator have the same angular dependence as the magnetic single dipole operator, and the spin-only, spindifferential field gradient tensor has eigenvalues which correspond to the expectation value of the electron magnetic dipole operator. The expectation values of the spin-differential field gradient tensor were calculated from density functional wave functions at the B3LYP/ 6-311G** level. This calculation yielded expectation values for effective distances between nuclei and the delocalized spin density. The use of these effective distances in the SolomonBloembergen equation in place of distances measured from the crystal structure greatly improved the correlation for a plot of experimental relaxation rates versus r - 6 for I5Nresonances Walkert6*has reviewed and assessed the optimization of 1-D and 2-D NMR experiments on paramagnetic proteins. The use of gradient pulses and ROESY spectroscopy are recommended as being particularly effective for the accumulation of high quality spectra. Bondon and Mouro203have described a simple yet effective method for recording spectra of resonances near the paramagnetic center of paramagnetic proteins. They applied broadband homonuclear decoupling (or Waltz16) to the proton region and acquired after a short delay. The paramagnetically-perturbed resonances relaxed faster than the unperturbed protons, and thus this technique, which they refer to by the acronym PASE (PAramagnetic Signals Enhancement), effectively suppressed peaks in the complex diamagnetic region. PASE background suppression was incorporated into NOESY spectra, for which examples with sperm whale myoglobin are given. Todorovic et aZ.204have reported a novel method of distance measurement paramagnetic proteins. They studied the drug isoniazid ((38),an anti-tuberculosis agent), bound at the active site of KatG, a catalase-peroxidase of Mycobacterium tuberculosis consisting of two 82 kD subunits, each containing an Fe(II1)-protoporphyrin IX cofactor. Using ISN-labeled isoniazid, they measured the amide N---Fe(III)distance from a determination of the 15N relaxation time. A direct inversion-recovery was not feasible, even with the

Nuclear Magnetic Resonance

512

38

isotopically-labelled substrate, because of low sensitivity. Indirect detection of 15N through the attached proton resonances was not feasible because of rapid proton exchange with the solvent, and indirect detection through the the phenyl protons was precluded because of the very small scalar coupling constants, which required very long evolution times that were precluded by the paramagnetic center. A novel method was therefore developed based on the detection of proton-nitrogen two-spin order of the amide "N and the phenyl protons. An analysis is given showing that the 15N TI in a paramagnetic complex can be extracted from the proton Tl's and the zz-order of remotely coupled spins. From this measurment and the Solomon-Bloembergen equation, they determined the 15N-- .Fe(III) distance. A related approach to the measurement of I5N self-relaxation rates, R1 and R2 or RIP, in a paramagnetic protein was described by Felli et who measured the mobility of I5N labeled proteins through measurements of the cross-correlation rates that govern the conversion of Zeeman order of an amide 15N nucleus into longitudinal two-spin order involving the amide 15N and 'H nuclei. The rate of interconversion between Zeeman order and 2-spin order is due to cross-correlation between fluctuations of different interactions and is not affected by a variety of relaxation mechanisms that contribute to the self-relaxation rates. The methods were applied to the reduced High-Potential Iron-Sulfur Protein is0 I from E. halophila. Boisbouvier et aL206have described a new kind of correlated 15N relaxation experiment that has the potential of providing long-range distance constraints in paramagnetic proteins. In systems where the electron spin relaxation is rapid compared to Brownian reorientation, the paramagnetic center produces a local field at near-by nuclear spins due to Curie magnetisation (ie., equilibrium magnetisation averaged over the electron spin states). The correlation between the Curie field and the 15N-'H magnetic dipolar field produces differential relaxation rates in the two components of the 15N-IH spin doublet. This effect 9)h3for the different amide nitrogens was used to measure the quantity P~(COS of ferrocytochrome c', for which the solution structure is known. ( P ~ ( xis) the second-order Legendre Polynomial, 0 is the angle defined by the spin triangle (N-H-S), and S is the electron spin; r is the H-S distance). In demonstration experiments, a sensitivity-enhanced TROSY pulse sequence was modified to include a transverse 'H relaxation period prior to detection. The relaxation decay of the individual doublet lines was monitored by recording a series of 2D IH-15N correlation spectra. Intensity ratios measured as a function relaxation delay for 91 N-H cross-peaks confirmed the theoretical analysis. In paramagnetic molecules, correlations between Curie spin-'H magnetic dipole-dipole coupling and I H- H dipole-dipole coupling can induce cross-

14: Paramagnetic N M R

513

relaxation leading to 'relaxation-allowed' coherence transfer between the two nuclear spins. Desvaux and Gochin207 have demonstrated these effects in a complex containing a fragment of double-stranded DNA, two chromomycin molecules, and a Co(I1) ion. Relaxation-allowed coherence transfer produced characteristic signals in DQF-COSY, but not in TOCSY, experiments. The analysis of these phenomena is discussed. The interaction between Curie spin magnetization and the Zeeman magnetic field can produce a small degree of residual orientation in paramagnetic molecules, an effect which increases as the square of the Zeeman field strength. At very high Zeeman fields, tensor quantities, including hyperfine chemical shifts, are expected riot to average to the value measured in isotropic solution. Bertini et aL2O8 report an experimental investigation of these effects in which the 'H hyperfine shifts of Dy(II1)-DOTA- were compared to the 'H hyperfine shifts of Ni(II)(bipy)32'. Dy(II1)-DOTA- is magnetically highly anisotropic, Ni(II)(bipy)32' essentially isotropic, so that a magnetic field dependence was expected for former but not latter. The hyperfine chemical shifts were measured as a function of temperature in 500 MHz and 800 MHz spectrometers, and a significant field dependence (a few percent maximum) was found for certain resonances of Dy(II1)-DOTA- but not for Ni(II)(bipy)32'. A quantitative description of the effect is given. In solid-state 2H NMR, Curie spin magnetisation causes serious distortion of the spectral lineshapes relative to the reference diamagnetic system. In a diamagnetic system, the quadrupolar lineshape of the S = 1 deuterium nucleus is a Pake doublet, consisting of two mirror-image transitions symmetrically reflected about a center frequency. In the presence of the Curie dipolar field of a paramagnetic ion, the transition frequency depends on a sum of dipolar and quadrupolar interactions, which depends in turn on the relative orientations of the dipolar and quadrupolar tensor axes. As a consequence, the lineshape contains information about electron-nuclear couplings. For systems in fast conformational exchange, severely distorted lineshapes are expected. Lee et al. 209 have obtained 2H MAS-NMR spectra of various deuterated complexes of V(III), Cu(II), Mn(II1) and Fe(III), and have confirmed this expectation. Quantitative analysis of the spectra provided dynamical and spin hamiltonian information. The general features of the spectra were explored in spectral simulations of model compounds. Dangi et a1.210have reported a detailed study of the use of 15N TI and T2 relaxation to probe redox-induced changes in the backbone mobility in rat cytochrome b5. 15N relaxation data sets were collected and analysed at two magnetic field strengths, 11.7 T and 17.6 T (500 and 750 MHz *H).The motion of the protein was taken to be anisotropic and was modelled in both the oxidized and reduced oxidation states by an axially-symmetry rotational diffusion tensor, an assumption which provided a satisfactory fit of the data. The analysis accounted for paramagnetic relaxation by the heme center. The contribution of changes in the backbone mobility to the total redox-dependent entropy change of the protein was calculated and found to account for most of AS.

514

Nuclear Magnetic Resonance

Grobner et aZ.211describe an NMR method for probing the location of membrane-embedded components NMR nuclei with respect to the membrane surface through the measurement of NMR relaxation enhancements produced by Dy3+ ions bound at the membrane surface. The technique, which involves CP-MAS ss-NMR using pelleted membrane dispersions, was calibrated in experiments in which the 13C intensities of the membrane lipid (dimyristoylphosphatidyl choline) was measured as Dy3+was titrated into the sample. The intensities dropped with added Dy3+, apparently the effect of paramagnetic relaxation enhancment; at 5 mM Dy3+, the 13C intensities correlated with position along the lipid backbone. The method was tested using samples in which the 50 residue-long M 13 coat protein, 3C labeled at carbonyl of Val-29 and C, of Val-31, was embedded in the bilayer. No paramagnetic effects were observed in these resonances, suggesting that these resonances are located relatively far from the membrane surface. Yuan et aL212 have used Pr3+ and Gd3+ions and their DTPA chelates in a similar manner to study the structures of dipalmitoylphosphatidylcholinebilayer membranes.

9

Chemically-Induced Dynamic Nuclear Polarization (CIDNP)

In CIDNP experiments, a pair of radicals is produced, usually, by laser-induced photolytic cleavage of a bond in a precursor molecule, which is frequently a quinone species. The radical products are spin doublets In solution phase, a common fate of the radical pair is recombination to form one or more product species, which may be an electron singlet or triplet species, depending on the relative phases of the electron spins at the recombination event. Immediately after photolytic cleavage, the total electron spin wave function of the two radical doublets is a spin singlet, but as the radicals diffuse apart, the doublet spin wavefunctions de-phase due to magnetic hyperfine interactions, and in consequence, triplet character grows into the overall spin wavefunction. Radical pair recombination can thus produce product molecules in either a singlet or triplet electronic state. The singlet decays very rapidly to the ground state, while the triplet is longer-lived (typically 0.1- 1 ns). The triplet product produces, through nuclear-electron hyperfine interactions, large polarization of the nuclear spin populations. The NMR spectrum of the diamagnetic product, measured long after the decay of the triplet precursor (1-10 sec), exhibits strongly enhanced peaks, both positive and negative, due to nuclear polarization. The relative pattern of intensities (the ‘polarization pattern’) of the product NMR spectrum is directly related to and quite frequently identical with the corresponding relative hyperfine coupling constants in the paramagnetic intermediates. CIDNP thus provides hyperfine information that would be obtained from an ESR spectrum of the triplet product, although direct ESR observation is usually not feasible due to the short triplet lifetime. The polarization pattern can be regarded as a frozen signature of the intermediates. It provides detailed information concerning the identities, lifetimes and magnetic properties of the reaction intermediates. Recent CIDNP studies of

14: Paramagnetic N M R

515

free radical reaction mechanisms include the following: (1) the free radical reaction of triethylaluminum with chloroform, catalysed by N i ( a ~ a c ) ~ ;(2) ~l the reaction of triethylaluminum with n i t r ~ b e n z e n e ;(3) ~ ~ the ~ reaction of diisobutylaluminum hydride with CC14;215and (4) the electron tranfer reaction between dicyclopentadiene and chloranil;21 (5) nitration reactions of phenolic compounds with nitric and nitrous acid.217 CIDNP polarization patterns are sensitive to transformations of the initial radical pair into secondary radical pairs (‘pair substitution’) when these transformations occur on the time scale of the CIDNP effect. Electron selfexchange may be involved.218The evolution of the electron spin state provides an intrinsic clock against which the reaction rates can be measured. Goez et aZ.2193220 have used this dependence to study the reaction pathway initiated by the photolysis of methionine to form, in a first-step, the methonyl radical cation, which subsequently interconverts among different forms (open-chain protonated, open-chain deprotonated, and cyclic, with a two-center-threeelectron bond between sulfur and nitrogen). The rates and intermediates of these reactions have been characterized. In addition, the authors have extended the theory of pair substitution in CIDNP to cover reversible reactions with arbitrary equilibrium constants. Time-resolved CIDNP techniques (in which a variable delay of several microseconds is inserted between the laser flash and the NMR aquisition pulse) have been used in the laboratory of Goez to study sequential radical pair intermediates in the photosensitized photodecomposition of triphenylsulfonium hexafluoroantimonate221and of diphenyl iodonium salts.222Key intermediates of the first reaction, in which naphthalene and 9,lOdimethylanthracene were used as photosensitizers, are radical pairs consisting of the sensitizer radical cation and the phenyl radical that are formed by photoinduced electron transfer and bond cleavage. CIDNP has been used to study photolysis reactions of cyclic precursors which involve biradical intermediates. Biradical species, generated by photolysis of a cyclic precursor, are constrained with respect to diffusion and tend to have relatively efficient recombination pathways leading to CIDNP. Morozova et aZ.223have used variable field proton CIDNP techniques to study the biradicals generated during the photolysis of 2,12-dimethy1-2,12-dihydroxydodecanone (39). The primary acyl-ketyl biradical formed by photolysis of the dashed carbon-carbon bond of (39) was followed by a rapid decarbonylation step, producing a secondary bis-ketylbiradical. Both biradicals can combine or disproportionate producing numerous diamagnetic products that exhibit H CIDNP. These experiments demonstrated a ‘memory effect’ between the phases of the electron spin in the primary and secondary biradicals, described as the first such observation in a biradical system. Yurkovskaya et aZ.224 studied the photolysis reactions of 2,2,12,12-tetramethylcyclododecanone(40) in which successive biradical intermediates (acyl-alkyl, followed by bisalkyl) are formed. The yield of radical pairs and the efficiency of CIDNP generation can be influenced by a laboratory magnetic field. Fujiwdra et aZ.225have measured the effect of applied magnetic fields up to 13 T on the lifetime of radical pairs

516

Nuclear Magnetic Resonance

"--qH 0

39

40

generated by the photoreduction of anthraquinone and n-alkyl anthraquinone2-carboxylates in micellar solutions. The lifetimes increased strongly with increasing magnetic field strength in the region below 2 T, then leveled off or decreased slowly at higher field strengths. Taraban et aZ.226have studied the magnetic field dependence of CIDNP polarizations generated during the photolysis reaction of 7,7'-dimethyl-7-silanorbornadieneand have proposed paramagnetic intermediates for the reaction. Fedin et al. 227 have developed experimental techniques for generating CIDNP in a switched external magnetic field. In this experiment, the initial magnetic field at the time of the laser flash (which generates the radical pairs) is switched to a final value after a variable delay time, the order of a hundred nanoseconds. Because the rate of singlet-triplet conversion differs in the initial and final magnetic fields, the polarization pattern depends on the delay and provides an additional dimension for studying the reaction kinetics of radical pairs. Experimental results are reported for C-13 CIDNP generated in micellized solutions containing the precursors (1) dibenzyl ketone, (2) amethyldeoxybenzoin, and (3) benzophenone. CIDNP has previously been employed to probe rapid events in protein folding, and Lyon et aZ.228report further developments of these techniques. A photosensitive dye (usually a flavin) is photoexcited by a laser flash and forms radical pairs by reacting with solvent-exposed histidine, phenylalanine, or tryptophane. The resulting CIDNP enhancements in 'H NMR are typically an order of magnitude above the thermal equilibrium intensity, the magnitude and sign of the enhancement depending on the magnitudes of hyperfine coupling constants within the radical. The authors report the results of I5N CIDNP experiments on hen lysozyme, using 2D I5N-lH HSQC spectroscopy for detection. The experiment was found to provide higher resolution and greater polarization enhancement than the corresponding proton-CIDNP experiment.

Dynamic Nuclear Polarization (DNP). - DNP experiments monitor the nuclear polarization enhancements produced in radical-containing solutions when the electron spin system is saturated by microwave radiation. The experiment, which requires simultaneous NMR observation and microwave saturation, is experimentally demanding. Wies et al. 229 have described a triply tuned (IH, 13C, and microwaves, the latter at 140 GHz.), in which the body of the microwave cavity simulaneously acts as an NMR coil, thus providing more efficient irradiation and detection. Wind and Ardenkjaer-Lar~en~~O have described a new DNP probe operating at 60 MHd40 GHZ microwaves that is suitable for DNP-enhanced proton imaging in water at 1.4 T. To minimize 9.1

14: Paramagnetic N M R

517

microwave heating of aqueous samples, an existing DNP probe was modified with a screening coil wound around the sample capillary with its axis perpendicular to the electric component of the microwave field. Experimental results are described for aqueous samples containing a stable, water-soluble radical related tri-arylmethyl. This system appears to be suited for oximetric imaging of physiological samples.23 Odintsov et al.232 have studied heterogeneous DNP in aqueous suspensions of carbon chars. In these studies, pulsed field gradient techniques were used to measure translational diffusion of water in the porous structure of the char. 9.2 Overhauser Magnetic Resonance Imaging (OMRI). - OMRI is a technique in which the proton NMR signal in an imaging experiment is enhanced by microwave saturation of electron spin levels of an added free radical contrast agent. The experiment is highly sensitive to concentrations of dissolved oxygen, which, as a paramagnetic triplet species, is an efficient agent of electron spin relaxation of the label, and thus OMRI is potentially useful for oximetric NMR imaging. Ardenkjm-Larsen et al. 233 have measured parameters of relevance to this experiment for three single electron contrast agents of the triphenylmethyl type, all of which are stable and water soluble, at 9.5 mT in water, plasma, and saline samples. The results were very encouraging. The dynamic nuclear polarization of the proton signal was large, almost at the dipolar limit, and was highly sensitive to oxygen concentration in the physiological range.

10

Heterogeneous Media

Bulk magnetic susceptibilities are much larger for paramagnetic than for diamagnetic samples. Local demagnetisation fields produced by susceptibility changes at surfaces of paramagnetic powder samples act to broaden resonances in CP-MAS experiments and to distort the pattern or envelope of spinning side-bands. These effects complicate the measurement of principal values of hyperfine and/or chemical shift tensors. Kubo et al.234present a theoretical analysis of the phenomenon, which they use to simulate line broadening in 13C MAS spectra of two lanthanide compounds, Ln(C2D5SO4)3.8H 2 0 (Ln = Pr(I1I), Yb( I1I)). 10.1 Porous Media. - Several studies of water relaxation in porous systems containing paramagnetic ions have been reported. Zavada et al. 235 have studied the solution dynamics of solvents confined in the pore space of porous glass and fine particles using proton relaxometry (field-cycling T1 relaxation measurements of solvent protons). A pronounced TI dispersion was observed at low NMR frequencies that was attributed to molecular reorientation of the water coupled to translational displacements. The nature of this kind of solvent motion in which reorientations are mediated by translational displacements enhanced by Levy steps is discussed. Bryar et al.236have studied the

518

NucIear Magnetic Resonance

proton N M R relaxation of water in quartz sand and silica gel samples of known porosity. The water proton R 1 was highly sensitive to the presence of adsorbed Fe(III), such that samples in which only 0.01% of the surface sites were occupied exhibited an order of magnitude increase in R1 over control samples in which paramagnetic ions were excluded. A similar result was found by Korb et who studied water relaxation in microporous reactive powder concrete. The frequency-dependence of proton R1 demonstrated a strong influence due to paramagnetic metal ions located on the surface of the pore. Capitani et aZ.238have studied water adsorbed in cellulose papers by ESR and N M R 'relaxation measurements. An analysis the temperature dependence of the 'H lineshape provided information about the porosity, including the distribution of pore diameters. As in other studies of porous systems, T2 was found to be very sensitive to the amounts and types of adsorbed paramagnetic impurities. The addition of lanthanide ions to bicellar systems results in membrane bilayer dispersions oriented with their planes perpendicular to the applied magnetic field and the magnetic axis parallel to the field. The preparations, properties, and use of these systems in solid-state NMR and in various nonresonance experiments was reviewed by Prosser et aZ.239At Yb3+/lipid mole ratios as low as 1:100, a single oriented bilayer phase may be obtained in which the average bilayer normal is found to be parallel to the magnetic field. In other studies of the effects of cations in bilayer systems, Wu et aZ.240have used 'H N M R to study the effects of the trivalent cations Eu3+and A13+on the gel to liquid crystal transition of DPPC bilayers, and Amirov and S a p r i k ~ v a ~ ~ ' have used 'H Ti's to study the effects of Mn2+ on micellation in binary mixtures of anionic (SDS) and non-ionic (TWEEN) surfactants.

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15 NMR of Liquid Crystals and Micellar Solutions BY ALI KHAN

1

Introduction

This chapter covers the contribution of NMR to the study of the structures, conformations and dynamics of liquid crystals and micellar solutions formed by amphiphilic systems during the last two years (June 1998-May 2000). The liquid crystalline phases are characterized by fast molecular motions and longrange order. Thus the intermolecular interactions are averaged, while dipolar and quadrupolar NMR splittings can be observed from intramolecular couplings. The partial averaging of static effects gives rise to complex NMR relaxation behaviour, while the static effects are sensitive to the type of molecular alignment. Here, ‘the liquid crystals’ include both the lyotropic and the thermotropic liquid crystalline mesophases. The two classes of liquid crystals, namely, the lyotropic and thermotropic mesophases, are similar in that macroscopic structure corresponds to some space group, describing the symmetry properties of the crystalline phase. Microscopically, the two classes are also equal, since they are disordered giving rise to the liquid character of the phases. However, the thermotropics are usually considered as close packed assemblies of more or less rigid molecules while the lyotropics are regarded as built up by molecular aggregates being arranged in space. The ‘micellar solutions’ include normal micelles, reversed micelles and microemulsions. In isotropic solution phases, the absence of long-range order and fast molecular motions average to zero static nuclear effects, such as direct dipole-dipole couplings or nuclear quadrupolar couplings; thus an NMR spectrum usually consists of narrow lines. Nuclei such as ‘H, 2H, 13C, 31P,I9F etc. are employed extensibly in this study. Inorganic counterions, notably 23Na+, 133Cs+,7Li+, 19F- and a limited number of organic counterions as well as some paramagnetic ions are also used. The format of this chapter is similar to that used in previous volumes. The review articles published are included in a section followed by theoretical models and experimental techniques. The lyotropic polymorphism and the thermotropic mesomorphism constitute the largest sections of the chapter. Each of these topics is divided systematically and is described under the appropriate section. Polymer/protein-surfactant interactions in solution which is, currently, a popular research area is also included. The water (solvent) and ions that bring about important changes in the phase behaviour and aggregate Nuclear Magnetic Resonance, Volume 30

0The Royal Society of Chemistry, 2001 527

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microstructure by their association at the interface, is reviewed, followed by a short description of syntheses of few liquid crystalline materials.

2

Reviews

Many review papers dealing with natural and synthetic amphiphilic systems have been published. A review article' emphasising the use of the NMR techniques to characterize supramolecular structure of lyotropic liquid crystals is published. Being exquisitely sensitive to interfacial curvature, NMR lineshapes and relaxation rates can monitor the structural variations that occur at or near phase transitions. NMR derived diffusion coefficients can be used to establish topology, while low-frequency relaxation rates can provide information about the elastic properties and interactions of surfactant aggregates. Dynamic properties of nematic liquid crystals are reviewed including order fluctuations in isotropic phase, dielectric relaxation, pretransitional dynamics near nematic-smectic A transition, dynamics of nematics in microdroplets, microcylinders, and randomly constrained nematics2. An article covering the very recent work on mixed surfactant systems is reviewed. Topics such as polymorphism, micellar growth, micelle-to-vesicle transition, and structure of liquid crystals are emphasised3. A detailed report on the micellar solution, lyotropic and thermotropic mesophases is published4. The physico-chemical properties of anionic surfactans, such as carboxylated-, sulfated-, sulfonated-, amino- and phosphorylated anionic surfactants, are investigated by N M R methods5. A review on the experimental methods is available, for probing biaxiality in oriented or disoriented samples in the hopes of helping the discovery of a really biaxial nematic phase in the thermotropic liquid crystalline family. The relative advantages and problems that arise with biaxial tests based on the NMR techniques are discussed. To probe biaxiality, at least two types of disclination lines should be observed with typical variation of the zigzag angles, contrasts and relaxation rates as function of temperature6. N M R relaxation experiments with field cycling techniques proved to be a valuable tool for studying molecular motions in liquid crystals, allowing a very broad Larmor frequency variation, sufficient to separate the cooperative motions from the liquid-like molecular diffusion. The application of the techniques on the nematic liquid crystals is narrated7. The study of lipid bilayers has been greatly devoted to the structure and thermodynamic properties as the lipids are varied both in the chain length and headgroup. Changes of these properties, on adding a third component to the binary systems, are reported in this review paper8. Domain formation in lipid bilayer membranes can occur through electrostatic interactions between charged polyelectrolytes, such as proteins. The review describes a novel method for examining such domains in the bilayers, based on 2H NMR lineshapes. Spectral details, such as quadrupolar splittings and the relative intensity of spectra, permit complete analysis of the domain composition, size

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and lifetimesg. A review on studies, based on several NMR techniques on polymer-surfactant systems in nonaqueous solution, in liquid crystals and in the solid state is available10. Interactions between lipids and membrane proteins or peptides containing up to 20 amino acids are reviewed, with special emphasis on the solid-state NMR. The physico-chemical properties of the lipid bilayers play an important role in many biomembrane processes, such as, membrane folding and peptide induced membrane fusion' l-12.The stability of the casein micelle is dependent on the presence of K-casein on the surface of the micelle where it functions as an interface between the hydrophobic caseins of the micelle interior and the aqueous environment. NMR is used to examine the micelle stabilty with casein structure 3.

3

Models and Methods

Physico-chemical properties of a DPC micelle are analysed by a molecular dynamics (MD) simulation study containing 60 DPC molecules in 5295 H20 molecules. The micelle is found to be slightly prolate. The dynamics of the micelle is probed by the trans-gauche conformational transition rates and from the time correlation function of the C-H bonds. The order parameters and the correlation times for the internal motions are also obtained14. Similar MD simulations are also available for a hydrated liquid crystal of diphytanolphosphocholine bilayer15 and that of DPPC bilayer containing halothane16. The calculated unconstrained area density per lipid, bilayer profile and acyl chain order parameters are ~ b t a i n e d ' ~A. subtle structural change of the lipid bilayer in the presence of the drug compared with the pure lipid bilayer is observed. No significant change in the hydrocarbon chain conformation is detected. The calculated parameters for these systems are found to be in good agreement with those from NMR studies. A self-consistent field theory17 is developed to calculate the bilayer thickness and an average orientational order in the liquid crystalline phase of a fully hydrated PC bilayer with increased pressure at constant temperature as well as with increase of both pressure and temperature. The predicted results are in agreement with those measured experimentally. A study employing a lattice Monte Carlo dynamics algorithm that rests on the assumption, that the conformational dynamics of chain molecules can be described as the superposition of local structural rearrangements involving short chain segments, is carried out for the POPC bilayer containing a small amount of C12E4. The simulations reproduce the principal features of the order parameter profiles of the surfactant molecules found in pure bilayers and bilayers of POPC/C12E4 mixtures and provide a physical framework for understanding the experimental observations *. The Fourier approach and theories of space groups and colour symmetries are used to systematically generate and compare bicontinuous cubic structures in the framework of a Ginsburg-Landau model for ternary surfactant systems. The authors show how and why single structures can be made to approach very closely the triply

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Nuclear Magnetic Resonance

periodic minimal surfaces. The single gyroid G turns out to be the most stable bicontinuous cubic phase since it has the smallest p o r ~ s i t y ' ~A. molecular simulation in combination with H(water) NMR measurements of the chemical shift indicate that the solubilized water can exist in a different environment in reversed micelles. The modelling shows that water molecules can be localized in channels within the surface of rod-like micellar aggregates, thereby confirming the open water-channel of reversed micelles20. A method is developed to track the distributional preferences of phospholipids in polymorpism based on sideband analyses of the 31P MAS NMR spectra, The method is applied to lipid mixtures containing PC (phosphatidylcholine), PE (phosphatidylethanolamine), cholesterol, diolein, etc. When compositional heterogeneity among coexisting phases is observed, it appears that pre-existing lateral microheterogeneities led to the compositionally distinct transition clusters, such that the distributional preferences that result are not those of the individual phospholipids2'. The theory describing nuclear crosspolarization from a spin - 1 to spin - 1/2 is generalized to sweep of the rf field amplitude. This allows efficient cross-polarization over the entire width of the 2H NMR spectrum of partially oriented systems, such as liquid crystals. Such experiments permit the assignment and measurement of the 2H quadrupolar couplings and hence, the quantification of the order parameters which characterize of such systems22. A numerical procedure is presented which permits to derive a formal distribution of collective fluctuation modes from experimental field-cycling NMR relaxometry data of an ordered system. The method is capable to distinguish true order-fluctuation modes from local reorientation mechanisms. The evaluation scheme is demonstrated using simulated as well as experimental data23. The application of magic angle spinning (MAS) NMR to uniformly aligned biomembrane samples is demonstrated as a new tool toward structural studies of membrane proteins, peotides and lipids. The dramatic resolution improvement for protons which are achieved in a lipid sample at only 220 Hz spinning speed in a 9.4 T field is slightly better than any data published to date using ultra-high fields (17.6 T, 14 kHz spinning speed)24.This section is concluded by citing a method which is capable of converting second-order 19F spectra of liquid crystalline samples into first-order spectra. This is achieved by recording a 19F COSY spectrum with a multiple pulse dipolar reducing sequence operating in the tl period, leading to a F1 projection which is first order25. 4

Lyotropic Polymorphism

Surfactants (both synthetic and biological) are often rich in polymorphism. The water-soluble surfactants form normal micellar aggregates with water followed by different liquid crystalline phases with increased surfactant concentrations. Insoluble amphiphiles cannot form a micellar solution phase but they often form mesophases. Addition of other components, such as another amphiphile, a polymer, a protein, a polar or a non-polar substrate,

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53 1

etc. to a binary aqueous system may result in a very complex phase behaviour. The identification of phases and the determination of phase diagrams constitute a very important task for the surfactant systems. The following section covers the reported phase equilibria for the surfactant systems during the last two years.

4.1 Phase Diagrams and Phase Structures. - 1 -dodecylpyridinium bromide forms a normal micellar solution phase and a hexagonal, HI liquid crystalline phase. These two phases can solubilize a limited amount of dodecane. In addition, dodecane induces the formation of a cubic phase within the triangle. A complete phase diagram is obtained26. Both hexagonal and lamellar phases are characterised for the ternary system based on the quarternary ammonium surfactant with pentan01~~. The binary (composition vs. temperature) phase equilibria of bile acid salts comprising Na deoxycholate, Na taurodeoxycholate, Na chenodeoxycholate and the glycine conjugate Na glycodeoxycholate28929is studied. The equilibrium phases identified for each of the four systems are a micellar solution phase followed by a hexagonal liquid crystalline phase. Several unusual kinetics effects, such as very slow equilibriation times and the formation of transient spherulitic crystals in the biphasic regions, are observed. A rich polymorphic behaviour is described for the aqueous solution of the double tail alkylbenzenesulfonate system. The important finding is a vesicle region between the isotropic solution and lamellar phases. The binary (composition vs temperature) phase diagram is also determined30. The ternary system AOT-water-octanol forms, at low water content, a reversed cubic phase which melts into a reversed solution phase on dilution with water. The two phases are reported to consist of a similar type of microstr~cture~~. The phase equilibria of non-ionic surfactants, alkyl oxyethylene glycol (CnHn+1(OCH2CH2),0H, (termed CnE,) C12E632,C12E833 and C17E8434, with water are reported. Within WO={D20]/{C&} = 5.6-14.9, a smectic lamellar phase exists below a certain given temperature. Cooling induces the isotropic phase to become an almost completely isooriented lamellar phase with high magnetic field strength and the phase is stable out of magnetic field for several months. Isooriented lamellar phase coexists with the isotropic phase. The phase diagram of C&8 with mixed solvents water and glycerol is determined and is found that the isothermal phase diagram with mixed solvents is similar to the one with water as a function of temperature. The hexagonal liquid crystal phase destabilises with increasing alcohol concentration. The surfactant, C17E84, when mixed with water, forms a micellar phase below 13% and a cubic phase between 20-60% surfactant. It is concluded that the cubic phase is composed of discret aggregates and the micellar building blocks of the cubic phase are non-spherical in shape. The lifetime of a monomer in the micelles is also determined. The influence of the length of the ethoxylates on the phase behaviour of phytosterol e t h o ~ y l a t e sin~ ~water is examined. For the hydrophobic surfactants, large lamellar, La and for the hydrophilic surfactants, cubic, I and hexagonal, HI liquid crystalline phases are formed. Other physico-chemical properties are also studied. Nonyl

532

Nuclear Magnetic Resonance

P-glucoside forms an La phase with water at high surfactant concentration and on dilution, the succession of phases are La-to-I-HI-to-micelles; on the other hand the sequence of phases for the n-decyl P-glucoside is La-to-micelles-totwo coexisting solutions. Mixing of these two surfactants also exhibits two coexisting solutions at high water content. The solution structures are investigated36. The phase behaviour of the catanionic mixtures Na+ C12SO4(SDS)+(C&N+(CH& Br- (DDAB) is reported. Emphasis is given on the dilute part of the ternary phase At equimolar ratio, the system precipitates producing a catanionic surfactant. On adding either excess SDS in the anionic-rich side, or excess DDAB in the cationic-rich side, the precipitate redissolves forming spontaniously thermodynamic equilibrium vesicles. The vesicles coexist with micelles as well as with the lamellar phase. Addition of polyacrylic acid to the mixed cetyltrimethylammonium bromide and hexadecylphosphocholine in water40 leads to the formation of an L, phase at high temperature which transforms into a HI phase on decreasing the temperature. The phase diagram of the ternary system DDAB-PEO (poly ethylene oxide)water is dominated by multiphase heterogeneous regions. The two lamellar phases of the binary DDAB-water system solubilizes small amounts of PEO, but the DDAB vesicular phase can solubilize a substantial amount of the polymer prior to the phase destabilization4*. The ternary phase diagram for the system C7FISCOONH4+ + NH4Cl + 2H20 is obtained and the obtained results, namely an isotropic phase, a nematic phase and a tricritical point at the nematic-tolamellar phase transition line is rationalised in a phenomenological way42. The phase equilibria of several lipids are studied in water under different experimenta1 conditions. The reported systems are - Dipalmit oylphosphat idylethanolamine (DPPE)43, palmitoyloleoyl phosphoserine (POPS), palmitoyloleoylphosphatidylcholine (POPC)U, soybean ph~sphatide~~, dipalmitoylphosphatidylcholine (DPPC)46, POPC + POPS47, palmitoylarachidonoylphospha- tidylcholine (PAPC)48, dielaidoylphosphatidylethanolamine (DEPE)49v50dihexadecylphosphatidyl- choline (DHPC), DPPC, DMPC5 , phospholipids with terminal diene groups52 and lecithin with fatty acid53, cy~lohexane~~, and Triton X1 1455. Addition of feline virus peptide lowers Lato-HII phase transition temperature of the DPPE system43. The phase diagrams of POPS and POPC with palmitoyloleoylglycerol(POG) are constructed between 0-45 "C. Depending on composition and temperature, both the systems exhibit La,HI1 and isotropic phases? Furthermore, the system is employed to study the activation of protein kinase C. The physical state of the membrane is shown to effect the interaction of the protein with the membrane47. The liquid crystalline phases formed by the PAPC system become more ordered when cholesterol is added to the system. The chain order and dynamics of bilayers are also obtained48. The effects of changes in bilayer phase structure on the permeability of acetic acid and trimethylacetic acid are studied in large unilamellar vesicles composed of DPPC/cholesterol, DHPC/ cholesterol, as well as mixed DPPClDMPC lipids. The observations are used

'

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to suggest that trans-bilayer permeability measurements can be used to construct lipid bilayer phase diagrams5'. The effect of temperature and pressure is reported on the phase behaviour of DPPC with and without added cholester01~~. In the absence of cholesterol, the system displays at least three transitions as temperature is lowered from 90°C to 0°C at a fixed pressure of 210 MPa and with cholesterol, the changes are continuous from fluid to ordered phases. Addition of diacylglycerol and unsaturated fatty acids to the bilayers synergistically increases the tendency of the lipids to form non-bilayer phases with a concomitant increase in protein kinase C activity to a mximum after which the non-bilayer structure is formed with decreased protein activity56.A gel with lamellar structure, an La phase and an HII phase are found to form with increasing temperature for the phospholipids with terminal diene groups52. Both c e r a m i d e ~and ~ ~antineoplastic ether lipids50are found to modulate the polymorphic properties of the lipid DEPE. A major effect is the decrease of the transition temperature from the L, phase to the HI1 phase. The presence of a drug in the phospholipid systems may cause the phase transitions, such as liposomes-to-micelles. The phase transitions are explained in terms of a theoretical A model based on the self-assembly process is presented. The phase equilibrium of the ternary systems lecithin-water with Triton X 11455, c y ~ l o h e x a n e ~and ~ , alkanephosphodiethylesteric acid53 are studied. The system with cyclohexane exhibits four liquid crystalline phases - a lamellar, a reversed nematic, a reversed cubic, a reversed hexagonal and two solution phases. The phase structure is also investigated. Lamellar, hexagonal and isotropic phases are formed with esteric acid and the coexistence of two lamellar phases are identified with the non-ionic surfactant. Salinity and pH are found to effect the phase behaviour of the soybean phosphatide-alcoholwater systems45. The phase behaviour of several globular proteins, such as, lysozyme, plactoglobulin, ovalbumin, bovine serum albumin with surfactants SDS and DOTAC are reported. For the oppositely charged protein-surfactant systems, formation of a precipitate-, a gel- and a solution region are common phenomena and only the solution phase is formed for the similarly charged systems. The microstructures of selected phases are also examined5*.The phase behaviour of bovine heart cardiolipin in presence of divalent cations and the kinetics of cardiolipin vesicle fusion induced by these cations are i n ~ e s t i g a t e d ~ ~ . The phase transition temperatures as well as stabilization vs. fusion of vesicles as affected by the cations Ca2 , Mg2+, Ba2+, Sr2 are examined. The phase behaviour of the mixed DDAB and PEO is studied in detail. A segregativetype of phase separation is detected and the DDAB-lamellar phases can solubilize very limited amount of the polymer. At high water content, DDAB forms vesicles and the addition of polymer leads to the fusion of vesicles60. +

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Nuclear Magnetic Resonance

Isotropic Micellar Solution Phases

Isotropic micellar solutions are formed in two- or multi-component surfactant systems. In normal micellar solutions, the aggregates adopt a positive curvature (the aggregate interface is curved towards oil) and in most cases the solution phase contains an excess aqueous part whereas the reversed micelles which contain an excess non-aqueous component(s) have a negative curvature. A microemulsion phase which contains oil as one of the components, can have either a positive or a negative curvature. But for a balanced surfactant, the microemulsions obtained with a few percent of surfactant, may have solution structures varying from the normal- to-bicontinuous- t o-reversed type microstructures. Due to rapid and isotropic motions, narrow lines in the NMR spectra are normally obtained in these systems. Where broad lines do appear their origin may be due to the presence of rather large non-spherical micellar aggregates. This section is presented as follows: normal micelles, solubilization and mixed micelles, and reversed micelles and microemulsions. 5.1 Normal Micelles. - The pulsed field gradient spin echo technique is used to study the micellization processes as well as the growth, size, and shape of micelles by measuring components’ self-diffusion coefficients for the surfactant systems, (CH3(CH2)n-lS03- N a + , (n = 5-9,l l), C12S04Na+, C16N+(CH3)3 Br- and Triton X-10O6l. This method in combination with NMR relaxation values and chemical shift measurements provide informations, such as, the dynamics of the alkyl chains within the micellar core and the polar headgroups at the interfidce62.The lateral surface diffusion coefficient of the Br- ions that are electrostatically confined to the vicinity of the oppositely charged surface of C16N+(CH3)3 micelles is one order of magnitude faster than the self-diffusion coefficient of the micelles. This is due to a high symmetry for the counterion e n ~ i r o n m e n t ~For ~ . the systems C12E5 and C12E8@, a substantial micellar growth is observed with C12E5 whereas a concentration-independent small micelles are detected for C12E8. The surfactant curvilinear diffusion65 in giant wormlike micelles is measured for various observation times t and the surfactant mean-squire displacement is found to scale to t1/2.NMR velocimetry and rheometry are used to measure the flow dynamics of the wormlike micelles of the system c16 pyridinium Cl--Na ~ a l i c y l a t e - N a C l ~Two ~ - ~ ~classes . of behaviour are observed; one, the slow migration governed by stress relaxation effects, and the other, the rapid migrations that are flow-driven and arise from interfacial instability. Na dodecanoyl prolinate68 having two conformational isomers is shown to micellise on the basis of stereochemical code and the presence of Na cholate dimers can facilitate the micellization of bile salts68. It is observed for the system dipropionyl (diC3)-to-dioctanoyl (dicg) phosphatadylcholine (PC)69 that the lipid molecules in a micelle experience more conformational restriction than in the case of a monomer and the monomeric molecules start to become more rigid for chains longer than diC4. Small, quasispherical micelles are formed by phospholiponucleosides dioctanoylphosphatadyl-adenosine and

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dioctanoylphosphatadyl uridine70. The micelles are found not to grow with lipid concentration, but charging of the polar headgroup is observed to be pHdependent. The micellar properties of an equimolar mixture of the two lipids also deviate from the ideal behaviour. NMR line-width of 35Cland 23Na are used to suggest that the anion C104- and not the cation N a + is incorporated into the micelles of tetradecyldimethylammonio propanesulfonate containing NaC10471.Hydrophobic interactions play a dominant role in the self-association of a,l-casein, but the extent to which the casein interaction sites are influenced by local secondary structure is examined in this article under various experimental condition^^^. Aggregation properties of gemini surfactants of the type (phenylenedimethylene) bis(octyldimethylammonium)dibromides73,dimethoxy bis(hexadecy1dimethy1ammonium)butane d i b r ~ m i d eare ~ ~ reported. Mode of linkage, i. e., ortho, meta or para position of the spacer, phenylenedimethylene with the two quarternary ammonium ions has strong effect on the micellization processes and the conformation of the surfactant molecules. Gemini chiral ~ u r f a c t a n t ~ ~ is not soluble in water, but forms large aggregates with water. The surfactant is also capable of aggregation in non-aqueous solvents C2HC13 and C(2H2)302H2.The exchange between the bulk solution and micelles occurs slowly on NMR time scale75.The bolaform surfactant based on w-hydroxy quarternary ammonium75 forms much smaller micelles compared to the conventional one. From the measured micellar size, it is concluded that both the quarternary ammonim and the terminal hydroxyl are positioned at the micelle-water inter face. Cmc of the fluorinated surf act ant^^^.^^ such as CF3(CF2)7S03- Li+ is obtained by following I9F NMR chemical shift. Cmc is found to increase with increase in temperature and a decrease in aggregation number is also measured with increased temperature. The system78exhibits viscoelastic behaviour when perfluorooctylsulfonate micelles is mixed with N (C2H5)4 and Li ions. The mixtures form threads which further organised into dendritic networks. Relaxation data indicate that the m i ~ e l l e shave ~ ~ a microstructure with the fluoroalkyl chain extending straight outwarldly to the bulk of water phase. The surface of fluorinated micelles exhibits a greater protective effect than the hydrocarbon analog micelles. The binding affinity and stoichiometry of the cyclodextridsurfactant complexes depend on the type of c y ~ l o d e x t r i n ~ ~ . However, the size of the micelles is weakly affected by the different counterionss0. Simulation of the decay curves of 'H relaxation of cationic, anionic and non-ionic micelles shows that the spin-lattice relaxation follows single exponential whereas spin-spin relaxation is bi-exponential. These observations are explained in terms of packing of the surfactant molecules in the micelles8'. +

+

5.2 Mixed Micelles and Solubilization. - The physico-chemical properties of mixed surfactant-surfactant solutions are studied by different NMR techniques. Cmc of mixed micelles, the maximum surface excess, minimum area per molecufe, the interaction parameter that measures the deviation of the mixed system from ideal solution behaviour, the thermodynamic parameters of

Nuclear Magnetic Resonance

536

micellization and adsorption, are obtained for several systems, The mixed systems reported during the last two years are C12SO4- Na +/CloE5, C12SO4Na+/CI2 amido Pr betaine82, C12SO4- Na+/Na+ cholates3, C12SO4- Na+ with C12E5, C12E8, or C12N+(CH3)3 Cl-84, C12S03- Na+/Triton X-10085, c6s03- Na+/C6E586,N a + Clzbenzene S03-/C12E10s7, C4 benzene SO3-/C16 pyridinium C1+88, C7F15COO- Na + /Cl2 ether hydroxypropyl ~ u l f o n a t e ~ ~ , gemini Br - C12N ( C H ~ ) ~ - ( C H ~ ) ~ - (CH3)2 C I ~ N Br -/C1 C12E lddecanoyl methylgl~camine~ ', diC8 phosphatidyl-adenosine, and diC8 phosphatidyl~ridine~~. The diffusion coefficients and relaxation rates of the cationic and anionic surfactants of the mixed systemss3 are used in the description of the two surfactants in the various environments. The partition coefficient of the added PEG between the micelles and solvent for the system87is reported to be almost zero. The measured self-diffusion coefficient of C12E5 and the weaker mixing ratio with C12E8 for the mixed systems4are explained, respectively, in terms of micelle-micelle repulsion and molecular geometry effect. Addition of polyacrylamide to the mixed anionichon-ionic systems5causes the reduction in the crnc of the ionic surfactant without having any influence on the cmc of the nonionic one. The short chain hydrotropes interact electrostatically as well as hydrophobically with the cationic surfxtant88 giving rise to rod-type micelles and vesicles. Mixed hydrocarbon-fluorocarbon surfactants show high surface activity and the system also forms mixed micelless9. For the mixed geminihonionic surfactant systemg0,the polar group of the gemini surfctant is incorporated near the surface of the ethylene oxide chains of the non-ionic surfactant in the mixed micelles whose aggregation numbers are smaller than those of the respective parent surfactants. It is observed that in the mixed micelles formed by the phosphatidyl nucleosidesg2, both stacking and hydrogen bonding interactions are present between the bases at the micellar surface. The headgroup size and headgroup packing are important contributors to non-ideal behaviour exhibited by the mixed surfactant systems. The theoretical model based on the van Laar interaction energy can successfully account for the nonideal behaviour of the mixed systems. NMR techniques are used to reveal the dispersion mechanisms of stratum corneum in the micellar solutions of the mixed C12SO3- N a +/C12 dimethylamineoxide with and without solubilized limonene. The intercellular lamellar structure of stratum corneum is disrupted. However, considering the micellar size and the interaction with micelles, the interaction mechanisms in these systems are considered to be differentg3. The chain length of aliphatic alcohols is reported to exert strong effects on the C12SO4- Na micelles containing NaCl. Propanol successively breaks down the micelles, pentanol leads to the formation of wormlike aggregates and butanol can increase or decrease the micellar size depending on alcohol c o n c e n t r ~ t i o n ~The ~ . organic perfume solubilizates of phenols, anisoles, cineole and limonene types possess different functional groups with variable degrees of hydrophobic/hydriphilic character and are thereby solubilized at different locations of C 12S04- Na m i c e l l e ~ The ~ ~ . micellar solubilization of butanol for the system C12SO4- Na+/PE096starts at cac, but the solubiliza+

+

+

+

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tion capacity is low until the surfactant concentration reaches its free cmc. 2D NOESY technique is found to provide significant insight in terms of the solubilizdtion of butanol and benzene molecules in the interior of the ionic micelles of C12S04- Na+ and C12N+(CH3)3 Br-97. The solubility of short chain aliphatic alcohols in C16N +(CH3)3 Br- micelles9*,and that of benzyl alcohol in C16N+(CH3)3Br- and c16 pyridinium Br- m i ~ e l l e are s ~ ~reported. The location of solubilizates and their effects on micellar structures are also examined. P-cyclodextrin and its derivatives are solubilized by complexing with Na cholate and Na deoxycholatelOO.A mechanism of complexation is suggested. The non-ionic surfactant decanoyl methylglucamide, MEGA- 10"' causes a pronounced decrease in the solubility of phenot in water. The additive also induces a sphere-to-rod transition by incorporating the additive into the surfactant micelles. The partition coefficient of pentanol between the outer aqueous bulk and the inner micellar pseudo-phase of the C12E23 (Brij 35) system is obtained by the NMR self-diffusion methodlo2. The distribution of stable nitroxyl radicals between the aqueous and micellar pseudo-phase and in the micellar interior is measured for the cationic, anionic, zwitterionic and non-ionic micelles. It is observed that the paramagnetic fragments are located in the interface and there is no clear correlation between the partition coefficient and the micelle chargelo3. In recent years, NMR is found to be an important tool to investigate surfactant adsorption layers on solid surfaces. NMR diffusion study performed on C12E5 on polystyrene latex particles deals with the equilibrium exchange dynamics of surfactant molecules between the adsorbed layers and surfactant in souloitn@ '! Adsorption layers of C12E5on colloidal Si02 are also investigated by 2H NMR of selectively deuterated surfactant. Results for surface aggregates are compared with different types of bulk aggregates, such as micelles which show isotropically averaged Lorentzian line shapes and with liquid crystalline phases, which show a Pake pattern spectrum. Possible mechanisms of isotropic averaging of the quadrupole interaction in surface aggregates and the implications for the structural arrangement are dis-

5.3 Reversed Micelles and Microemulsions. - NMR self-diffusion technique is used to investigate structural aspects of microemulsions formed by the surfactants ((C12)2N+(CH3)2)2 Br+ (DDAB)lo7, ((C12)2N+(CH3)2)2S042(DDAS)"*, Ca2+ AOTlo9, sucrose alkanoateslIO,lecithin'", C12E5112 and octylmonoglucoside and geraniol' l 3 with different oils and co-surfactants. DDAB forms water-in-oil microemulsion at high oil and low water content. By replacing monovalent Br c o ~ n t e r i o n with ' ~ ~ the divalent S042- ionlo*, the system forms microemulsion with oil; the discrete and spherical aggregates at low, and a bicontinuous-type microemulsion at high surfactant-to-oil ratios are characterized. Water-in-oil spherical droplets are reported to exist at low volume fraction of the disperse phase on adding isooctane to Ca2+ AOTIo9. The occurrence of transient fusion-fission processes among the droplets is also indicated. The results are discussed in terms of existing models. The micro+

538

Nuclear Magnetic Resonance

emulsion of the Na AOT with isooctant and water is also reported114.Sucrose monododecanoate mixed with hexanol forms middle phase microemulsion which coexists with excess water. A large solubilization capacity of oil is attained by mixing with mono- and polydodecanoate. For this system, a lamellar liquid crystalline phase is also identified1I0. In another study" l, the effect of addition of sucrose monododecanoate, alkyl maltosides and Na taurocholate on the microemulsion formed by soybean lecithin-waterpropanol system is investigated. The microstructure is found to change from an oil-continuous structure to oil-swollen micelles to a bicontinuous structure. The system C12E5-water-decane with small amount of an ionic surfactant forms reversed type hexagonal lyomesophase with aggregates consisting of essentially infinite cylinders' 12. Microemulsions are prepared by adding toluene/butanol' and heptane/ isopropanol' l 6 to C12SO4- N a + /brine. The microstructure is studied by NMR relaxation method and data are rationalized by invoking the bicontinuous structure for the Winsor I11 microemulsion. The system also forms a lamellar liquid crystalline phase. On adding Cl2N (CH3)3Br- or C12E5 to DDAB/heptane microemulsions causes a synergistic effect and the water solubilization capacity of the microemulsion is increased. However, with SDS, there is a destabilization, indicating the surfactant antagonism. Observed changes in microemulsion phase behaviour and properties are a direct consequence of preferred curvature, which may be varied by surfactant mixing in the interface117. It is reported'18 that the structure of the middle phase microemulsion made with Na C12 benzene sulfonate /C&Il&&€-€/brine undergoes oil-in-water to bicontinuous to water-in-oil droplets. The microemulsion made with octylribonamide/decane/C40H/brine1 l 9 is shown to have existing structures, namely, normal, reversed and bicontinuous-type microstructures. The dependence of lecithin self-diffusion coefficient and the apparent molar volume on the lecithin concentration for the system lecithin/ cyclohexane is studied. Pre-micellar aggregation, with formation of small micellar aggregates are suggestedI2O. Over a wide concentration range, the water diffusion is found to be Gaussian with a well defined, time independent, single diffusion coefficient. The dominating mechanism for the water diffusion is suggested to be the motion inside giant wormlike reversed aggregates mediated by an interaggregate exchange with a characteristic time on the order of microseconds'21. +

6

Lyotropic Mesophases

As described earlier, surfactants are often rich in mesomorphism. They form liquid crystals with water as well as in multicomponent systems. The usual mesophases - lamellar, hexagonal (normal and reversed) and cubic liquid crystals have been studied extensively. The so-called 'intermediate' liquid crystals are attracting the interest of increasing number of research scientists. Vesicles are used extensively as models for biological membranes. The vesicles

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prepared mechanically are thermodynamically unstable. Spontaneous formation of vesicles which are thermodynamically stable are reported for many surfactant systems. Natural systems, mainly biological membranes, are also included. Polymer-surfactant interactions which are also included here, are becoming important topics for NMR studies. Water and ions, important constituents in amphiphilic systems, are also included in this review article. 6.1 Vesicles. - Steric stabilization of small unilamellar phospholipid vesicles is carried out either by sonicating soybean lecithin with added triblock copolymer or by adsorbing the copolymer onto performed vesicle surfaces. In both ways, the large hydrophilic polymer chains coat the phospholipid surface exhibiting enhanced stability. Examination of 31PNMR linewidths of phosphocholine (PC) groups and that of 13C of the Me groups (PPO) suggest that the coploymer is incorporated inside the bilayer122*123. 2H NMR spectra recorded for the multilamellar vesicles at 37.5 "C consist of two components, the gel-type and the pseudofluid-type; the latter one does not correspond to the L, phase, but it originates from the ripple structure124.The membrane viscosity calculated from the lateral diffusion rates on diester PC vesicles indicates that at 55 "C, the viscosity is comparable to that of the lipids in the L, phase, while the value at 30°C is cosiderably higher than that in the liquid crystalline phase'25. The stability of liposomes, in terms of the retention of carboxyfluorescein, made from total polar lipids of Methanosarcina mazei is shown to be strongly dependent on the ratio between mono- and divalent inorganic ions as well as on the methods of preparation and temperature126. The effect of membrane curvature and segmental headgroup motion on the vesicle size is also studied by I4N NMR method*27.It is claimed that simultaneous analysis of 2H NMR spin relaxation rates of lipid bilayer as a function of both frequency and sample orientation is decisive in evaluating different models for the dynamics of the membranes'28. The orientational order parameter determined from 2H NMR spectra of magnetically partially oriented PC liposomes is tested by a numerical method based on Tikhonov regularization algo~ - i t h r n ' ~The ~ . shear-induced orientation of the L, phase of C12E4 shows a continuous transition from a state of aligned layers to close packing of multilamellar vesicles130. Several fluorinated cationic surfactants are also reported to form vesicles upon sonication. The size and microstructure of vesicles are determined' 31. There are several studies concerning the effects of c h ~ l e s t e r o l ' ~ganglio~, carbohydrate^'^^, thym01'~~ and dextran sulfate136on PC vesicles. The co-operativity of lipid alkyl chain, miscibility and thermotropic phase behaviour are investigated with added cholesterol to the vesicles. Incorporation of gangliosides to the vesicles destabilizes the vesicle structure leading to the formation of micelles. The destabilization process can be reverted on addition of cholesterol to cylindrical micelles. The carbohydrates trehalose, glucose and hydroxyethyl starch effect the motional properties of the phosphate headgroup primarily by forming H-bonding between the OH groups of the sugar and the polar headgroups of phospholipids. Thymol is easily

Nuclear Magnetic Resonance

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interpregnated into PC vesicles causing increased dynamics of the lipid chains and eventually, destroys vesicles to mixed micelles or other aggregates. Dextran sulfate binds to vesicles by the formation of Ca2+ bridge between S042- groups of dextran and phosphate groups of the lipid vesicles. The binding is modulated by electrolyte, NaCl and Ca2 concentration. By following the physico-chemical properties of phospholipd vesicles, the interactions between vesicles and lung type I1 myosin protein137, alamethicin' 38, 8-lysine' 39 and fusion peptideI4O are studied. An addition of myosin to DMPC vesicles neither alters the 2H NMR splitting nor the spinlattice relaxation times of deuterated lipid. But on adding charged DMPG to DMPC vesicles affects both the splitting and relaxation rates revealing a direct electrostatic interaction. A comparison of equilibrium positions of the transmembrane migration rates of the two peptides, the aative alamethicin and a more hydrophobic analog L1,show that L1 sits 3-4 A deeper in the membrane than does the native one. 31P NMR study shows that at low 6-lysine concentration, the bilayer remains intact, but on increasing peptide concentration, an isotropic signal, indicating the phase transition, is recorded. A peptide corresponding to the 23 N-terminal amino acid residues of the human virus type 1 has the capacity to induce intervesicular lipid mixing in large unilamellar vesicles, composed of DOPC, DOPE and cholesterol. The interactions lead to the transformation of lamellar-to-nonlamellar-to-isotropic phases. The catanionic mixtures of SDS/DDAB form thermodymically equilibrium vesicles in both SDS-rich and DDAB-rich side at a very high water content. With excess SDS, fairly monodisperse small unilamellar vesicles are obtained while with excess DDAB, the vesicles are farely polydisperse and predominantly multilamellar in s t r u ~ t u r e ~The ~ - ~system ~. is alsc used to prepare vesicles at a given concentration by following different preparation methods. Method-independent stable vesicles are always obtained141. The vesicles are formed for the SDS/CTAPTS (cetyltrimethylammonium toluenesulfonate) pair, but not for the SDBSKTAPTS mixtures; instead, a precipitation is obtained. Surfactant packing parameters are used to explain these differe n c e ~ ~Thermodynamically ~*. stable unilamellar vesicles are reported to be formed spotaneously by simple hydration of fluid phase phospholipid bilayer films containing low molar ratio of PEO-based amphiphiles. The existence of such structures is also predicted by mean-field theory for the systems143.A mixture of dodecylbetaine, ammonium salt of perfluoropolyether carboxylate and cytochrom c forms stable vesicles with size that ranges from 83 to 113 nm. Cytochrom c is suggested to be surrounded by a shell of water, the so-called, water shell mode1I4. Synthetic archaeal phospholipids bearing zero, one or four double bonds in each chain form a variety of self-organized structures in water, in particular vesicles and tubules. The properties of these structures are studied145.In the presence of dicetylphosphate, the gel phase monostearoylglycerol forms stable vesicles. Interaction between P-lactoglobulin and vesicles are studied as a function of pH. At pH 4, protein-vesicle pair is oppositely charged and at pH 7, the pair is similarly charged. At both pH, protein interacts with vesicles and the binding affinity of the protein determined is in micromolar +

15: N M R of Liquid Crystals and Micellar Solutions

54 1

range146.We conclude the sub-section by citing two reports where vesicles are suggested to be used as drug carrier. In one the preferentially trapped benzyl alcohol in the interfacial region near the carbonyl group of the PC and the delivery of the alcohol are monitored by following suitable proton NMR signals. The other report deals with the distribution of phospholipids and triglyceride molecules in the membranes forming the vesicular network within multivesicular lipid particles148. 6.2 Liquid Crystals. - With magic angle spinning (MAS) all orientationdependent line broadining is averaged and liquid-like NMR spectra of 'H and 13C are observed in the liquid crystalline phase of monoolein-water system. Following this method, the structural investigation provides insight into lipid/ water interactions at the molecular level. A slow exchange of water protons with the OH protons as well as spatial proximity of water to non-exchangeable protons of the glycerol moiety is observed149.A mechanism to describe the isotropic solution-to-cubic (Q223) liquid crystalline phase transition for the palmitoyllysolecithin is proposed in terms of the local order in concentrated solution phase and of the long-range order in the lower concentration limit of the cubic phase150.The effects of lipid chain packing and permeant size and shape on permeability across lipid bilayers are investigated in the gel and liquid crystalline state of DPPC with short-chain fatty acids (C = 1-6). The results suggest that in the bilayer interior the permeants prefer to move with their long principal axis along the bilayer normal. Based on these results, a permeability model combining the effects of bilayer chain packing and permeant size and shape on permeability across lipid membranes is developed151.The chain order and slow orientation is also studied for the DPPC bilayers mixed with the anionic lipid DPPG. The anionic lipid broadens the chain-melting transition in presence of Ca2+ . It is found that an addition of a small amount of the non-ionic surfactant C12E4 to the liquid crystalline DPPC bilayers leads to drastic changes of the lipid dynamics in the frequency domain which is dominated by diffusive motions of the whole molecule. The surfactant also causes a reduction of the order parameter of the lipid chains152.The surfactant chain penetrates the lipid membrane and at surfactant-lipid molar ratio of 0.2, rigidization of the POPC membrane is observed. However, the membrane is transformed to the liquid crystalline state on increasing the molar ratio to I , due to the mismatch of the hydrocarbon chain lengths between the surfactant and lipid153.Phosphatidylethanolamine (PE) bilayers with a high content of saturated fatty acids assume two backbone conformations, with a slow exchange between the two. The two conformations originate from the different domains, which have different fatty acid corn position^^^^. Application of hydrostatic pressure on the liquid crystal of DMPC causes the reduction of splitting of the a and p choline deuterons and an increase of the quadrupole decay times; these parameters are, however, less sensitive for the gel phase155.The cross-peak between the lipid resonances in 2D NOE lH NMR spectra, recorded with MAS for DMPC bilayers contain valuable information about the structure and dynamics of the bilayer. The results reflect

542

Nuclear Magnetic Resonance

the high degree of the lipid motional disorder and the substantial variations in location of neighbouring lipid molecules s6. The surface electrostatic charge of POPC membrane is found to be effected in the presence of the anionic lipid POPG mixed with the cationic polyelectrolyte poly(viny1benzyltrimethylammonium chloride). 2H NMR (Pake powder-pattern) spectra indicate the existence of two coexisting lipid domains with only slow exchange of lipids between the two. There is no evidence of non-bilayer structure. The two domains differ with respect to the overall surface charge environment and the polyelectrolyte-bound domain is rich with anionic lipids while the polyelectrolyte-free domain is depleted. The results suggest that the oppositely charged poly(vinylbenzyltrimethylammonium chloride) and POPG pair forms a complex at the surface which undergoes swelling by a solvent POPC and penetrates more deeply into the lipid bilayer than does POPG alone'57. The effects of polyadenylic acid on three aminomethyl-deuterated cationic amphiphiles are investigated by 2H NMR. When mixed with POPC and incorporated into lipid membranes, Pake powder type spectra of the amphiphile is observed. Analysis of the spectra results an electrostatic binding of the acid to the cationic lipid bilayer surface, accompanied by the formation of a stoichiometric complex between the acid and cationic amphiphileIs8. There are three reports concerning the effect of cholesterol on the phospholipid bilayer. 2H NMR spectra of oriented bilayers of DMPC and 30 mol% deuterated cholesterol provide a set of very accurate quadrupolar splitting for eight C-2H bonds. These data are used to accurately obtain the cholesterol orientation and dynamics*s9.The presence of Eu3-t ions raises the gel-to-liquid crystalline phase transition temperature of DMPC bilayer. In another study'60, the results suggest that the axial rotation of the DMPC molecules with cholesterol occurs at a somewhat higher rate than in pure lipid bilayers, as a consequence of the higher ordering and reduction of the chain entanglement. On the other hand, the rigid cholesterol molecule appears to undergo slower axial rotation160.It is reported that an eqimolar mixture of cholesterol and phospholipid give identical 2H NMR quadrupole splittings for cholesterol, i. e., same molecular orientation, with different phospholipid bilayers, consisting of 18:O-18:l PC, 18:O-22:6 PC or 18:O-20:4 PC161. The presence of an ionophore can lead to a non-ideal mixing with phospholipids in fluid bilayer due to the hydrophobic mismatch of the methylene groups'62. It is observed that the quadrupolar splitting in 2H NMR spectra obtained from a sample with natural abundance 'H isotope is considerably different from specifically deuterated samples of a lamellar phase'63. The order parameter profile determined in the lamellar phase of the Cs+ C7FlSCOO- system shows that the perfluorinated surfactant chains are more rigid than their hydrogenated counterpart^'^^. The simulation of 2H NMR spectra recorded on the nematic liquid crystals of PBLGlm-cresol as a function of time allows to characterize the director field pattern and to evaluate most of the viscoelastic properties of the system. A good agreement with classical rheometry is observed165. With solid-state 31P and 2H NMR, the formation of bicelles (bilayered micelles) is characterized for the system of DMPC and DHPC in presence of

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either cationic DMTP or anionic DMPG lipids. The stability range of the bicelles and factors that lead to the formation of bicelles are given1@'. In another report, the stability of bicelles is studied as a function of pH167.It is shown that the incorporation of PEG-PE to DMPC bilayer (pegylation) does not interfear with the orientation of bicelles with or without the presence of lanthanide ions. The presence of PEG stabilizes the samples against aggregationi6*.A stable smectic PC bilayer phase aligned with the director parallel to the magnetic field can be generated by the addition of Eu3+,Er3+, Tm3+ or Yb3+ to a bicellar solution of DMPC and DHPC. As a membrane mimic, these new positively ordered phospholipid phases possess high potential for structural studies using a variety of t e c h n i q u e ~ lThe ~ ~ . solid-state 2H and 31P NMR experiments illustrate how magnetically aligned neutral and acidic phospholipid bicelles can be used to characterize the ordering and mode of binding of proteins and peptides' 707126.

7

Surfactant-Protein and Surfactant-Polymer Systems

The peptide from the C2 domain of the antihemophilic factor VIII in dodecylphosphocholine (DPC) micelles assumes an amphiphatic structure comprising an extended N-terminal region and a C-terminal helix. Most of the amino acid side-chains that penetrate the DPC micelle are hydrophobic. Thus, the long axis of the peptide lies parallel to the micelle surface and the hydrophobic face of the a-helix provides the hydrophobic membrane interactionl7I. The dynamic behaviour of DPC phosphocholine groups at low temperature (12 "C) corresponds to that of a phosphocholine interface at high temperature (51 "C). In the presence of helical peptides17*, the DPC local dynamics are not effected, whereas the DPC aggregation number is increased to accommodate the bound peptides. Moreover, measurements of paramagnetic relaxation enhancement provides a meaningful insight on the location of the different fragments of the peptides. The sequence-specific effects of micelle association are also observed in the homonuclear nonselective spin-lattice relaxation times; these, in combination with spin-spin relaxation times, are used to calculate correlation times for the backbone amide protons173. Positively charged protein lysozyme strongly interacts with the anionic surfactant SDS forming in, succession, a solution (very limited stability), a precipitate, a gel and another large solution region, with increasing surfactant concentration. NMR relaxation of a-deuterated SDS and line-width measurements of perdeuterated surfactant show that the gel consists of large microstructure. The solution is composed of small protein-surfactant complex at low SDS content and free surfactant aggregates coexist with the proteinsurfactant complex at high surfactant c ~ n c e n t r a t i o n l ~The ~ . hormone peptides175are significantly partitioned in the negative charged SDS micelles and possess definite secondary structure, as opposed to random structures in water. The residence time of halothane molecules in SDS micelles is measured by 'H and I9F NMR. The data are explained in terms of 60% of the total halothane

544

Nuclear Magnetic Resonance

molecules in the micelles are restricted to the a-position and 40% to the pposition at saturated drug c ~ n c e n t r a t i o n ' ~The ~ . hydrophobic segment of the peptide is partitioned in the DPC micelles, whereas the charge segment prefers to remain in the aqueous region. The amyloid p-peptide varies slightly in length and exists in two predominant forms. The SDS micelles prevent the aggregation of the shorter form and encourages the folding of the longer one, into predominantly a-helical structures177.The interaction of SDS with two simple tripeptides are studied by following chemical shift and components selfdiffusion coefficient. The presence of peptides does change the surfactant cmc to any great extent. There is an interaction between the peptides and surfactant as deduced from the chemical shift data and the interaction is stronger with more hydrophobic ~ e p t i d e ' ~ ~ . 2H NMR quadrupolar splitting of heavy water and a-deuterated surfactant measured in the lamellar phase for the system dodecyldimethylammoniumoxide-gramicidin D is found to be consistent with an electrostatically stabilized lamellar phase with low surface charge density, though none of the components are ionic. The surface charge is likely due to either ionic impurities, that bind to the mouth of the gramicidin or partial protonation of the surfactant in the membrane'79. Multinuclear NMR methods are used to examine the effect of gramicidin A on the dynamics of the membranes of DMPC in the lamellar phase and DOPC in the hexagonal phase. It is found that the presence of the peptide does not significantly affect the fast motions of the lipid acyl chains but increase slightly the fast motions of the polar head group180.The natural abundance I3C NMR spectra of gramicidin A in a lipid membrane are recorded under MAS conditions. High resolution spectra are obtained and the spinning side-bands in backbone carbonyls is used to determine the residual chemical shift tensor. The results are used to propose the single-stranded righthanded p helix structure for gramicidin A in the lipid membranelg1. The interactions between the non-ionic surfactant CI2E8 and gramicidin S is explained in terms of binding of surfactant monomers and cooperative binding of micelles at low and high surfactantlpeptide molar ratios, respectively' 82. The presence of the peptide mastoparenls3 causes the phase transition of the mixed DMPC/DMPG oriented samples from lamellar to non-lamellar phases as deduced from 31PNMR spectra. Moreover, the peptide interacts strongly and selectively with the charged DMPG head group. The interaction of a new class of antineoplastic agents derived from arylchloroethylurea and DMPC membrane is investigated by 2H NMR. The drug incorporates in the bilayer and causes an increase of the lipid acyl chain order, this being greater close to the interfacial region of the bi1aye1-l~~. Nisin, an antimicrobial peptidels5, significantly affects the DPPC membrane from both permeability and the structural points of view. These perturbations are modulated by the lipidic species in the bilayer. The dynamic simulation of halothane at a molar fraction of 50% in the liquid crystalline phase of DPPC shows that halothane molecules preferentially segregate to the upper part of the lipid acyl chains. Halothane induces a contraction in the bilayer thickness and a change in the acyl chain segmental

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order parameters186.31PNMR shows that the amide derivative of amphotericin B can be incorporated into the single, stable liquid crystalline phase of DPPC. 2H NMR recorded one 2H quadrupolar- and one 2H-1H dipolar splitting in the liquid crystalline phase. Variations of quadrupolar and dipolar splittings as a function of macroscopic sample orientation and temperature indicate that the amide derivative undergoes fast rotation about a motional axis that is paralell to the bilayer The phase transition temerature of the DPPC membrane is noted to be lowered on adding cannabinoid peptide and both peptide and lipid molecules intercalcate into the membrane bilayers' 88. The molecular organization and dynamics are studied in POPC membrane containing various ratios of a transmembrane a-helical peptide (L24) in order to gain insights into how the transmembrane portions of membrane proteins are mixed with PC and organized in biological membranes. The results suggest that the lipid exchange rates among the bulk, boundary and L24-rich regions are fast with a time scale shorter than 0.2 pslS9.A highly oriented sample, which is needed to obtain individual bond orientations from 2H NMR (selectively deuterated) is prepared by forming hydrated lamellar films of purple membranes on glass slides. A Monte Carlo method is developed to accurately simulate the NMR line shape. The data indicate that the Me groups on the polyne chain point toward the cytoplasmic side of the membrane and the N-H bond of the Schiff base to the extracellular sidelgO.The effect of hydrophobic polypeptides (WALP peptides) on the thickness of phospholipid bilayers with different alkyl chain lengths are reported. The bilayer thickness, which can increase, decrease or remain unchanged depend on both the alkyl chain lengths and the number of amino acid residues of a peptide19*.13CNMR chemical shifts indicate that alamathicin forms a-helical structure in the lipid bilayers and is oriented along the bilayer normal. The result further suggests that the peptide exists as a monomeric form in the absence of membrane potential19*. The micellization of SDS containing a constant amount of PEO is investigated at three temperatures by different NMR methods. The formation of surfactant-polymer aggregates as well as the existence of free surfactant micelles are obtained. A crude calculation of the Gibbs' energy of the two aggregation processes suggest that the energetics of both the processes are of similar magnitude193.In another star-PEO is used to examine the binding with SDS. The onset of the binding takes place at the same concentration for both linear and star-PEO, but the saturation of the star occurs at lower concentration. The star decreases in size at normal cmc of SDS but increases at higher SDS concentration. H NMR chemical shift measurements indicate that, for the mixture SDS-ethyl(hydroxyethy1)cellulose (EHEC), EHEC molecules do not incorporate in the core of SDS micelles, instead, they reside just inside the micellar surface. Both TI and T, relaxation times imply that the molecular motions are slow as the system aggregates. Moreover, T2 monitors slower dynamics for SDS-EHEC system compared to SDS-PEO system 195. The presence of poly(vinylpyro1idone) in Li C8F17C00- -2H20 system strongly modifies 19Fchemical shift compared to that in the absence of +

Nuclear Magnetic Resonance

546

the polymer in the system indicating strong surfactant-polymer interactions. With the knowledge of free monomers vs. bound monomers, bound micelles vs. free micelles, a structural model for the micelle-polymer supramolecular aggregates is presented195. 'H NMR measurements in the fringe field of a superconducting magnet is used to measure the long range self-diffusion of lipo-fullerenes intercalcated in oriented DPPC liquid crystal. The lipo-fullerenes' diffusion is found to be one order of magnitude slower than that of the DPPC in the host bilayer. The diffusion behaviour largely remained unchanged under the condition that the bilayer is in solid-like gel state, indicating a decoupling of lipo-fullerenes and DPPC motion196. 8

Water and Counterions

The effect of sodium decyl, dodecyl and tetradecyl sulfates on the composition and stability of Ni2+, Cu2+, Mn2+, Gd2+, and Fe2+ complexes with cheleating agents is studied by the NMR relaxation method. Computer simulation is used to compute the association constants of the ions with micelles and the values obtained are nearly equal for equicharged ions'97. The effect of N a + , K + , and NH4' counterions on the formation and stabilty of micellar aggregates and liquid crystalline phases is studied for the perfluorinated carboxylate surfactant systems198.It is reported that the solubilization of benzene in micellar solution and the size of a micelle in the presence of benzene are strongly dependent on the nature of the counterion for the quarternary ammonium surfactant with a chain length of c16. The micellar solution becomes saturated with benzene having no micellar growth at a molar ratio of benzenehurfactant to about 3 with C1-, MeS03- and S042- counterions, but with Br- and NO3- ions, small amounts of benzene can saturated the solution and also leads to the formation of sphere-to-rod type m i ~ e l l e s ' The ~ ~ . degree of binding of Li' counterion to micellar surface of CgF17S03- surfactant is found to be much smaller than that of (CZH5)4NH4+ ion. Diffusion studies indicate that micelles with Li+ are small whereas with (C2H5)4NH4+ ion, the system forms threadlike micelles. The competition of counterion binding to micelles between these two ions are also examined200.201. An increase in the degree of counterion binding with increased surfactant concentration is observed for the system Cs+ C7F15COO-. The observation is explained in terms of the two-dimensional growth of micelles202. Fractionation factors of interfacial H20 at micellar surfaces of ionic surfidctants are determined by NMR methods. For micelles, interfacial water is more structured than bulk H20. Water structuring increases with increasing the bulkiness of the headgroup for the cationic betaine surfactants, but for the anionic Na alkyl sulfate surfactants, H20 structuring factor is unaffected due to the size of the headgroup or the length of the alkyl chain203.A weak 'H-19F cross-relaxation between the water protons and fluorines in the fluoroalkyl chains and a strong 19F-19Fcross-relaxation within the fluoroalkyl chain are measured for the micellar solution of Cs+ C7FlsC00-. It is claimed that the

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fluorine hydration penetrates more than the first CF2 segment of the chain2@‘. The bilayers of stratum corneum containing equimolar amounts of ceramide and cholesterol is shown to bind 11 molecules of water per molecule of both ceramide and cholesterol205. An ultra high resolution *H MAS NMR is employed to study the nature and extend of the lipid hydration as well as the arrangement of nuclei at the membrane/water interface for multilamellar phospholipid vesicles. Among others, exchange between bulk and interlamellar water is found to be slow206. The composition of the unfrozen phases at freezing is determined in phospholipid lamellar phases containing DMSO, sorbitol, sucrose and trehalose. The presence of DMSO or sorbitol increases the hydration of the lamellar phases and the increase is as expected from the pure osmotic effect. However, the increased hydration measured with sucrose or trehalose cannot be explained by the osmotic effect207.

9

Thermotropic Mesomorphism

Many crystalline organic compounds that form liquid crystals upon heating have elongated molecules and this class of crystalline solids are very rich in mesomorphism. The vast majority of liquid crystalline substances that yield smectic and nematic liquid crystals are based on the structure

where they possess: (a) two or more aromatic rings, usually benzene rings; (b) one or more bridging groups, A-B, that bind the rings together, e.g. -CH = N-; and (c) two terminal groups, X and Y, usually on the long axis of the molecule, e.g. X = CH30-, (OR) and Y = C4H9. In addition to smectic and nematic liquid crystals, one may obtain cholesteric mesophases by adding chiral solutes to the nematic phases, discotic mesophases consisting of disk-like rather than rodlike molecules. This article reports the papers published on these liquid crystalline phases 9.1 Relaxation Studies. - Deuteron Tlz and TI, spin-lattice relaxation times and quadrupolar splittings are measured for all the stable mesophases in mixed of perdeuterated 60CB (4-n-hexyloxy-4’-cyanobiphenyl)and 80CB at two different Larmor frequencies and compared with those for a pure 60CB. Results show that both the dynamic and static behaviours appear to be similar for pure and mixed systems. The tumbling motion of 60CB molecules, however, show quite two different behaviours in the two systems208. Similar experiments performed on the columnar phase of ring deuterated 1-fluorohexahexyloxy-triphenylenesuggest that the disk-like molecules are stacked with only short range position order into columns which are arranged on a 2D

548

Nuclear Magnetic Resonance

hexagonal lattice. Spectral density results are in consistent with the picture that the motion towards or away from the local director tends to disrupt the packing of molecules within the column209.In another report210,2H Zeeman and TI D times are measured on two mesogens 4,4'-bis(hexy1oxy)azoxybenzene and its Pd(I1) acetylacetonate derivatives by the Wimperis pulse sequence. The results are discussed in terms of internal and overall motions. 'H NMR has been performed on the liquid crystalline phases of N-(p-methoxybenzi1idene)p-n-butylaniline (MBBA)21 and 4-pentyl-4'-~yanobiphenyl~~~. TI times measured on the nematic phase confined in a porous matrix show a strong influence of the restricted geometry on the character of the 7'1 dispersion. The results are in good agreement with theory. Relaxation data are also used to establish a connection between dynamics and phase transitions. In a slow cooling experiment, both the slow Et group motion together with reorientation of the Me groups, and after reheating a quenched sample, only Me rotation contribute to the relaxation. The dipolar order relaxation times measured in the nematic phase of Me deuterated p-azoxyanizole over a broad frequency range detect a faster relaxation than predicted by the two-spin model. The non-agreement of the result is discussed in terms of the basic poor assumptions of the model2*3. The Jeener-Broekaert pulse sequence combined with fast-field cycling NMR technique are used to measure the dipolar relaxation times (TID) at low frequencies. TI,-, values obtained at frequencies > lo5 Hz, are characteristics of order fluctuation of the director in nematics. In contrast, the Zeeman relaxation is driven by faster and less correlated motions, especially in the MHz frequency range. It is suggested that the technique is useful to measure slow molecular dynamics in me so phase^^^^. The temperature behaviour of the director order fluctuation coherence length in the nematic phase of butyl(cyanopheny1)-cyclohexane is measured by following spin-lattice relaxometry profile. The significance of the plateau in the profiles at low frequencies is discussed215 . Fast field-cycling in combination with conventional NMR techniques are employed to study molecular dynamics on the liquid crystal of discotic molecules exhibiting an ordered columnar phase216. The spin-lattice relaxation results are analysed considering the structure of the mesophase and the types of movements which are expected to influence significantly the relaxation rate for high, medium and low Larmor frequency. The outstanding features of the molecular dynamics behaviour as observed by NMR in different columnar phases, such as mono- and poly-molecular transverse sections, are compared. The biforked molecules that form both lamellar and columnar mesophases also allow the comparison of the molecular movements between the two phases217. 9.2 Bandshapes-Order Parameters. - 2H NMR is performed on 4-octyl-4cynobiphenyl (8CB) liquid crystal confined to the randomely interconnected pores of Si02 aerogel as a function of temperature and SiO2 density. The spectral patterns are consistent with powder line shapes characterized by a single orientational order which is suppressed from the bulk's with no enhancement upon the onset of the smectic A phase2'*. In another study2I9,2H

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NMR is used to monitor the magnetic field induced alignment of the directors in the smectic A phase of 8CB by recording 2H spectra over a period of time and the results are reported for the pure mesogen. The effect of several new homonuclear dipolar decoupling methods on 13Cspectra of 5CB liquid crystals is also recorded and the results are compared with the results published previously22o.The orientational order in the nematic and smectic A, B, C phases formed by 4-(2-methylbutyl)phenyl-4-n-heptylphenyl-4-carboxylate is determined221.Lee-Goldburg decoupling method is applied to obtain the order parameters of the aromatic core of the liquid crystal N-(4-ethoxybenzylidene)4-n-butylaniline at different temperature. The values are in agreement with those obtained from the 2H NMR spectra222.The liquid crystal phases of 4cyanobenzoyloxy-(4-octylbenzoyloxy)-p-phenyleneundergo phase transitions I-N-SAd-RN-SAI and there is highly disordered chain structure in the SAd mesophase. The phase behaviour is proposed to be crystal to SAI lSt order, SAI to N 2"d order, N to SAd 1st order, SAd to N lSt order, and N to I 1'' order2237224. The existence of three bilayer smectic mesophases SA2, SC2 and SC? is reported. Experimental dipolar splittings measured for the SC2 and SC? are explained in terms of theoretical models225. The liquid crystals, in which two mesogenic groups are linked by a flexible spacer, a-(4'-cynobiphenyl-4-yloxy)-o-(4-alkylanilinebenzylidene-4'-oxy)alkane d i m e r ~ ~ ~ ~ ? ~ ~a~rich e x hsmectic i b i t polymorphism for both symmetric and asymmetric dimers which differ in the nature of the mesogenic groups. For example, smectic phases having monolayer, interdigitated and intercalcated structures are discovered. The liquid crystalline compound containing four rings in the main core, two lateral hexyloxy chains and a terminal dioxyethylene Me ether chain presents a wide nematic range starting from room temperature. The lateral chains fold back along the main core and the dioxyethylene chain assumes extended conformation228.Laterally dialkylated nematogens containing a short terminal polyoxyethylene chain present a wide nematic range starting from room temperature. The order parameter detemined from 13C NMR in the liquid crystalline phase decrease monotonically along the oxyethylene chain229.A wide enantiotropic nematic phase is identified in several other laterally substituted liquid crystalline compounds. The order parameter and its dependence on temperature are obtained by 13C NMR. The conformation of the chains and aromatic rings in the liquid crystals are discussed230.Laterally substituted two isomeric compounds containing a dodecyloxy chain is also studied. On entering the nematic phase, chemical shift shows a positive jump for the oxomethylene attached to the core, but a negative jump for the oxomethylene on the aromatic lateral branch. However, the carbons in dodecyloxy chain have negative order parameter 127. Dialkylaminomethylene lateral substituents can be used to reduce the melting temperatures of nematogens having a long rigid core but the thermodynamic stability due to this substituent does not increase231.Terminally alkyl- and alkyloxysubstituted 4,ti-dichloro-1,3-phenylene bis(4-(phenyliminomethyl)-benzoates) forms nematic and smectic phases, although comparable diesters of resorcinol and monochlororesorcinol exhibit B phases typical for banana-shaped meso-

550

Nuclear Magnetic Resonance

gens232.The local molecular structure in nematic and smectic C phases of p heptyl azoxyanizole is modelled from nuclear quadrupole resonance spectra. Both the phases consist of a huge number of bimolecular units and biaxial unit 6,7-bis(hexyloxy)-2,3-dihydrophthalazine1,4-diones self-assemble into trimaric disks in solution and with suitable substituents these disks self-organise into a thermotropic, columnar, discotic phase234. Bis(hydroxydimethylsily1)methane compound forms columnar liquid crystal over an extensive range of temperature. Molecular mechanics calculations also confirms the formation of columnar phase that consists of supramolecular H-bonded associations235.A new liquid crystalline phase, induced by the addition of small amounts of a non-mesogenic solute to a quaternary ammonium salt is detected by NMR. In some cases, there is a coexistence of nematic and smectic phases. The ordering of the phases are also examined236.The structure, conformation and orientational order of fluorinated liquid crystals are studied from I3C NMR237-238. The theoretical model predicts that the conformer probabilities for the molecules in the nematic phase are substantially different from those in the isotropic phase at the same temperature. The magnetohydriodynamics of a chiral nematic liquid crystal made from a mixture of PBLG and deuterated CHC13 determined by 2H NMR show two critical speeds, the first critical speed is obtained using a dynamical approach and the second one is revealed by the viscous and magnetic torques2".

10

Synthesis

Two biosurfactants, one of geminal-type and the other is glycolipid-type, are produced. Geminal cationic Na-acyl-arginine biodegradable surfactant having two symmetric residues of N"-acyl-L-arginine (10- 12 carbon atoms in the chain) joined by amide bonds through a,o-alkyldiamine chains of variable length are prepared240. The surfactants are structurally analogous to the bis(Quat)s. The glycolipid-type b i o ~ u r f a c t a n t ~is~ produced ' from Candida species, SY16. The hydrophilic moiety of the lipid consists of mannosylerythritol and that of hydrophobic moiety are fatty acids of variable chain length (C6, C12 and C14). The cmc of newly synthesized quaternary pyridinium surfactants are lower than those of corresponding monoquats having higher emulsification e f f i ~ i e n c e y ~16-fluoro ~~. palmitic acid is used to synthesise a fluorinated analog of DPPC. The phase transition temperature of F-DPPC occurs near 50°C about 10°C higher than DPPC. The solid state NMR demonstrates that F-DPPC forms a fully interdigitated bilayer in the gel phase243. Tetra-(4-octanoyloxyphenyl)porphyrin molecules are similar to discotic liquid crystal. This compound forms thermotropic liquid crystal which undergoes phase transition at around 27 0C244.a,w-bis(4-(4'-($)-(+)-2-methylbutylbiphenyl-4-carboxy)phenoxylalkanes have been synthesised. The chain length of the central polymethylene spacers of the chiral twin compounds is varied. A

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detailed study demonstrates that the formation of different liquid crystals and their transition temperatures are strongly dependent on the length of the spacer and at higher value of spacer, the compound shows the so-called oddeven effect245.The mesogenic compounds of the type alkoxybiphenyl resorcylate and vanillate derivatives with chiral moiety246,2-ring and 3-ring compounds containing o,a,a-trihydroperfluoroalkoxyterminal tails247as well as 3,6-bis(3,4-dialkyloxybenzylidene)-pipera~ine-2,5-diones~~~ are also synthesized. These compounds form smectic phases. The correlation of phase behaviour and molecular shape is also discussed. The liquid crystallinity of the is attributed to the presence of intermolecular H-bonds involving NH groups of the heterocyclic rings. The syntheses of liquid crystalline dendrimers of generation 1-4 containing 8, 16 and 32 terminal methoxyphenyl benzoate mesogenic groups are reported for the first time. These compounds form smectic mesophases as well as crystalline phases over wide temperature range249.The formation of binary liquid cystal mixtures from a thermal cyclization process at a constant temperature is also reported250.The liquid crystalline materials reported here are characterized by several techniques, such as elemental analysis, IR, NMR, DSC and microscopy.

References 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

B. Halle and I. Furo in Phase Transitions Complex Fluids, (eds.) P. Toledano and N. A. M . Neto, World Scientific, Singapore, 1998. R. Blinc and I. Musevic, Handb. Liq. Cryst., 1998, 2A, 170. A. Khan and E. F. Marques, Curr. Opinion Colloid Inter- Sci., 2000,4,402. A. Khan, Nucl. Magn. Reson, 1999, 28, 522. L. C. M. V. Gorkom and A. Jensen, Surfactant Sci. Ser., 1998,73, 169. Y. Galerne, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A, 1998,323, 21 1 . R. C. Zamar, C. E. Gonzalez and 0. Mensio, Braz. J. Phys., 1998,28,314. G . Chidichimo and F. P. Nicoletta in Phase Transitions Complex Fluids, (eds.) P. Toledano and N. A. M. Neto, World Scientific, Singapore, 1998. P. M. Macdonald, K. J. Crowell, C. M. Franzin, P. Mitrakos and D. J. Semchyschyn, Biochem. Cell Biol., 1998,76,452. P. Slilbs, Surfactant Sci. Ser., 1998,77,239. G. Lindblom and P.-0. Quist, Curr. Opin. Colloid. Interface Sci., 1998,3,499. A. Watts, Biochim. Biophys. Acta, 1998, 1376,297. L. K. Creamer, J. E. Plowman, M. J. Liddell, M. H. Smith and J. P. Hill, J. Dairy. Sci., 1998, 81, 3004. T. Wymore, X. F. Gao and T. C. Wang, J. Mol. Struct., 1999,485-486, 195. T. Hwsslein, D. M. Newns, P. C. Pattnaik, Q. Zhong, P. B. Moore and M. L. Klein, J. Chem. Phys., 1998, 109,2826. K. Tu, M. Tarek, M . L. Klein and D. Scharf, Biophys. J . , 1998,75,2123. M. D. Whitmore and J. P. Whitehead, Can. J. Phys., 1998,76, 883. G . KIose and Y. K. Levine, Langmuir, 2000, 16,671. U. S. Schwarz and G. Gompper, Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Rrlat. Interdiscip. Top., 1999, 59, 5528. R. D. Neuman and T. H. Ibrahim, Langmuir, 1999,15, 10.

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