The Structures of Cellulose. Characterization of the Solid States 9780841210325, 9780841211834, 0-8412-1032-2

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 9780841210325, 9780841211834, 0-8412-1032-2

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The Structures of Cellulose

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS

SYMPOSIUM

SERIES

The Structures of Cellulose Characterization of the Solid States Rajai H. Atalla, EDITOR Institute of Paper Chemistry

Developed from a symposium sponsored by the Cellulose, Paper, and Textile Division at the 190th Meeting of the American Chemical Society, Chicago, Illinois, September 8-13, 1985

American Chemical Society, Washington, DC 1987

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

340

Library of Congress Cataloging-in-Publication Data The structures of cellulose. (ACS symposium series; 340) Includes bibliographies and indexes. 1. Cellulose—Congresses. I. A l a l i a , Rajai H . , 1935. II. American Chemical Society. Cellulose, Paper, and Textile Division. III. American Chemical Society. Meeting (190th: 1985: Chicago, Ill.) IV. Series. TS933.C4S77 1987 ISBN 0-8412-1032-2

547.7'82

87-11537

Copyright © 1987 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright L a w . This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by A C S of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law.

PRINTED IN THE UNITED STATES OF AMERICA

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ACS Symposium Series M. Joan Comstock, Series Editor 1987 Advisory Board Harvey W. Blanch University of California—Berkele Alan Elzerman Clemson University

W. H. Norton J. T. Baker Chemical Company

John W. Finley Nabisco Brands, Inc.

James C. Randall Exxon Chemical Company

Marye Anne Fox The University of Texas—Austin

E. Reichmanis AT&T Bell Laboratories

Martin L. Gorbaty Exxon Research and Engineering Co.

C. M . Roland U.S. Naval Research Laboratory

Roland F. Hirsch U.S. Department of Energy

W. D. Shults Oak Ridge National Laboratory

G. Wayne Ivie USDA, Agricultural Research Service

Geoffrey K. Smith Rohm & Haas Co.

Rudolph J. Marcus Consultant, Computers & Chemistry Research

Douglas B. Walters National Institute of Environmental Health

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Foreword The ACS SYMPOSIUM SERIES was founded in 1974 to provide a medium for publishin symposi quickl i book form Th format of the Serie IN CHEMISTRY SERIES except that, in order to save time, the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Preface "We must love them both, those whose opinions we share and those whose opinions we reject. For both have labored in the search for truth and both have helped us in the finding of it." —Saint Thomas Aquinas

CONTROVERSY HAS BEEN A CONSTANT IN THE STUDY OF CELLULOSE since it began in the middle of of industrial use of cellulosic raw materials, and with advances in plant biology. The early controversies involved many hypotheses concerning the chemical nature of cellulose and culminated in the acceptance of the polymer hypothesis during the first decades of this century. More recent controversies have dealt with hypotheses concerning the physical structures of cellulose as well as the mechanisms of its biogenesis. The symposium upon which this book is based was an attempt to promote convergence among the structural hypotheses toward a useful paradigm. Such a model would be helpful to practitioners in other areas of cellulose science who are seeking to organize chemical or biological data concerning cellulose in relation to its structure. The symposium also sought to bring together reports from the leading laboratories active in structural studies on the many forms and complexes of cellulose. The objective was to incorporate discussion of new methodologies that have been applied to cellulose in the past decade and to include the most recent results based on more traditional methods of structural investigation. Furthermore, the organizers of the symposium wanted to include some presentations representative of the uses of structural studies to complement investigations of other aspects of cellulose. Another objective of the symposium, in addition to promoting further studies in the field, was to provide those in related fields with a sense of the origin of the controversies and the questions that remain open. In a review published in 1970, D. W. Jones wrote, "After extensive studies by many crystallographers over the last 50 years.. .many uncertainties remain about the crystal structures of the celluloses and their derivatives." Later in the same review he added, "When evidence from spectroscopy and stereochemistry is taken into account, the X-ray data from cellulose I, modest as they are, have not been shown to be consistent with any conventional crystal structure." The diversity of views represented

ix In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

in the chapters of this book suggests that the observations made by Jones remain valid today, though perhaps some convergence of views can be perceived. Investigators attempting to interpret the diffractometric data seek the simplest structure consistent with the data. On the other hand, neither recent spectroscopic results nor some of the earlier data concerning allomorphic transformation could be rationalized in terms of the simple structures that were fitted to the diffractometric data. Thus, to the extent that a structural model is to be used to organize and interpret patterns of behavior, the simple structural models are inadequate. Yet the diffractometric data have not provided a sufficient basis for refinement of a structural model with a larger number of internal degrees of freedom. The chapters in this book are at the leading edge of the effort to resolve the questions that remain in this area. Readers familiar wit terminology cellulose structures will note that, in many chapters, the commonly used polymorph has been replaced with allomorph. This term was suggested by A. D. French, who pointed out that this differentiation is more consistent with correct usage in crystallography. This usage has not been required of authors, however, so both forms occur in the book. The cooperation of many individuals has been central to completion of this volume. R . St. John Manley first suggested the symposium when he was program chairman for the Cellulose, Paper, and Textile Division. More recently, as chairman of the division, he supported publication of this volume in the ACS Symposium Series. The referees provided an important measure of refinement for the manuscripts beyond the initial drafts. A. D. French was willing to undertake a disproportionate share of the review process and provided helpful editorial assistance for a number of manuscripts. The Institute of Paper Chemistry provided valuable assistance with respect to correspondence and preparation of a number of manuscripts not originating at the Institute; Grace Kessler was particularly helpful. Robin Giroux of the ACS Books Department provided support and helpful suggestions at many stages during the publication process. Finally, the authors, who invested time and effort in the preparation of the manuscripts, are the individuals without whom the publication would not be possible. I extend to all my deepest appreciation. RAJAI

H.

ATALLA

Institute of Paper Chemistry Appleton, WI March 1987

x In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 1

Structures of Cellulose Rajai H. Atalla Institute of Paper Chemistry, Appleton, WI 54912

An overview of studies of the structure of cellulose is presented and begins with a historical perspective, developed with particular emphasis on the early diffractometric studies. More recent studies are then described, and the key questions confronted in any analysis of diffractometri central question the assumption that the unit cells of cellulose belong to space group P2 , and whether the twofold screw axis associated with this space group coincides with the molecular chain axes. The diversity of the interpretations which occur in the literature and in following chapters is noted. More recent spectroscopic investigations are then discussed, with emphasis on the degree to which they may provide additional information concerning structure. It is noted that although both Raman spectroscopy and CP-MAS C NMR cannot provide direct information concerning the positions of molecules in the unit cells, they are sensitive to the values of the internal coordinates. Thus, they provide information complementary to the diffractometric data in that i t serves to constrain the acceptable structural models to a smaller subset than that otherwise admissible on the basis of diffractometric observations alone. In this respect, the spectroscopic information complements the diffractometric data in the same way as the assumptions concerning the symmetry of the unit cell. Furthermore, i t appears that the structures suggested by the spectroscopic studies represent relatively small although significant departures from those derived on the basis of diffractometry alone. In anticipation of future directions in studies of celluloses, i t is noted that multidisciplinary approaches, similar to some described in later chapters, hold great promise for future progress in understanding the structural diversity that is characteristic of cellulose. 1

13

0097-6156/87/0340-0001 $06.00/0 © 1987 American Chemical Society

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2

THE STRUCTURES OF CELLULOSE

S i n c e the o c c u r r e n c e o f c e l l u l o s e as a d i s t i n c t s u b s t a n c e was first r e c o g n i z e d by Anselme Payen i n 1842 the e v o l u t i o n o f i d e a s c o n c e r n i n g i t s s t r u c t u r e has been c l o s e l y r e l a t e d t o advances i n s t r u c t u r a l c h e m i s t r y and i t s m e t h o d o l o g i e s . The p a t t e r n o f c l o s e r e l a t i o n c o n t i n u e s i n t o the p r e s e n t time and i s w e l l r e f l e c t e d i n the f o l l o w i n g c h a p t e r s which i n c l u d e c o n t r i b u t i o n s from most o f the major l a b o r a t o r i e s a c t i v e i n the f i e l d . In t h i s c h a p t e r , we d i s c u s s the s t r u c t u r a l problem i n g e n e r a l and p l a c e t h o s e o f the f o l l o w i n g c h a p t e r s which are concerned w i t h the problem i n p e r s p e c t i v e r e l a t i v e t o r e c e n t developments i n the f i e l d , w i t h p a r t i c u l a r emphasis on the p a s t decade. The p r o c e d u r e s f o r s t r u c t u r a l s t u d i e s on c e l l u l o s e have much i n common w i t h i n v e s t i g a t i o n s of s t r u c t u r e i n polymers i n g e n e r a l . In most i n s t a n c e s d i f f r a c t o m e t r i c d a t a are not s u f f i c i e n t f o r a s o l u t i o n o f the s t r u c t u r e i n a manner a n a l o g o u s t o t h a t p o s s i b l e f o r lower m o l e c u l a r weight compounds which can be made t o form s i n g l e crystals. It becomes n e c e s s a r y therefore t o complement d i f f r a c tometric data with s t r u c t u r a c a r r i e d out on the monomer Kakudo and K a s a i have summarized the c e n t r a l problem w e l l (_1_): "There are g e n e r a l l y l e s s t h a n 100 i n d e p e n d e n t l y o b s e r v a b l e d i f f r a c t i o n s f o r a l l l a y e r l i n e s i n the x - r a y diagram o f a f i b r o u s polymer. T h i s c l e a r l y imposes l i m i t a t i o n s on the p r e c i s i o n which can be a c h i e v e d i n polymer s t r u c t u r e a n a l y s i s , e s p e c i a l l y i n c o m p a r i s o n w i t h the 2000 o r more d i f f r a c t i o n s o b s e r v a b l e f o r o r d i n a r y s i n g l e crystals. However, the m o l e c u l a r c h a i n s o f the h i g h polymer u s u a l l y p o s s e s s some symmetry o f t h e i r own, and i t i s o f t e n p o s s i b l e t o d e v i s e a s t r u c t u r a l model o f the m o l e c u l a r c h a i n t o i n t e r p r e t the f i b e r p e r i o d i n terms o f the c h e m i c a l c o m p o s i t i o n by comparison w i t h s i m i l a r o r homologous s u b s t a n c e s o f known s t r u c t u r e . Structural i n f o r m a t i o n from methods o t h e r t h a n x - r a y d i f f r a c t i o n ( e . g . , i n f r a r e d and NMR s p e c t r o s c o p y ) a r e a l s o sometimes h e l p f u l i n d e v i s i n g a s t r u c t u r a l model o f the m o l e c u l a r c h a i n . The m a j o r i t y of the s t r u c t u r a l a n a l y s e s which have so f a r been performed a r e based on models d e r i v e d i n t h i s way. T h i s i s , o f c o u r s e , a t r i a l and e r r o r method". S i m i l a r p e r s p e c t i v e s have been p r e s e n t e d by A r n o t t (2), A t k i n s (3_), and Tadokoro (4,_^). An a c c e p t a b l e f i t t o the d i f f r a c t o m e t r i c d a t a i s not the u l t i mate o b j e c t i v e , however. R a t h e r i t i s the development o f a model t h a t p o s s e s s e s a s i g n i f i c a n t measure o f v a l i d i t y as the b a s i s f o r o r g a n i z a t i o n , e x p l a n a t i o n and p r e d i c t i o n o f e x p e r i m e n t a l o b s e r v a tions. With r e s p e c t t o t h i s c r i t e r i o n , the models of c e l l u l o s e which have been d e v e l o p e d so f a r l e a v e much t o be d e s i r e d , f o r t h e i r c a p a c i t y to i n t e g r a t e and u n i f y the v a s t a r r a y o f i n f o r m a t i o n c o n cerning c e l l u l o s e i s l i m i t e d indeed. One o f the o b j e c t i v e s o f t h i s symposium i s to f a c i l i t a t e i d e n t i f i c a t i o n o f p o i n t s o f d e p a r t u r e f o r f u r t h e r s t u d i e s i n s e a r c h o f models which a r e more u s e f u l . To h e l p p l a c e the p r o c e e d i n g s i n p e r s p e c t i v e we b e g i n w i t h a b r i e f h i s t o r i c a l r e v i e w , and c o n t i n u e w i t h a d i s c u s s i o n o f r e c e n t c o n t r i b u t i o n s based on the key m e t h o d o l o g i e s which have been used. The m e t h o d o l o g i e s are i n t h r e e broad, complementary c a t e g o r i e s , which i n c l u d e d i f f r a c t o m e t r y , s p e c t r o s c o p y , and t h e o r e t i c a l model b u i l d i n g on the b a s i s o f c o n f o r m a t i o n a l a n a l y s i s . Although, s i g n i f i c a n t s t r u c t u r a l i n f o r m a t i o n i s i n f e r r e d from p a t t e r n s o f c h e m i c a l

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Structures of Cellulose

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r e a c t i o n s under a wide range o f c o n d i t i o n s , we l i m i t t h i s c h a p t e r t o s t u d i e s based on p h y s i c a l methods. In o r d e r t o a c h i e v e g r e a t e r c l a r i t y i n t h e f o l l o w i n g d i s c u s s i o n , i t i s w e l l t o note that q u e s t i o n s o f s t r u c t u r e a r i s e at three different levels. The f i r s t , t h a t o f t h e c h e m i c a l s t r u c t u r e , r e f l e c t s t h e p a t t e r n o f c o v a l e n t b o n d i n g i n c e l l u l o s e m o l e c u l e s and i s generally well established. While t h e e v o l u t i o n o f c o n c e p t s a t t h i s l e v e l i s o f h i s t o r i c a l i n t e r e s t , i t i s not under d i s c u s s i o n i n these proceedings. The next l e v e l o f s t r u c t u r e i s t h a t o f t h e r e l a t i v e o r g a n i z a t i o n o f t h e r e p e a t u n i t s i n an i n d i v i d u a l m o l e c u l e , under c o n s t r a i n t s o f c o n f o r m a t i o n a l energy c o n s i d e r a t i o n s , as w e l l as c o n s i d e r a t i o n s o f p a c k i n g o f t h e m o l e c u l e s i n a p a r t i c u l a r s t a t e of aggregation. This l e v e l o f s t r u c t u r e i s p a r t i c u l a r l y important i n s p e c t r o s c o p i c s t u d i e s w h e r e i n t h e energy l e v e l s between which t r a n s i t i o n s a r e o b s e r v e d a r e d e t e r m i n e d by t h e v a l u e s o f the i n t e r n a l c o o r d i n a t e s which d e f i n e m o l e c u l a r c o n f o r m a t i o n s . The f i n a l l e v e l o f s t r u c t u r e i s t h a t r e f l e c t i n g t h e arrangement o f the mole c u l e s r e l a t i v e t o each o t h e whether i t be amorphous t a l l i n e a l l o m o r p h s which o c c u r because o f t h e polymorphy c h a r a c t e r i s t i c o f the c r y s t a l l i n i t y o f c e l l u l o s e . T h i s i s t h e l e v e l o f s t r u c t u r e probed by d i f f r a c t o m e t r i c measurements which a r e i n h e r e n t l y most s e n s i t i v e t o t h e t h r e e d i m e n s i o n a l o r g a n i z a t i o n r e p r e s e n t e d by a p a r t i c u l a r s t a t e o f a g g r e g a t i o n . Historical

Overview

The e v o l u t i o n o f i d e a s c o n c e r n i n g t h e n a t u r e o f c e l l u l o s e and the models o f i t s c h e m i c a l s t r u c t u r e have been d e s c r i b e d by P u r v e s (6^) i n an e x c e l l e n t o v e r v i e w , b e g i n n i n g w i t h t h e f i r s t observations by Payen and l e a d i n g up t o t h o s e which f i n a l l y won a c c e p t a n c e o f t h e polymer h y p o t h e s i s i n t h e decade i m m e d i a t e l y p r e c e d i n g t h e Second World War. A n o t h e r v a l u a b l e p e r s p e c t i v e i s p r e s e n t e d by F l o r y (7) i n h i s general review of the e v o l u t i o n o f the polymeric h y p o t h e s i s , h i g h l i g h t i n g i n v e s t i g a t i o n s o f the t h r e e common n a t u r a l homopolymers: s t a r c h , c e l l u l o s e , and n a t u r a l r u b b e r . F i n a l l y , the f i r s t c h a p t e r i n t h e t r e a t i s e by Hermans (8) f o c u s e s on the p h y s i c a l chemi c a l a s p e c t s o f t h e e a r l y s t r u c t u r a l s t u d i e s , i n an a c c o u n t which i s an e x c e l l e n t complement t o t h e r e v i e w by Purves w i t h i t s emphasis on the c l a s s i c a l o r g a n i c c h e m i c a l phase i n t h e s t r u c t u r a l s t u d i e s . Among more r e c e n t r e v i e w s o f s t r u c t u r e , t h o s e by Jones (90, and by Tonessen and E l l e f s e n (lOjJjO a r e t h e most comprehensive. P r e s t o n (12) and F r e y - W y s s l i n g (13) i n t h e i r r e s p e c t i v e t r e a t i s e s on p l a n t c e l l w a l l s , have a l s o t o u c h e d upon t h e p r o b l e m o f t h e s t r u c t u r e o f c e l l u l o s e . The r e a d e r i s r e f e r r e d t o t h e s e s o u r c e s f o r comprehensive p r e s e n t a t i o n s o f t h e range o f p r o p o s a l s c o n c e r n i n g t h e s t r u c t u r e s o f c e l l u l o s e which have been under d i s c u s s i o n i n r e c e n t decades. A r e p r e s e n t a t i v e s u b s e t w i l l be p r e s e n t e d h e r e as a p o i n t of departure f o r following d i s c u s s i o n s . Quite e a r l y i n the x-ray d i f f r a c t o m e t r i c s t u d i e s o f c e l l u l o s e i t was r e c o g n i z e d t h a t i t s c r y s t a l l i n i t y i s p o l y m o r p h i c . I t was e s t a b l i s h e d t h a t n a t i v e c e l l u l o s e , on t h e one hand, and b o t h r e g e n r a t e d and m e r c e r i z e d c e l l u l o s e s , on t h e o t h e r , r e p r e s e n t two d i s t i n c t c r y s t a l l o g r a p h i c allomorphs (14). L i t t l e has t r a n s p i r e d

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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THE STRUCTURES OF CELLULOSE

s i n c e the e a r l y s t u d i e s t o change t h e s e p e r c e p t i o n s . There has been, however, l i t t l e agreement r e g a r d i n g the s t r u c t u r e s o f the two forms. For example, P e t i t p a s et a l . (15) have s u g g e s t e d on the b a s i s o f e x t e n s i v e a n a l y s e s o f e l e c t r o n - d e n s i t y d i s t r i b u t i o n s from x - r a y d i f f r a c t o m e t r i c measurements t h a t c h a i n c o n f o r m a t i o n s a r e d i f f e r e n t i n c e l l u l o s e s I and I I . In c o n t r a s t , Norman (16) has i n t e r p r e t e d the r e s u l t s o f h i s e q u a l l y comprehensive x - r a y d i f f r a c t o m e t r i c s t u d i e s i n terms o f s i m i l a r c o n f o r m a t i o n s f o r the two a l l o morphs. At a more b a s i c l e v e l t h a n the comparison o f c e l l u l o s e s I and I I , the s t u c t u r e o f the n a t i v e form i t s e l f has remained i n q u e s t i o n . Among r e c e n t s t u d i e s , f o r example, B l a c k w e l l and Gardner ( 1 7 ) , i n t h e i r a n a l y s i s o f the s t r u c t u r e o f c e l l u l o s e from V a l o n i a v e n t r i c o s a , assumed a l a t t i c e b e l o n g i n g t o the ?2\ space group, w i t h the t w o f o l d screw a x i s c o i n c i d e n t w i t h the m o l e c u l a r c h a i n a x i s . Hebert and M u l l e r ( 1 8 ) , on the o t h e r hand, i n an e l e c t r o n d i f f r a c t o m e t r i c study o f a number of c e l l u l o s e s i n c l u d i n g V a l o n i a , c o n f i r m e d the findings of e a r l i e r i n v e s t i g a t o r o f the odd o r d e r r e f l e c - t i o n P 2 j , and c o n c l u d e d t h a t the c e l l u l o s e u n i t c e l l s do not b e l o n g t o t h a t space group. Even when P2^ i s t a k e n t o be the a p p r o p r i a t e space group, the q u e s t i o n o f c h a i n p o l a r i t y remains. As n o t e d by Jones ( 1 9 ) , and by Howsmon and S i s s o n ( 2 0 ) , the s t r u c t u r e i n i t i a l l y p r o p o s e d by Meyer and Mark (21) assumed t h a t the c h a i n s were p a r a l l e l i n p o l a r i t y . The s t r u c t u r e l a t e r proposed by Meyer and M i s c h (22) was based on the r e a s o n i n g t h a t the r a p i d i t y o f m e r c e r i z a t i o n , and i t s o c c u r r e n c e w i t h o u t d i s s o l u t i o n r e q u i r e d t h a t the p o l a r i t y o f the c h a i n s be the same i n b o t h c e l l u l o s e s I and I I . It was r e a s o n e d f u r t h e r t h a t r e g e n e r a t i o n o f c e l l u l o s e from s o l u t i o n i s most l i k e l y t o r e s u l t i n p r e c i p i t a t i o n i n an a n t i p a r a l l e l form, and t h a t the s i m i l a r i t y between x - r a y d i f f r a c t i o n p a t t e r n s of m e r c e r i z e d and r e g e n e r a t e d c e l l u l o s e r e q u i r e d t h a t they have the same p o l a r i t y . It was thus i n f e r r e d t h a t n a t i v e c e l l u l o s e must a l s o have an a n t i p a r a l l e l s t r u c t u r e . A l t h o u g h the argument t h a t r e g e n e r a t i o n i n the a n t i p a r a l l e l mode i s more p r o b a b l e was found i n v a l i d w i t h i n a decade o f i t s f i r s t p r e s e n t a t i o n ( 2 3 ) , the r e l a t i v e o r g a n i z a t i o n of m o l e c u l e s s u g g e s t e d by Meyer and M i s c h remained the p o i n t o f d e p a r t u r e f o r most subsequent i n v e s t i g a t o r s . When the models i n c o r p o r a t i n g a n t i p a r a l l e l arrangement o f the c h a i n s are extended t o n a t i v e c e l l u l o s e , they pose s e r i o u s q u e s t i o n s c o n c e r n i n g proposed mechanisms f o r the b i o s y n t h e s i s o f c e l l u l o s e . It i s d i f f i c u l t t o e n v i s i o n a p l a u s i b l e mechanism f o r s i m u l t a n e o u s s y n t h e s i s and a g g r e g a t i o n o f a n t i p a r a l l e l c h a i n s . It i s perhaps f o r t h i s r e a s o n t h a t more r e c e n t p r o p o s a l s o f p a r a l l e l s t r u c t u r e s f o r n a t i v e c e l l u l o s e have been embraced by i n v e s t i g a t o r s o f the mechanism of b i o s y n t h e s i s . The c o n t r i b u t i o n of s p e c t r o s c o p y t o the e a r l y s t u d i e s of s t r u c t u r e was q u i t e l i m i t e d . An i m p o r t a n t c o n t r i b u t i o n was made i n t h e s t u d i e s by L i a n g and M a r c h e s s a u l t (24-26) w h e r e i n measurements o f d i c h r o i s m i n i n f r a r e d a b s o r p t i o n o f o r i e n t e d specimens l e d to p r o p o s a l o f a p a r t i c u l a r h y d r o g e n - b o n d i n g scheme. The d i f f e r e n c e s between the s p e c t r a o f c e l l u l o s e s I and I I were e x p l a i n e d i n terms o f d i f f e r e n c e s i n the p a c k i n g o f m o l e c u l a r c h a i n s and a s s o c i a t e d

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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v a r i a t i o n s i n the h y d r o g e n - b o n d i n g p a t t e r n s . In a n o t h e r a p p l i c a t i o n , i n f r a r e d a b s o r p t i o n measurements were used as the b a s i s o f a c r y s t a l l i n i t y i n d e x by N e l s o n and O'Connor (27,28). More r e c e n t l y , a number o f new s t r u c t u r e s e n s i t i v e t e c h n i q u e s have been d e v e l o p e d , and they have been a p p l i e d t o s t u d i e s o f c e l l u lose. These i n c l u d e Raman s p e c t r o s c o p y and S o l i d S t a t e ^ C Nuclear M a g n e t i c Resonance, i n the e x p e r i m e n t a l a r e n a , and c o n f o r m a t i o n a l energy c a l c u l a t i o n s i n the t h e o r e t i c a l domain. These are more r e c e n t c o n t r i b u t i o n s and are the s u b j e c t s o f subsequent s e c t i o n s i n t h i s c h a p t e r and l a t e r c h a p t e r s i n t h e s e p r o c e e d i n g s . Diffractometric

Studies

As n o t e d by Kakudo and K a s a i , the p r i m a r y d i f f i c u l t y i n s t r u c t u r a l s t u d i e s on p o l y m e r i c f i b e r s i s t h a t the number o f r e f l e c t i o n s u s u a l l y o b s e r v e d i n d i f f r a c t o m e t r i c s t u d i e s are q u i t e l i m i t e d . In the c a s e o f c e l l u l o s e i t i s g e n e r a l l y d i f f i c u l t t o o b t a i n more t h a n 50 r e f l e c t i o n s . C o n s e q u e n t l number of s t r u c t u r a l c o o r d i n a t e a d o p t i n g p l a u s i b l e assumptions c o n c e r n i n g the s t r u c t u r e o f the monomeric e n t i t y . The l i m i t e d s c a t t e r i n g d a t a a r e t h e n used t o d e t e r mine the o r i e n t a t i o n o f the monomer u n i t s w i t h r e s p e c t t o each other. In the m a j o r i t y o f d i f f r a c t o m e t r i c s t u d i e s o f c e l l u l o s e p u b l i s h e d so f a r , the monomeric e n t i t y has been chosen as the anhydroglucose u n i t . Thus, s t r u c t u r a l i n f o r m a t i o n from s i n g l e c r y s t a l s o f g l u c o s e i s i m p l i c i t l y i n c o r p o r a t e d i n the a n a l y s e s o f the s t r u c t u r e o f c e l l u l o s e . The c o o r d i n a t e s which are a d j u s t e d i n s e a r c h o f a f i t t o the d i f f r a c t o m e t r i c d a t a i n c l u d e t h o s e o f the p r i m a r y a l c o h o l group at C6, t h o s e o f the g l y c o s i d i c l i n k a g e , and t h o s e d e f i n i n g the p o s i t i o n s o f the c h a i n s r e l a t i v e t o each o t h e r . In a d d i t i o n t o s e l e c t i o n o f the s t r u c t u r e o f the monomer as the b a s i s f o r d e f i n i n g the i n t e r n a l c o o r d i n a t e s o f the r e p e a t u n i t , the p o s s i b l e s t r u c t u r e s a r e u s u a l l y f u r t h e r c o n s t r a i n e d by t a k i n g advant a g e o f any symmetry p o s s e s s e d by the u n i t c e l l . The symmetry i s d e r i v e d from the s y s t e m a t i c absence o f r e f l e c t i o n s which are f o r b i d den by the s e l e c t i o n r u l e s f o r a p a r t i c u l a r space group. In the c a s e o f c e l l u l o s e , the s i m p l i f i c a t i o n u s u a l l y i n t r o d u c e d i s the a p p l i c a t i o n o f the symmetry o f space group P 2 i , which i n c l u d e s a t w o f o l d screw a x i s p a r a l l e l t o the d i r e c t i o n o f the c h a i n s . The v a l i d i t y o f t h i s s i m p l i f i c a t i o n remains the s u b j e c t o f controversy, however, because the r e f l e c t i o n s which a r e d i s a l l o w e d under the s e l e c t i o n r u l e s o f the space group a r e i n f a c t f r e q u e n t l y o b s e r v e d . In most o f the s t u d i e s t h e s e r e f l e c t i o n s , which are u s u a l l y weak r e l a t i v e to the o t h e r main r e f l e c t i o n s , are assumed t o be n e g l i gible. The c o n t r o v e r s y c o n t i n u e s because the r e l a t i v e i n t e n s i t i e s can be i n f l u e n c e d by e x p e r i m e n t a l c o n d i t i o n s such as the p e r i o d s o f e x p o s u r e o f the d i f f r a c t o m e t r i c p l a t e s . F u r t h e r m o r e , the d i s a l l o w e d r e f l e c t i o n s tend t o be more i n t e n s e i n e l e c t r o n d i f f r a c t o m e t r i c measurements than i n x - r a y d i f f r a c t i o n measurements. Thus, more o f t e n t h a n not, i n v e s t i g a t o r s u s i n g e l e c t r o n d i f f r a c t i o n c h a l l e n g e the v a l i d i t y o f the a s s u m p t i o n o f t w o f o l d screw a x i s symmetry. The key assumption w i t h r e s p e c t t o symmetry, however, i s not the e x i s t e n c e of the t w o f o l d screw a x i s as an element o f the symmetry o f the u n i t c e l l , but r a t h e r the a d d i t i o n a l a s s u m p t i o n t h a t

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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t h i s a x i s c o i n c i d e s w i t h the a x i s o f the m o l e c u l a r c h a i n s o f c e l l u lose. T h i s l a t t e r assumption has, i m p l i c i t i n i t , a number o f a d d i t i o n a l c o n s t r a i n t s on the p o s s i b l e s t r u c t u r e s which can be d e r i v e d from the d a t a . It r e q u i r e s t h a t a d j a c e n t a n h y d r o g l u c o s e u n i t s are r e l a t e d t o each o t h e r by a r o t a t i o n o f 180 d e g r e e s about the a x i s , accompanied by a t r a n s l a t i o n e q u i v a l e n t to h a l f the l e n g t h o f the u n i t c e l l i n t h a t d i r e c t i o n ; i t i s i m p l i c i t , t h e r e f o r e , t h a t a d j a c e n t a n h y d r o g l u c o s e u n i t s are s y m m e t r i c a l l y e q u i v a l e n t and, c o r r e s p o n d i n g l y , t h a t a l t e r n a t i n g g l y c o s i d i c l i n k a g e s a l o n g the c h a i n are s y m m e t r i c a l l y equivalent. I f the assumption c o n c e r n i n g c o i n c i d e n c e o f the t w o f o l d screw a x i s and the m o l e c u l a r c h a i n a x i s were e x c l u d e d f o r example by l o c a t i n g the t w o f o l d screw a x i s between the m o l e c u l a r c h a i n s though s t i l l p a r a l l e l t o the c h a i n axes, the d i f f r a c t o m e t r i c p a t t e r n s would admit n o n e q u i v a l e n c e o f a l t e r n a t e g l y c o s i d i c l i n k a g e s a l o n g the m o l e c u l a r c h a i n , as w e l l as the n o n e q u i v a l e n c e o f a d j a c e n t anhydroglucose units. T h i s p o s s i b i l i t y has been i g n o r e d however i n large p a r t because i t r e q u i r e d i n a t e s which have t o b F u r t h e r m o r e , i t e x c l u d e s the p o s s i b i l i t y o f a n t i p a r a l l e l a l i g n m e n t o f c h a i n s i n the u n i t c e l l . The assumptions t h a t the u n i t c e l l p o s s e s s e s the symmetry o f space group P2\ and t h a t the t w o f o l d a x i s i s c o i n c i d e n t w i t h the c h a i n a x i s , do i n f a c t meet a c r i t e r i o n l o n g honored i n s c i e n t i f i c s t u d i e s , namely, W i l l i a m o f Ockham's p r i n c i p l e o f economy, which r e q u i r e s t h a t the most s i m p l e h y p o t h e s i s c o n s i s t e n t w i t h o b s e r v a t i o n s s h o u l d always be adopted. C l e a r l y the s t r u c t u r e based on the a n h y d r o g l u c o s e as the r e p e a t u n i t i s the most s i m p l e s t r u c t u r e t h a t a c c o u n t s f o r the m a j o r i t y o f the d i f f r a c t o m e t r i c d a t a . F u r t h e r m o r e , the d i f f r a c t o m e t r i c d a t a a v a i l a b l e a r e not s u f f i c i e n t t o a l l o w r e f i n e m e n t o f a s t r u c t u r e p o s s e s s i n g many more d e g r e e s o f freedom, as would be the c a s e i f the t w o f o l d a x i s were not assumed c o i n c i d e n t w i t h the c h a i n a x i s . The assumptions c o n c e r n i n g the symmetry o f the u n i t c e l l n o t e d above have been the b a s i s o f r e c e n t r e f i n e m e n t s o f the s t r u c t u r e o f c e l l u l o s e I. In one such r e f i n e m e n t (17) the f o r b i d d e n r e f l e c t i o n s were simply assumed n e g l i g i b l e , and the i n t e n s i t y d a t a from V a l o n i a c e l l u l o s e were used t o a r r i v e a t a f i n a l s t r u c t u r e . In a n o t h e r s t u d y , the i n a d e q u a t e i n f o r m a t i o n a l c o n t e n t o f the d i f f r a c t o m e t r i c d a t a was complemented w i t h a n a l y s e s o f l a t t i c e p a c k i n g e n e r g i e s ( 2 9 ) ; the f i n a l s t r u c t u r e s were c o n s t r a i n e d t o m i n i m i z e the p a c k i n g energy as w e l l as o p t i m i z i n g the f i t t o the d i f f r a c t o m e t r i c d a t a . Here the assumptions i m p l i c i t i n the w e i g h t i n g o f the p o t e n t i a l f u n c t i o n s which are used i n the energy c a l c u l a t i o n s , f u r t h e r c o m p l i c a t e the i n t e r p r e t a t i o n s . As n o t e d by F r e n c h , e t a l . i n a subsequent c h a p t e r i n t h e s e p r o c e e d i n g s , the s t r u c t u r e s d e r i v e d i n t h e s e two s t u d i e s , though b o t h based on p a r a l l e l c h a i n arrangements, are n e v e r t h e l e s s v e r y d i f f e r e n t c r y s t a l s t r u c t u r e s . When the same c o n v e n t i o n i s a p p l i e d i n d e f i n i n g the axes o f the c r y s t a l l a t t i c e , the s t r u c t u r e most f a v o r e d i n one a n a l y s i s i s s t r o n g l y r e j e c t e d i n the o t h e r . Furthermore, n e i t h e r of these i s s t r o n g l y favored over yet a t h i r d , a n t i p a r a l l e l s t r u c t u r e (30). The relevant

s t r u c t u r e s o f o l i g o m e r s are a n o t h e r i m p o r t a n t s o u r c e o f i n f o r m a t i o n c i t e d by Kakudo and K a s a i . The i m p l i c a t i o n s

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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the s t r u c t u r e s o f t h e d i s a c c h a r i d e s have been c o n s i d e r e d by A t a l l a (31) and were t h e b a s i s f o r r e a s s e s s m e n t o f t h e second a s s u m p t i o n c o n c e r n i n g symmetry n o t e d above. S t r u c t u r e s w i t h a l t e r n a t i n g none q u i v a l e n t g l y c o s i d i c l i n k a g e s were found more c o n s i s t e n t w i t h s p e c t r o s c o p i c data (32). S t u d i e s o f o l i g o m e r s have been e x t e n d e d i n two c h a p t e r s i n t h e p r e s e n t volume, w i t h t h e comparisons made p r i m a r i l y w i t h s t r u c t u r e s proposed f o r c e l l u l o s e I I . S a k t h i v e l , e t a l . a p p l i e d the R i e t v e l d c r y s t a l s t r u c t u r e method t o c e l l o t e t r a o s e . Their r e s u l t s favor a p a r a l l e l arrangement o f c h a i n s i n t h e u n i t c e l l , w i t h i n d i v i d u a l c h a i n s p o s s e s s i n g near t w o f o l d screw a x i s symmetry. In a study o f a number o f o l i g o m e r s , H e n r i s s a t , et^ a_l_. used a m u l t i d i s c i p l i n a r y approach t o examine t h e m a t t e r o f t h e v a l i d r e p e a t unit. T h e i r c o n f o r m a t i o n a l a n a l y s e s and NMR s p e c t r a were i n t e r p r e t e d i n terms o f n o n e q u i v a l e n t g l y c o s i d i c l i n k a g e s i n t h e i n d i v i d u a l c h a i n s , b u t t h e d i f f r a c t i o n d a t a were found most cons i s t e n t w i t h an a n t i p a r a l l e l s t r u c t u r e Spectroscopy S p e c t r o s c o p i c s t u d i e s are u s e f u l i n s t r u c t u r a l i n v e s t i g a t i o n s because they p r o v i d e i n f o r m a t i o n which i s complementary t o t h a t der i v e d from d i f f r a c t o m e t r i c d a t a . The i n f o r m a t i o n d e r i v e d from s p e c t r a i s not d i r e c t l y r e l a t e d t o the c o o r d i n a t e s o f molecules i n the u n i t c e l l . The s p e c t r a a r e , however, s e n s i t i v e t o t h e v a l u e s o f i n t e r n a l c o o r d i n a t e s which d e f i n e m o l e c u l a r s t r u c t u r e . Thus they provide a b a s i s f o r t e s t i n g the degrees o f equivalence o f s t r u c tures. Very o f t e n a l s o , s p e c i f i c s p e c t r a l f e a t u r e s c a n be i d e n t i f i e d w i t h p a r t i c u l a r f u n c t i o n a l groups d e f i n e d by d i s t i n c t i v e s e t s of i n t e r n a l coordinates. Two c l a s s e s o f s p e c t r a l s t u d i e s have been a p p l i e d f o r t h e f i r s t time d u r i n g t h e p a s t decade as t h e b a s i s o f s t r u c t u r a l s t u d i e s o f cellulose. These a r e Raman s p e c t r o s c o p y , and s o l i d s t a t e NMR u s i n g t h e CP/MAS t e c h n i q u e . Both have r a i s e d q u e s t i o n s c o n c e r n i n g the assumptions about symmetry i n c o r p o r a t e d i n t h e d i f f r a c t o m e t r i c studies. And w h i l e they cannot p r o v i d e d i r e c t i n f o r m a t i o n c o n c e r n i n g t h e s t r u c t u r e s , they e s t a b l i s h c r i t e r i a t h a t any s t r u c t u r e must meet t o be r e g a r d e d as an adequate model. The i n f o r m a t i o n from s p e c t r o s c o p i c s t u d i e s r e p r e s e n t s one o f t h e major p o r t i o n s o f t h e phenomenology t h a t any a c c e p t a b l e s t r u c t u r a l model must r a t i o n a l i z e . A l t h o u g h t h e new s p e c t r a l methods have a l s o found a p p l i c a t i o n i n i n v e s t i g a t i o n s o f s t r u c t u r a l changes i n d u c e d by m e c h a n i c a l t r e a t ments o r by t r e a t m e n t s w i t h s w e l l i n g a g e n t s , t h e f o l l o w i n g d i s c u s s i o n w i l l be l i m i t e d t o s t u d i e s which have f o c u s e d on q u e s t i o n s o f structure. The r e s u l t s o f such s t u d i e s have t o be r a t i o n a l i z e d by any model d e r i v e d from c r y s t a l l o g r a p h i c i n v e s t i g a t i o n s and thus p r o v i d e t e s t s o f c o n s i s t e n c y complementary t o t h e d i f f r a c t o m e t r i c d a t a , i n t h e sense s e t f o r t h by Kakudo and K a s a i . Raman S p e c t r o s c o p y . Raman s p e c t r o s c o p y i s the common a l t e r n a t i v e to i n f r a r e d spectroscopy f o r i n v e s t i g a t i n g molecular v i b r a t i o n a l s t a t e s and v i b r a t i o n a l s p e c t r a . I t has e n j o y e d a s i g n i f i c a n t r e v i v a l s i n c e t h e development o f l a s e r s o u r c e s f o r e x c i t a t i o n o f t h e spectra. I t s key advantage i n t h e p r e s e n t c o n t e x t i s t h a t i t i s

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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THE STRUCTURES OF CELLULOSE

p r i m a r i l y s e n s i t i v e t o t h e s k e l e t a l v i b r a t i o n s o f the c e l l u l o s e m o l e c u l e , w i t h t h e mode o f p a c k i n g i n t h e l a t t i c e h a v i n g o n l y s e c o n dary e f f e c t s . T h i s f e a t u r e i s a consequence o f the dependence o f Raman s p e c t r a l a c t i v i t y o f m o l e c u l a r v i b r a t i o n s on changes i n t h e p o l a r i z a b i l i t y o f v i b r a t i n g bond systems, r a t h e r t h a n changes i n associated molecular d i p o l e s . The most i n t e n s e c o n t r i b u t i o n s t o t h e s p e c t r a a r e due t o bond systems w h i c h a r e p r e d o m i n a n t l y c o v a l e n t i n c h a r a c t e r , w i t h t h e more p o l a r systems r e s u l t i n g i n much weaker bands. In t h e f i r s t d e t a i l e d comparison o f t h e Raman s p e c t r a o f c e l l u l o s e s I and I I , i t was c o n c l u d e d t h a t t h e d i f f e r e n c e s between the s p e c t r a , p a r t i c u l a r l y i n t h e low f r e q u e n c y r e g i o n , c o u l d n o t be a c c o u n t e d f o r i n terms o f c h a i n s p o s s e s s i n g t h e same c o n f o r m a t i o n but packed d i f f e r e n t l y i n t h e d i f f e r e n t l a t t i c e s ( 3 3 ) . As n o t e d above, t h a t had been t h e g e n e r a l i n t e r p r e t a t i o n o f d i f f r a c t o m e t r i c s t u d i e s o f the two most common a l l o m o r p h s . The s t u d i e s o f the Raman s p e c t r a l e d t o t h e p r o p o s a l t h a t two d i f f e r e n t s t a b l e con f o r m a t i o n s o f the c e l l u l o s morphs. In o r d e r t o e s t a b l i s h t h e d i f f e r e n c e s between t h e conformat i o n s , i n f o r m a t i o n from o t h e r s o u r c e s was c o n s i d e r e d . The r e s u l t s o f p u b l i s h e d c o n f o r m a t i o n a l energy c a l c u l a t i o n s s u g g e s t e d two s t a b l e c o n f o r m a t i o n s f o r t h e g l y c o s i d i c l i n k a g e s (34,35). These r e p r e s e n t r e l a t i v e l y s m a l l l e f t - h a n d e d and r i g h t - h a n d e d d e p a r t u r e s from t h e conformation o f the g l y c o s i d i c linkage i n a twofold h e l i c a l s t r u c ture. They a r e w e l l approximated, r e s p e c t i v e l y , by t h e experiment a l l y observed conformations o f the g l y c o s i d i c linkages i n the c r y s t a l s t r u c t u r e s o f the model d i s a c c h a r i d e s c e l l o b i o s e (36) and methyl-p-cellobioside (37). An a n a l y s i s o f the v i b r a t i o n a l s p e c t r a i n t h e OH s t r e t c h i n g r e g i o n f o r both t h e model d i s a c c h a r i d e s and f o r c e l l u l o s e s I and I I suggested that nonequivalent g l y c o s i d i c l i n k a g e s a l t e r n a t e along the molecular chains (31). The s o l i d s t a t e l ^ C NMR s p e c t r a were found c o n s i s t e n t w i t h t h i s model (38), a l t h o u g h a l t e r n a t i v e i n t e r p r e t a tions are also possible. F i n a l l y t h e Raman s p e c t r a i n t h e methylene b e n d i n g r e g i o n i n d i c a t e d t h a t t h e C6 c a r b o n s o c c u r i n two n o n e q u i v a l e n t environments i n c e l l u l o s e I b u t appear merged i n t o a s i n g l e s e t in c e l l u l o s e II (39). The r e s u l t s o f t h e s p e c t r o s c o p i c s t u d i e s were i n t e r p r e t e d i n terms o f n o n e q u i v a l e n c e o f a d j a c e n t a n h y d r o g l u c o s e u n i t s i n t h e m o l e c u l a r c h a i n s , r e q u i r i n g t h e b a s i c r e p e a t u n i t o f s t r u c t u r e t o be t a k e n as the d i m e r i c a n h y d r o c e l l o b i o s e u n i t . The d i f f e r e n c e between c e l l u l o s e I and I I was a s s o c i a t e d w i t h the l o c u s o f the n o n e q u i v a lence. In c e l l u l o s e I I i t was thought t o be a t t h e g l y c o s i d i c l i n k ages, w h i l e i n c e l l u l o s e I i t was t a k e n t o be c e n t e r e d a t C6 and t h e a d j a c e n t segment o f t h e pyranose r i n g s . To r e c o n c i l e t h e c o n c l u s i o n s o u t l i n e d above w i t h t h e r e q u i r e ments o f c h a i n p a c k i n g , t h e p r o p o s a l was made t h a t c e l l u l o s e c h a i n s p o s s e s s a l t e r n a t e l e f t - h a n d e d and r i g h t - h a n d e d g l y c o s i d i c l i n k a g e s i n sequence a l o n g t h e c h a i n axes. The l e f t - h a n d e d and r i g h t - h a n d e d l i n k a g e s were e n v i s i o n e d as r e p r e s e n t i n g r e l a t i v e l y s m a l l d e p a r t u r e s o f the d i h e d r a l a n g l e s from t h o s e p r e v a i l i n g f o r a t w o f o l d h e l i x . The degree o f d e p a r t u r e from t h e parameters o f a t w o f o l d h e l i x was seen as somewhat g r e a t e r f o r c e l l u l o s e I I t h a n f o r c e l l u l o s e I . The

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

ATALLA

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Structures of Cellulose

model i s d i s c u s s e d i n somewhat more d e t a i l and A t a l l a , l a t e r i n t h i s volume.

i n the c h a p t e r

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S o l i d S t a t e C NMR Spectra. The second i m p o r t a n t s p e c t r o s c o p i c method which has been a p p l i e d i n i n v e s t i g a t i n g the s t r u c t u r e o f c e l l u l o s e d u r i n g the p a s t decade i s high r e s o l u t i o n NMR o f the s o l i d s t a t e based on the CP-MAS t e c h nique. In t h i s t e c h n i q u e , c r o s s p o l a r i z a t i o n (CP) i s used t o enhance the l ^ C s i g n a l , h i g h power p r o t o n d e c o u p l i n g t o e l i m i n a t e d i p o l a r c o u p l i n g s w i t h p r o t o n s , and magic a n g l e s p i n n i n g (MAS) of the sample about a p a r t i c u l a r a x i s r e l a t i v e t o the f i e l d t o e l i m i nate chemical s h i f t anisotropy. A p p l i c a t i o n o f t h i s method r e s u l t s i n a c q u i s i t i o n o f s p e c t r a o f s u f f i c i e n t l y h i g h r e s o l u t i o n so t h a t c h e m i c a l l y e q u i v a l e n t c a r b o n s which o c c u r i n m a g n e t i c a l l y n o n e q u i v a l e n t s i t e s can be d i s t i n g u i s h e d . Though the t e c h n i q u e has been used by a number o f d i f f e r e n t i n v e s t i g a t o r s (38,40-43) we f o c u s h e r e on the s t u d i e s by V a n d e r H a r t and A t a l l a as r e p r e s e n t a t i v (44,45). Some r e s o n a n c c a r b o n s o c c u r i n the s p e c t r a o f a l l the c e l l u l o s e s i n v e s t i g a t e d . The s p e c t r a o f h i g h c r y s t a l l i n i t y samples o f c e l l u l o s e I I showed c l e a r s p l i t t i n g s o f the r e s o n a n c e s a s s o c i a t e d w i t h C4 and CI. These have been i n t e r p r e t e d as e v i d e n c e o f n o n e q u i v a l e n t g l y c o s i d i c l i n k a g e s a l o n g the m o l e c u l a r c h a i n s ( 3 8 ) , though i t has a l s o been s u g g e s t e d t h a t the s p l i t t i n g s may be e v i d e n c e f o r n o n e q u i v a l e n t c h a i n s i n the u n i t c e l l ( 4 3 ) . The l a t t e r argument l e a v e s open the q u e s t i o n as to why the r e s o n a n c e s f o r carbons 2, 3, 5, and 6 do not display similar splittings. Perhaps the most s i g n i f i c a n t new i n f o r m a t i o n d e r i v e d from the CP-MAS s p e c t r a i s t h a t r e l a t i n g t o the n a t i v e c e l l u l o s e s . The s p e c t r a r e v e a l m u l t i p l i c i t i e s t h a t cannot be i n t e r p r e t e d i n terms o f a u n i q u e u n i t c e l l , even though they a r i s e from m a g n e t i c a l l y none q u i v a l e n t s i t e s i n c r y s t a l l i n e domains. The narrow l i n e s o b s e r v e d have r e l a t i v e i n t e n s i t i e s which a r e n e i t h e r c o n s t a n t among the samples o f d i f f e r e n t n a t i v e c e l l u l o s e s , nor are they i n the r a t i o s o f s m a l l whole numbers as would be e x p e c t e d i f they a r o s e from d i f ferent s i t e s w i t h i n a r e l a t i v e l y small u n i t c e l l . V a n d e r H a r t and A t a l l a proposed t h a t n a t i v e c e l l u l o s e s a r e c o m p o s i t e s o f two d i s t i n c t c r y s t a l l i n e forms ( 4 4 , 4 5 ) . S p e c t r a o f the two forms were r e s o l v e d t h r o u g h l i n e a r comb i n a t i o n o f the s p e c t r a o f n a t i v e c e l l u l o s e s p o s s e s s i n g the two forms i n d i f f e r e n t p r o p o r t i o n s . The two t y p e s were d e s i g n a t e d celluloses I and I g . The I form was found t o be dominant i n c e l l u l o s e s from lower p l a n t forms and b a c t e r i a l c e l l u l o s e s , w h i l e the Ig form was found dominant i n c e l l u l o s e s from h i g h e r p l a n t s . In s t u d i e s o f the Raman s p e c t r a o f d i f f e r e n t n a t i v e c e l l u l o s e s , A t a l l a (32) c o n c l u d e d t h a t the two forms I and Ig c o n s i s t o f m o l e c u l a r c h a i n s which have the same m o l e c u l a r c o n f o r m a t i o n . In the c h a p t e r by W i l e y and A t a l l a i n the p r e s e n t volume, e v i d e n c e i s p r e s e n t e d t o suggest t h a t though the m o l e c u l a r c o n f o r m a t i o n s a r e the same, the h y d r o g e n - b o n d i n g p a t t e r n s d i f f e r i n the two forms. VanderHart and A t a l l a a l s o p r e s e n t a d d i t i o n a l C NMR CP-MAS e x p e r i m e n t s i n a subsequent c h a p t e r . These p r o v i d e s t r o n g e v i d e n c e f o r the e x i s t e n c e o f the I and Ig forms i n n a t i v e c e l l u l o s e s , a

a

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In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE

10

p a r t i c u l a r l y t h o s e from t h e lower p l a n t s and b a c t e r i a l c e l l u l o s e . They do r a i s e , however, some q u e s t i o n s about t h e e a r l i e r e s t i m a t e s o f the amount o f I form i n t h e n a t i v e c e l l u l o s e s from t h e h i g h e r plants. In y e t a n o t h e r a p p l i c a t i o n o f t h e CP-MAS l ^ C NMR s p e c t r o s c o p y i n s t u d i e s o f t h e s t r u c t u r e o f c e l l u l o s e s , H o r i i , e t a l . have i n t r o duced c o r r e l a t i o n s between t h e c h e m i c a l s h i f t s and d i h e d r a l a n g l e s as the b a s i s o f d e v e l o p i n g new s t r u c t u r a l i n f o r m a t i o n . In a subsequent c h a p t e r i n t h e s e p r o c e e d i n g s they p r o v i d e an o v e r v i e w o f t h e i r s t u d i e s c o r r e l a t i n g t h e c h e m i c a l s h i f t s o f s p e c i f i c carbons w i t h t h e v a l u e s o f the d i h e d r a l a n g l e s about bonds i n v o l v i n g t h o s e carbons. By examining t h e v a l u e s o f c h e m i c a l s h i f t s f o r monomeric and o l i g o m e r i c compounds o f known s t r u c t u r e s , they have d e v e l o p e d c o r r e l a t i o n s which may be a p p l i e d i n t r a n s l a t i n g t h e s p e c t r a l i n f o r m a t i o n i n a manner t h a t i s complementary t o t h e d i f f r a c t o m e t r i e studies. a

Multidisciplinary

Studie

In a d d i t i o n t o t h e s t u d i e s o u t l i n e d above, w i t h p r i m a r y f o c u s on d i f f r a c t o m e t r y o r on s p e c t r o s c o p y , t h e r e have been, r e c e n t l y , a number o f s t u d i e s which r e c o g n i z e a t t h e o u t s e t t h e type o f cons t r a i n t s summarized by Kakudo and K a s a i , and which b e g i n w i t h an i n t e g r a t e d approach t o t h e i n v e s t i g a t i o n o f s t r u c t u r e . Perhaps t h e b e s t i l l u s t r a t i o n o f t h i s approach i s t h e work o f H e n r i s s a t and coworkers noted e a r l i e r and o u t l i n e d i n a l a t e r c h a p t e r , f o c u s i n g on o l i g o m e r s c l e a r l y r e l a t e d t o t h e s t r u c t u r e o f c e l l u l o s e I I . In t h e work o f H e n r i s a t , e_^ a l . , t h e x-ray d i f f ractome t r i e d a t a o f P o p p l e t o n and M a t h i e s o n ( 4 6 ) on e e l l o t e t r a o s e was complemented w i t h s t r u c t u r a l d a t a on o t h e r o l i g o m e r s , w i t h CP-MAS C NMR s p e c t r o s copy, w i t h c o n f o r m a t i o n a l energy c a l c u l a t i o n s , and w i t h m o l e c u l a r o r b i t a l c a l c u l a t i o n s t o d e t e r m i n e some o f t h e f a v o r e d c o n f o r m a t i o n s . The d i f f i c u l t y o f t h e problem o f the s t r u c t u r e s o f c e l l u l o s e i s perhaps b e s t i l l u s t r a t e d by some o f t h e r e m a i n i n g a m b i g u i t i e s c i t e d i n t h i s study. Yet another s e t o f i n t e r d i s c i p l i n a r y s t u d i e s a r e r e p r e s e n t e d by the work o f H a y a s h i and c o w o r k e r s , w h e r e i n they attempt t o shed l i g h t on t h e q u e s t i o n s o f r e v e r s i b i l i t y , o r l a c k t h e r e o f , i n t r a n s f o r m a t i o n s between t h e a l l o m o r p h s o f c e l l u l o s e and i t s d e r i v a t i v e s . In a d d i t i o n t o t h e i r d i f f r a c t o m e t r i c s t u d i e s r e p o r t e d i n p r i o r p u b l i c a t i o n s , they add i n t h e i r c o n t r i b u t i o n t o t h e p r e s e n t symposium a n a l y s e s o f the i n f r a r e d s p e c t r a as w e l l as a n a l y s e s o f t h e CP-MAS l ^ C NMR s p e c t r a . T h e i r t h e s i s i s not i n c o n s i s t e n t w i t h t h e p r o p o s a l s o f A t a l l a and coworkers c o n c e r n i n g d i f f e r e n c e s between t h e c o n f o r m a t i o n s o f c e l l u l o s e s I and I I . However, H a y a s h i and coworkers go beyond t h i s by p r o p o s i n g t h a t t h e d i f f e r e n c e s i n conf o r m a t i o n c a n be p r e s e r v e d i n t h e c o u r s e o f h e t e r o g e n e o u s d e r i v a t i z a t i o n r e a c t i o n s , and a l s o i n t h e p r o c e s s o f g e n e r a t i n g t h e o t h e r a l l o m o r p h s o f c e l l u l o s e , namely c e l l u l o s e s I I I and IV, from t h e two primary a l l o m o r p h s I and I I . 1 3

The a d o p t i o n o f m u l t i d i s c i p l i n a r y approaches i n t h e e f f o r t t o shed l i g h t on t h e complex q u e s t i o n s o f s t r u c t u r e s i s l i k e l y t o expand i n t h e f u t u r e . The p r o c e e d i n g s o f t h i s symposium a r e c l e a r e v i d e n c e b o t h f o r t h e need and t h e v a l u e o f such approaches.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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ATALLA

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11

Future D i r e c t i o n s The s t u d i e s r e v i e w e d b r i e f l y above p l a c e the problem o f the s t r u c tures of c e l l u l o s e i n a promising perspective. U n t i l the d e v e l o p ment o f the new s p e c t r o s c o p i c methods, the c r y s t a l l o g r a p h i c s t u d i e s were u n d e r t a k e n w i t h l i t t l e a d d i t i o n a l i n f o r m a t i o n from o t h e r s o u r c e s , w i t h the e x c e p t i o n o f some o f the c o n f o r m a t i o n a l energy calculations. These a r e u s e f u l , but they are s e n s i t i v e t o the n a t u r e o f the p o t e n t i a l f u n c t i o n s used i n the c a l c u l a t i o n s and p a r t i c u l a r l y t o the manner i n w h i c h the d i f f e r e n t p o t e n t i a l f u n c t i o n s are weighted. As n o t e d e a r l i e r , the c r y s t a l l o g r a p h i c s t u d i e s have sought the most s i m p l e model s t r u c t u r e c o n s i s t e n t w i t h o b s e r v a t i o n s . Clearly the s t r u c t u r e based on the a n h y d r o g l u c o s e as the r e p e a t u n i t i s the most s i m p l e s t r u c t u r e t h a t a c c o u n t s f o r the m a j o r i t y o f the d i f f r a c tometric data. F u r t h e r m o r e , the d a t a a v a i l a b l e d i d not p r o v i d e a basis for introducing departure suggestions for i t s r e v i s i o n The new i n f o r m a t i o i n two key a r e a s . The f i r s t i s r e l a t e d t o the c o m p l e x i t y o f the s t r u c t u r e s o f the n a t i v e c e l l u l o s e s . The second i s t h a t o f the r e l a t i o n s h i p between the s t r u c t u r e s o f c e l l u l o s e s I and I I . It has been known f o r some time t h a t the more c r y s t a l l i n e n a t i v e c e l l u l o s e s from a l g a e and from A c e t o b a c t e r x y l i n u m produce d i f f r a c t i o n p a t t e r n s t h a t have many f e a t u r e s i n common w i t h t h o s e o f the c r y s t a l l i n e c e l l u l o s e s from the h i g h e r p l a n t s , such as ramie, but t h a t cannot be i n d e x e d as simply o r on the b a s i s o f the same unit c e l l . The new i n f o r m a t i o n from the CP-MAS l^C NMR spectra, t o g e t h e r w i t h t h a t from the Raman s p e c t r a , s u g g e s t s some b a s e s f o r u n d e r s t a n d i n g t h e s e d i f f e r e n c e s , and d i r e c t i o n s f o r f u r t h e r e x p l o r a tions. The key c o n c l u s i o n t h a t i s r e l e v a n t h e r e i s t h a t the n a t i v e c e l l u l o s e s a r e c o m p o s i t e s o f more t h a n one c r y s t a l i n e form, but t h a t the d i f f e r e n c e between the two forms l i e s not i n the m o l e c u l a r conf o r m a t i o n but i n the h y d r o g e n b o n d i n g p a t t e r n s . Thus, i t i s poss i b l e t h a t the n a t i v e c e l l u l o s e s have u n i t c e l l s w i t h v e r y s i m i l a r a t o m i c c o o r d i n a t e s f o r the heavy atoms, but w i t h d i f f e r e n t c o o r d i n a t e s f o r the hydrogens. The s i m i l a r i t i e s i n the heavy atom l o c a t i o n s c o u l d a c c o u n t f o r the many c o m o n a l i t i e s i n the d i f f r a c t i o n p a t t e r n s , w h i l e the d i f f e r e n c e s i n the c o o r d i n a t e s o f the h y d r o g e n atoms c o u l d be r e s p o n s i b l e f o r the d i f f e r e n c e s between the p a t t e r n s . T h i s would account f o r the g r e a t e r i n c i d e n c e o f n o n a l l o w e d r e f l e c t i o n s i n the e l e c t r o n d i f f r a c t i o n p a t t e r n s . It i s not c l e a r t h a t a p o l y m e r i c system w i t h a c o m p o s i t e s t r u c t u r e , such as the one p r o p o s e d above, r e p r e s e n t s a t r a c t a b l e c r y s t a l l o g r a p h i c problem. However, any new i n s i g h t s c o n c e r n i n g the d i s c r e p a n c i e s between the p r o p o s e d s i m p l e s t r u c t u r e s and the o b s e r v a t i o n s a r e i m p o r t a n t , f o r they may s u g g e s t d e p a r t u r e s i n new d i r e c t i o n s f o r i n v e s t i g a t i o n . A v e r y s i g n i f i c a n t i m p l i c a t i o n o f the p r o p o s a l s u g g e s t e d above, t o e x p l a i n the d i s c r e p a n c i e s between the d i f f r a c t i o n p a t t e r n s , i s t h a t the p r i m a r y d e t e r m i n i n g f a c t o r i n the s t r u c t u r e o f the n a t i v e c e l l u l o s e s may be the shape o f the m o l e c u l e s r a t h e r t h a n the h y d r o g e n b o n d i n g p a t t e r n . The p r o p o s a l c l e a r l y i m p l i e s t h a t more t h a n one h y d r o g e n b o n d i n g p a t t e r n i s c o n s i s t e n t

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w i t h the o r g a n i z a t i o n o f the heavy atoms i n the m o l e c u l a r s k e l e t o n i n the u n i t c e l l . The p r o p o s a l has a number o f o t h e r i m p l i c a t i o n s f o r f u r t h e r i n v e s t i g a t i o n , d i s c u s s i o n o f which i s beyond the scope o f the p r e s e n t c h a p t e r . W i t h r e s p e c t t o the c o m p a r i s o n between c e l l u l o s e s I and I I , the s p e c t r a l d a t a l e a v e l i t t l e q u e s t i o n t h a t the m o l e c u l a r c o n f o r m a t i o n s a r e indeed d i f f e r e n t . The c h a p t e r by W i l e y and A t a l l a s e t s f o r t h some o f the e v i d e n c e based on Raman s p e c t r o s c o p y . The v a l i d i t y o f the t h e o r e t i c a l arguments d e v e l o p e d i n s u p p o r t o f the h y p o t h e s i s t h a t two d i s t i n c t c o n f o r m a t i o n s do i n d e e d o c c u r has been demons t r a t e d through i t s a p p l i c a t i o n i n s t u d i e s o f model compounds. The most comprehensive i s a study o f the v i b r a t i o n a l s p e c t r a o f t h e i n o s i t o l s ( 4 7 ) , w h e r e i n s p e c t r a o f seven o f the isomers were i n v e s t i g a t e d and the e f f e c t s o f c o n f o r m a t i o n a l d i f f e r e n c e s a c c o u n t e d for. The h y p o t h e s i s t h a t c o n f o r m a t i o n a l d i f f e r e n c e s o c c u r i s a l s o s u p p o r t e d by the d i f f e r e n c e s between the CP-MAS l^C ^MR s p e c t r a o f c e l l u l o s e s I and I I . I i n the c h e m i c a l s h i f t s o i n the degrees o f s p l i t t i n g s o f the CI and C4 resonances can be a c c o u n t e d f o r i n terms o f s t r u c t u r e s a d h e r i n g s t r i c t l y t o the assumption t h a t the t w o f o l d screw axes c o i n c i d e w i t h the axes o f the molecular chains. The d a t a a r i s i n g from b o t h s p e c t r o s c o p i c methods c l e a r l y p o i n t to the need t o e x p l o r e the degree t o which the d i f f r a c t i o n d a t a can be accounted f o r i n terms o f s t r u c t u r e s w h e r e i n the a n h y d r o c e l l o b i o s e u n i t i s assumed t o be the b a s i c r e p e a t u n i t i n the c r y s t a l l o graphic structure. The s p e c t r o s c o p i c s t u d i e s and the c o n f o r m a t i o n a l energy c a l c u l a t i o n s suggest t h a t the d e p a r t u r e s from e q u i v a l e n c e o f the two a n h y d r o g l u c o s e u n i t s need not be v e r y l a r g e ones. T h i s may i n d e e d be the r e a s o n why the d i s a l l o w e d r e f l e c t i o n s appear t o be weak i n the d i f f r a c t i o n p a t t e r n s . On the o t h e r hand, the s p e c t r o s c o p i c e v i d e n c e s u g g e s t s t h a t the n a t u r e o f these minor d e p a r t u r e s from symmetric e q u i v a l e n c e o f a d j a c e n t a n h y d r o g l u c o s e u n i t s may be the key t o some o f the a n o m a l i e s e n c o u n t e r e d i n the s t r u c t u r a l studies. I t i s c l e a r t h a t the new i n f o r m a t i o n d e v e l o p e d from s p e c t r o s c o p i c and m u l t i d i s c i p l i n a r y s t u d i e s p r o v i d e s a b a s i s f o r i n i t i a t i n g d i f f r a c t o m e t r i e s t u d i e s with a d i f f e r e n t set of c o n s t r a i n t s t h a n those used i n the p a s t . The r e f i n e m e n t s are l i k e l y t o be more complex, but the e x p e c t a t i o n i s t h a t the s t r u c t u r e s thus d e r i v e d w i l l more c l o s e l y a p p r o x i m a t e the m o l e c u l a r s t r u c t u r e o f c e l l u l o s e . Such models may then p r o v i d e more comprehensive r a t i o n a l i z a t i o n s o f t h e phenomenology o f c e l l u l o s e .

Literature Cited 1. 2. 3.

Kakudo, M.; Kasai, N. X-ray diffraction by polymers, Elsevier, New York, 1972, p. 285. Arnott, S. In Fiber Diffraction Methods; ACS Symposium Series No. 141, American Chemical Society, Washington, DC, 1980, p. 1. Atkins, E. D. T. In Fiber Diffraction Methods, ACS Symposium Series No. 141, American Chemical Society, Washington, DC, 1980, p. 31.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

1.

ATALLA 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.

Structures of Cellulose

13

Tadokoro, H. In Fiber Diffraction Methods, ACS Symposium Series No. 141, American Chemical Society, Washington, DC, 1980, p. 43. Tadokoro, H. Structure of crystalline polymers, Wiley, New York, 1979, p. 6. Purves, C. B. In Cellulose and Cellulose Derivatives, Pt. I; Ott, E.; Spurlin, H. M.; Graffline, M. W., Eds., Interscience, New York, 1954, p. 29. Flory, P. J. Principles of polymer chemistry, Cornell University Press, Ithaca, New York, 1953, p. 3. Hermans, P. H. Physics and chemistry of cellulose fibers, Elsevier, New York, 1949, p. 3. Jones, D. W. In Cellulose and Cellulose Derivatives, Pt. IV; Bikales, N. M.; Segal, L.; Eds., Wiley-Interscience, New York, 1971, p. 117. Ellefsen, O.; Tonessen, B. A. In Cellulose and Cellulose Derivatives, Pt. IV Bikales N M.; Segal L.; Eds. Wiley Interscience, New York Tonessen, B. A.; Ellefsen Derivatives, Pt. IV, Bikales, N. M.; Segal, L.; Eds., WileyInterscience, New York, 1971, p. 265. Preston, R. D. The physical biology of plant cell walls; Chapman and Hall, London, 1974. Frey-Wyssling, A. The plant cell wall; Gebruder Borntrager, Berlin, 1976. Howsmon, J. A.; Sisson, W. A. In Cellulose and Cellulose Derivatives, Pt. I; Ott, E.; Spurlin, H. M.; Graffline, M. W.; Eds., Interscience, New York, 1954, p. 231. Petipas, T.; Oberlin, M.; Mering, J. J. Polym. Sci. C 1963, 2, 423. Norman, M. Text. Res. J. 1963, 32, 711. Gardner, K. H.; Blackwell, J. Biopolymers 1974, 13, 1975. Hebert, J. J.; Muller, L. L. J. Appl. Polym. Sci. 1974, 18, 3373. Ref. 9; p. 138. Ref. 14; p. 237. Meyer, K. H.; Mark, H. Z. Physik. Chem. 1929, B2, 115. Meyer, K. H.; Misch, L. Helv. Chim. Acta 1937, 20, 232. Pierce, F. T. Trans. Faraday Soc. 1946, 42, 545. Liang, C. Y.; Marchessault, R. H. J. Polym. Sci. 1959, 37, 385. Liang, C. Y.; Marchessault, R. H. J. Polym. Sci. 1959, 39, 269. Marchessault, R. H.; Liang, C. Y. J. Polym. Sci. 1960, 43, 71. Nelson, M. L.; O'Connor, R. T. J. Appl. Polym. Sci. 1964, 8, 1311. Nelson, M. L.; O'Connor, R. T. J. Appl. Polym. Sci. 1964, 8, 1325. Sarko, A.; Muggli, R. Macromol. 1974, 7, 486. French, A. D. Carbohydrate Res. 1978, 61, 67. Atalla, R. H. Advances in Chemistry Series 1979, 181, 55. Atalla, R. H. In Structure, Function and Biosynthesis of Plant Cell Walls, Dugger, W. M.; Bartinicki-Garcia, S.; Eds.,

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33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.

THE STRUCTURES OF CELLULOSE

American Society of Plant Physiologists, Rockville, MD, 1984, p. 381. Atalla, R. H. Appl. Polym. Symp. 1976, 28, 659. Reese, D. A.; Skerrett, R. J. Carbohydrate Res. 1968, 7, 334. Melberg, S.; Rasmussen, K. Carbohydrate Res. 1979, 71, 25. Chu, S. S. C.; Jeffrey, G. A. Acta Crystallogr. 1968, B24, 830. Ham, J. T.; Williams, D. G. Acta Crystallogr. 1970, B29, 1373. Atalla, R. H.; Gast, J. C.; Sindorf, D. W.; Bartuska, V. J.; Maciel, G. E. JACS 1980, 102, 3249. Atalla, R. H. Proceedings of the International Symposium on Wood and Pulping Chemistry, SPCI Rept. No 38, Stockholm 1981, Vol. 1, p. 57. Earl, W. L.; VanderHart, D. L. JACS 1980, 102, 3251. Earl, W. L.; VanderHart, D. L. Macromol. 1981, 570. Maciel, G. E.; Kolodziejski, W. L.; Bertran, M. S.; Dale, B. E. Macromol. 1982 15,686 Fyfe, C. A.; Dudley Hamer, G. K.; Marchessault Atalla, R. H.; VanderHart, D. L. Science 1984, 223, 283. VanderHart, D. L.; Atalla, R. H. Macromol. 1984,17,1465. Poppleton, B. J.; Mathieson, A. McL. Nature 1968, 219, 1946. Williams, R. M.; Atalla, R. H. J. Phys. Chem. 1984, 88, 508.

RECEIVED March 5, 1987

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 2

X-ray Diffraction Studies of Ramie Cellulose I 1

2

2

A. D. French , W. A. Roughead , and D. P. Miller 1

Southern Regional Research Center, U.S. Department of Agriculture, P.O. Box 19687, New Orleans, LA 70179 Department of Physics and Astronomy, Clemson University, Clemson, SC 29631 2

Current fiber x-ray diffraction studies, including new calculations by the authors, are reviewed. Because of different convention structure of nativ structure describe "up y for Valonia corresponds to the "down" structure that was strongly rejected for ramie by Woodcock and Sarko. A variety of assumptions were tested, as were results from the different computer programs used for fiber diffraction. These results were compared with those from a computer program written for single crystal work, and the comparisons were satisfactory. A major reason for the differences in reported structures for celluloses comes from differences in the diffraction intensity data sets taken at the various institutions. C e l l u l o s e f i b e r s have been been s t u d i e d w i t h x - r a y d i f f r a c t i o n s i n c e 1913 ( 1 ) . Over t h e p a s t dozen y e a r s , a number o f f u l l f l e d g e d s t u d i e s o f n a t i v e c e l l u l o s e have been p u b l i s h e d , each without apparent f a u l t , but with c o n t r a d i c t o r y i n d i c a t i o n s o f c h a i n p a c k i n g mode. A t t h e same time, s p e c t r o s c o p i s t s have c h a l l e n g e d some o f t h e fundamental assumptions and a p p a r e n t l y c l e a r r e s u l t s o f the d i f f r a c t i o n s t u d i e s . T h i s r e p o r t i s p a r t o f a n e f f o r t by s e v e r a l f i b e r c r y s t a l l o g r a p h e r s t o b e t t e r u n d e r s t a n d t h e v a r i a b l e outcome from t h e s e s t u d i e s and s p e c i f i c a l l y t o a s c e r t a i n t h a t r e s u l t s a r e n o t dependent on t h e p a r t i c u l a r computer program u s e d . R. M i l l a n e a t Purdue and A. Sarko a t S y r a c u s e a r e a l s o a n a l y z i n g a d a t a s e t o b t a i n e d by Roughead and M i l l e r from d i f f r a c t i o n photographs t a k e n by F r e n c h ( t h e RMF d a t a ) (2) i n o r d e r t o l e a r n more about t h e methodology and about c e l l u l o s e i t s e l f . I n t h i s r e p o r t some r e s u l t s w i l l be d i s c u s s e d based on t h a t d a t a as w e l l as some c o m p a r a t i v e r e s u l t s based on o t h e r d a t a s e t s . S e l e c t i o n o f Ramie C e l l u l o s e f o r Study. Ramie c e l l u l o s e was s e l e c t e d f o r s t u d y because i t i s h i g h l y o r i e n t e d and f a i r l y

0097-6156/87/0340-0015$06.50/0 © 1987 American Chemical Society

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THE STRUCTURES OF CELLULOSE

crystalline. A l t h o u g h ramie i s not as c o m m e r c i a l l y i m p o r t a n t as c o t t o n , c o t t o n f i b e r s have a c o m p l i c a t e d e x t r a l e v e l o f s t r u c t u r e t h a t d e c r e a s e s the amount o f i n f o r m a t i o n a v a i l a b l e from a d i f f r a c t i o n study. I f t h i s f i b e r s t r u c t u r e i n cotton i s destroyed w i t h a Waring b l e n d e r (making x - r a y f i b e r d i f f r a c t i o n i m p o s s i b l e ) , an e l e c t r o n d i f f r a c t i o n p a t t e r n n e a r l y i d e n t i c a l t o t h a t from ramie may be o b t a i n e d ( 3 ) . I n c o n t r a s t , e l e c t r o n d i f f r a c t i o n p a t t e r n s (and x - r a y p a t t e r n s , t o o ) from a l g a l and b a c t e r i a l c e l l u l o s e s d i f f e r from ramie and c o t t o n , even though they a l l have the main f e a t u r e s i n common. I n f r a r e d s p e c t r a from ramie and c o t t o n a r e a l s o s i m i l a r t o each o t h e r and d i f f e r e n t from t h o s e o f the a l g a l and b a c t e r i a l c e l l u l o s e s ( 4 ^ . Thus, the s t r u c t u r e o f ramie s h o u l d a c c u r a t e l y resemble c o t t o n a t the m o l e c u l a r and c r y s t a l l i t e l e v e l s , w h i l e t h e r e i s some doubt as t o whether such c l o s e s i m i l a r i t y a p p l i e s between the s t r u c t u r e s o f a l g a l and c o t t o n c e l l u l o s e s . Review o f P r e v i o u s Work Unit C e l l . V a r i o u s worker d i m e n s i o n s f o r the ramie u n i t c e l l . As shown i n T a b l e I , however, the a_ and _b dimensions have ranges o f about 0.07 A. (The a_ and t> d i m e n s i o n s o f the MGW (Mann, Gonzalez and W e l l a r d ) and RMF c e l l s have been i n t e r c h a n g e d t o conform t o s t a n d a r d c r y s t a l l o g r a p h i c n o t a t i o n (7^) as d i s c u s s e d below.)

Authors

Table I.

U n i t C e l l s f o r Ramie

Mann* Gonzalez Wellard (MGW)

Woodcock Sarko (WS)

Roughead* Miller French (RMF)

Dimension a b c Y

7.846 8.171 10.34 96.38

7.78 8.20 10.34 96.5

7.794 8.248 10.33 96.77

* The a and b dimensions have been i n t e r c h a n g e d the r e p o r t e d v a l u e s f o r t h i s c o m p a r i s o n .

from

The base plane o f the u n i t c e l l f o r c e l l u l o s e I i s o f t e n drawn w i t h the o r i g i n o f the axes p l a c e d i n the lower l e f t c o r n e r , w i t h the x a x i s to the r i g h t and the y a x i s up and t o the l e f t with monoclinic angle being obtuse. However, Sarko's group i n S y r a c u s e has used s t a n d a r d c r y s t a l l o g r a p h i c c o n v e n t i o n , w i t h the o r i g i n i n the upper l e f t , the y a x i s t o the r i g h t , and the x a x i s down and t o the l e f t . I n both c a s e s , the z a x i s i s toward the viewer i f the x and y axes a r e i n the p l a n e o f the p a p e r . T h i s paper u s e s the s t a n d a r d c o n v e n t i o n , shown i n F i g u r e 1, as used i n S y r a c u s e . The a, b, and c

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2. FRENCH ET AL.

X-ray Diffraction Studies of Ramie Cellulose I

F i g u r e 1. Drawing of the c e l l u l o s e I u n i t c e l l , a c c o r d i n g to the s t a n d a r d c r y s t a l l o g r a p h i c c o n v e n t i o n i n r e f . 7, by B i l l Garner, Martin Marietta Corporation.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

18

THE STRUCTURES OF CELLULOSE

dimensions a r e the r e p e a t i n g d i s t a n c e s a l o n g the x, y, z axes, r e s p e c t i v e l y .

and

Chain Conformation. From computer m o d e l i n g s t u d i e s o f c e l l u l o s e ( 8 ) , we know t h a t the c e l l u l o s e m o l e c u l e has l i m i t e d f l e x i b i l i t y . Only a few r e g u l a r ( i n t e r n a l l y symmetric) c o n f o r m a t i o n s a r e a l l o w e d f o r c e l l u l o s e , a b e t a 1 , 4 - l i n k e d g l u c a n , and s t r u c t u r e s s i m i l a r t o the t r a d i t i o n a l 2 - f o l d h e l i x a r e the o n l y ones i n a c c o r d w i t h the observed z a x i s s p a c i n g . Such c h a i n s resemble f l a t r i b b o n s . C r y s t a l S t r u c t u r e . I n the c u r r e n t models from x - r a y d i f f r a c t i o n , a t o t a l o f two c e l l u l o s e c h a i n s h a v i n g a t o m i c p o s i t i o n s c l o s e t o t h o s e r e s u l t i n g from 2 - f o l d screw symmetry pass through each o f the unit cells. These c h a i n s a r e bound i n t o s h e e t s by hydrogen bonding between the edges o f the r i b b o n s ; t h e r e a r e no hydrogen bonds proposed t o c o n n e c t the s h e e t s t o each o t h e r ( 6 , 9 - 1 1 ) . The r e m a i n i n g c o h e s i v e n e s s i s a p p a r e n t l y p r o v i d e d by vander Waals attraction. I n o r d e r t o form t h e s r o t a t e d i n t o the t g p o s i t i o n , where i t forms both i n t r a - and i n t e r - m o l e c u l a r hydrogen bonds. A l t h o u g h the t g 06 p o s i t i o n i s somewhat c o n t r o v e r s i a l because i t i s r a r e l y found i n s i n g l e c r y s t a l s t u d i e s o f s m a l l e r c a r b o h y d r a t e s , i t has been r e p o r t e d i n a l l recent x-ray s t u d i e s of c e l l u l o s e I . P a c k i n g Mode. The s t r u c t u r e s f o r n a t i v e ramie c e l l u l o s e d e s c r i b e d by Woodcock and Sarko (6) (WS) and f o r V a l o n i a c e l l u l o s e by Gardner and B l a c k w e l l (11) have been c a l l e d " p a r a l l e l - u p " , as opposed t o " p a r a l l e l - d o w n " and t o " a n t i p a r a l l e l " arrangements ( 1 1 ) . However, t h e i r proposed s t r u c t u r e s used the d i f f e r e n t a x i a l c o n v e n t i o n s d e s c r i b e d above. I f both s t r u c t u r e s a r e d e s c r i b e d u s i n g the same c o n v e n t i o n , the p a c k i n g i n one o f the c e l l s i s p a r a l l e l - u p and and the o t h e r i s packed p a r a l l e l - d o w n . I n o t h e r words, the p a r a l l e l - u p s t r u c t u r e p r e f e r r e d by G a r d n e r and B l a c k w e l l c o r r e s p o n d s t o the p a r a l l e l - d o w n s t r u c t u r e s t r o n g l y r e j e c t e d by Woodcock and S a r k o . Work by F r e n c h (9) showed a s m a l l p r e f e r e n c e f o r an a n t i p a r a l l e l arrangement. I d e n t i c a l , hydrogen-bonded s h e e t s t r u c t u r e s can be formed i n each o f t h e s e p a c k i n g arrangements, but the between-sheet i n t e r a c t i o n s are d i f f e r e n t . C e l l Symmetry. F o r many y e a r s , i t was a c c e p t e d t h a t the c e l l u l o s e m o l e c u l e embodied 2 - f o l d screw symmetry ( s p a c e group P 2 ) . Later, Honjo and Watanabe (12) showed e l e c t r o n d i f f r a c t i o n diagrams t h a t i n d i c a t e t h a t V a l o n i a c e l l u l o s e c r y s t a l l i z e s i n the PI space group t h a t does n o t c o n t a i n symmetric c h a i n s . The u n i t c e l l a l s o c o n t a i n e d 8 c h a i n s . Other d i f f r a c t i o n work showed the p r e s e n c e o f f a i n t m e r i d i o n a l r e f l e c t i o n s on the odd l a y e r l i n e s t h a t a l s o i n d i c a t e t h a t the c h a i n s do not have e x a c t 2 - f o l d screw symmetry. While the v a l i d i t y o f t h i s e v i d e n c e has been a c c e p t e d f o r V a l o n i a , i t has not been used i n s t r u c t u r a l s t u d i e s because o f the low i n t e n s i t i e s o f the s p o t s t h a t i n d i c a t e the l a r g e c e l l and t h a t break the symmetry. I t has been argued t h a t the d e v i a t i o n s from symmetry must be r e l a t i v e l y s m a l l ( 1 1 ) ; assumptions r e g a r d i n g the c e l l s i z e have been n e c e s s a r y i n o r d e r f o r the needed computations to be manageable. 1

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2. FRENCH ET AL.

X-ray Diffraction Studies of Ramie Cellulose I

19

I n Woodcock and Sarko's work on ramie (6) and i n u n p u b l i s h e d work by F r e n c h , the improvement i n f i t between o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s was i n s i g n i f i c a n t when the model was a l l o w e d to d e v i a t e from 2 - f o l d symmetry. On the o t h e r hand, s p l i t t i n g i n the nmr peaks and o t h e r s p e c t r a l e v i d e n c e (13,14) has g i v e n s t r o n g i n d i c a t i o n t h a t the m o l e c u l e s a r e not n e a r l y as symmetric as thought. G i v e n the above c o n t r a d i c t i o n s , q u e s t i o n s o f g e n e r a l i n t e r e s t regarding c e l l u l o s e structure include: a.

b. c.

d.

e.

What i s the p a c k i n g mode o f the c h a i n s ? I s the p a c k i n g mode d i f f e r e n t i n c e l l u l o s e I and I I ? I f so, how i s the c o n v e r s i o n effected? What i s the e x t e n t o f d e v i a t i o n from a symmetric P2^ s t r u c t u r e ? What i s the n a t u r e o f f i b r i l s ? A r e they extended o r f o l d e d ; what i s the s o u r c e o f l e v e l i n g o f f degree o f p o l y m e r i z a t i o n ? Why i s i t d i f f e r e n t f o r c e l l u l o s e I and I I ? What i s the f i b r i l l a reactions? Which oxyge such as c r o s s l i n k i n g a f a b r i c ? I s the h y d r o g e n bonding scheme i m p o r t a n t i n d e t e r m i n i n g the r e a c t i v i t y ? What i s the cause o f d i f f e r e n c e s i n r e s u l t s among the o t h e r p h y s i c a l methods? What a r e the d i f f e r e n c e s among the v a r i o u s c e l l u l o s e s t h a t cause d i f f e r e n c e s i n t h e i r IR and nmr s p e c t r a ?

Some o t h e r a s p e c t s a r e p r i m a r i l y o f i n t e r e s t crystallographers: a. b.

to

fiber

What i s the s o u r c e o f the s u b s t a n t i a l e r r o r r e m a i n i n g i n our studies? What i s the meaning o f the temperature f a c t o r s t h a t r e s u l t ?

F i b e r x - r a y d i f f r a c t i o n can be e x p e c t e d to p r o v i d e a t l e a s t some e v i d e n c e r e l a t i v e t o a l l the above q u e s t i o n s . W i t h the a v a i l a b l e r a t i o o f d a t a t o p a r a m e t e r s , however, some q u e s t i o n s may w e l l r e m a i n unanswered. Samples o f f a r h i g h e r c r y s t a l l i n i t y would be needed to a c c o m p l i s h a complete d e t e r m i n a t i o n , and t h e r e i s some danger t h a t such samples would not c o m p l e t e l y c o r r e s p o n d t o samples o f more g e n e r a l i n t e r e s t . Computer T e c h n i q u e s Program D e s c r i p t i o n . I n o r d e r t o d e t e r m i n e polymer c r y s t a l s t r u c t u r e s , e a c h o f the p a r t i c i p a t i n g i n s t i t u t i o n s u s e s computer programs t h a t r e f l e c t d i f f e r e n t r e s o u r c e s and a b i l i t i e s as w e l l as philosophies. Models to be used f o r c a l c u l a t i o n o f i n t e n s i t i e s a r e c o n s t r u c t e d i n v a r i o u s ways, and each has a d i f f e r e n t s t r a t e g y f o r f i n d i n g the b e s t r e s u l t . E a c h o f the methods has some unique a b i l i t i e s and some d i s a d v a n t a g e s . The SHELX program (15) i s used a t Clemson U n i v e r s i t y . I t was d e s i g n e d f o r s i n g l e c r y s t a l s t u d i e s and can s w i f t l y r e a c h a good match between o b s e r v e d and c a l c u l a t e d d a t a but has no s t e r e o c h e m i c a l component i n the d e t e r m i n a t i o n and was n o t d e s i g n e d to accommodate polymers which have the same m o l e c u l e p a s s i n g t h r o u g h many u n i t c e l l s . I t was s l i g h t l y m o d i f i e d f o r o v e r l a p p e d

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

20

THE STRUCTURES OF CELLULOSE

intensities. S t a r t i n g models must be f a i r l y c l o s e , because t h e r e i s no s e a r c h i n g based on o v e r a l l parameters o f a polymer, such as chain r o t a t i o n or chain t r a n s l a t i o n . The program used a t the S o u t h e r n R e g i o n a l R e s e a r c h C e n t e r (SRRC) was w r i t t e n f o r s i m p l e c e l l u l o s e s t r u c t u r e s (9) m o s t l y by V. G. Murphy, now a t C o l o r a d o S t a t e U n i v e r s i t y . It quickly c a l c u l a t e s R v a l u e s f o r changes i n v a r i a b l e s such as c h a i n r o t a t i o n and c h a i n t r a n s l a t i o n . L i k e SHELX, i t has no s t e r e o c h e m i c a l component. M o d e l i n g i s based on the v i r t u a l - b o n d method ( 1 6 ) , and r e l i e s on s u b s t i t u t i o n o f d i f f e r e n t c o o r d i n a t e s e t s from s i n g l e c r y s t a l s t u d i e s o f model compounds such as c e l l o b i o s e i n o r d e r t o p r o v i d e f i n e i n t e r n a l a d j u s t m e n t s o f a t o m i c c o o r d i n a t e s . The SRRC program uses v a r i a t i o n space s e a r c h i n g t o f i n d the b e s t c o m b i n a t i o n of c h a i n r o t a t i o n and t r a n s l a t i o n i n s t e a d o f a l e a s t s q u a r e s minimization. Because i n d i v i d u a l a t o m i c p o s i t i o n s ( e x c e p t 06) a r e not v a r i e d d i r e c t l y by the program, such a s i m p l e program i s n o t w e l l - s u i t e d f o r determining structures with r e l a t i v e l y high r a t i o s of data to v a r i a b l e parameters e x a m i n i n g the e f f e c t s o f u r n i s h i n g s t a r t i n g models f o r a more e l a b o r a t e program. It i s a l s o v e r y easy t o p r e p a r e a new d a t a s e t . At S y r a c u s e and Purdue, the computer programs a r e h i g h l y s o p h i s t i c a t e d f o r s t u d y o f polymers i n g e n e r a l . Both c a n combine d i f f r a c t i o n i n t e n s i t y c a l c u l a t i o n s and s t e r e o c h e m i s t r y t o o p t i m i z e intermolecular i n t e r a c t i o n s f o r structures that simultaneously y i e l d a good f i t between the o b s e r v e d and c a l c u l a t e d d i f f r a c t i o n intensities. These s t u d i e s u s u a l l y weight the d i f f r a c t i o n d a t a and the s t e r e o c h e m i c a l s t u d i e s e q u a l l y . The PS-79 program used a t S y r a c u s e was w r i t t e n by P e t e r Zugenmaier and Tony Sarko ( 1 7 ) . I t uses t e c h n i q u e s d e s i g n e d f o r speed and i m p l e m e n t a t i o n on a r e l a t i v e l y s m a l l computer. The m o d e l i n g method a l l o w s v a r i a t i o n i n monomeric geometry t h r o u g h changes i n the d i s t a n c e between the l i n k a g e oxygen atoms. A l l the a t o m i c p o s i t i o n s c a n be v a r i e d w i t h c o n s t r a i n t s t o accommodate s t e r e o c h e m i c a l and x - r a y d a t a . The L i n k e d Atom L e a s t Squares (LALS) program (18,19) by A r n o t t and c o a u t h o r s i s used a t s e v e r a l i n s t i t u t i o n s , i n c l u d i n g Purdue, where i t was d e v e l o p e d , and Case Western Reserve U n i v e r s i t y , where i t was used by Gardner and B l a c k w e l l . I t i s b e s t implemented on a l a r g e computer; the C o n t r o l Data C o r p o r a t i o n supercomputer a t Purdue h a n d l e s i t v e r y r a p i d l y . S e t t i n g up a new model w i t h LALS i s more c o m p l i c a t e d t h a n w i t h the o t h e r programs, but LALS a p p e a r s to p r o v i d e r e a l i s t i c m o l e c u l a r f l e x i b i l i t y . R Factors. Each program d e t e r m i n e s the e x t e n t o f agreement between the o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s (the R f a c t o r ) i n a s l i g h t l y d i f f e r e n t way. The e x a c t method o f c a l c u l a t i o n i s i m p o r t a n t i n the magnitude o f R t h a t i s a t t a i n e d , making i t d i f f i c u l t t o compare r e s u l t s from d i f f e r e n t l a b o r a t o r i e s . While the minima i n t h e s e d i f f e r e n t R f a c t o r s u s u a l l y a r i s e from v e r y s i m i l a r s t r u c t u r e s , each a l g o r i t h m may, as seen below, i n d i c a t e a d i f f e r e n t p r e f e r e n c e among competing models. D i f f e r e n c e s i n the magnitude o f R a r i s e from d i f f e r e n t methods f o r c a l c u l a t i n g the c o n t r i b u t i o n s from s p o t s t h a t a r e too weak t o be o b s e r v e d b u t have a c a l c u l a t e d i n t e n s i t y g r e a t e r t h a n the t h r e s h o l d o f o b s e r v a t i o n . Such r e f l e c t i o n s a r e c a l l e d the unobserved d a t a . V e r y minor

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

2. FRENCH ET AL.

X-ray Diffraction Studies of Ramie Cellulose I

21

d i f f e r e n c e s r e s u l t from the t e c h n i q u e u s e d t o s u p p l y t h e s c a t t e r i n g f a c t o r s t o the c a l c u l a t i o n s . Two o t h e r s o u r c e s o f d i f f e r e n c e i n the magnitude o f r e p o r t e d R v a l u e s a r e the method u s e d t o s c a l e the o b s e r v e d i n t e n s i t i e s to the c a l c u l a t e d ones ( s c a l e f a c t o r ) , and the method f o r compensating f o r t h e r m a l m o t i o n and p o s i t i o n a l d i s o r d e r (temperature f a c t o r ) . The SRRC program c a l c u l a t e s 5 d i f f e r e n t R v a l u e s , shown i n T a b l e I I . They a r e : a s i m p l e R, ( R o b s ) , based on o n l y r e f l e c t i o n s t h a t a r e o b s e r v e d , an R f o r o n l y the unobserved d a t a (Runobs), t h e i r t o t a l ( R t o t ) , the w e i g h t e d t o t a l R" (R"wt) and the u n i t - ( o r un-) w e i g h t e d t o t a l R (R"unwt). A weighted Rtot i s c a l c u l a t e d i n some o t h e r programs. By k e e p i n g the Robs and Runobs s e p a r a t e , the e f f e c t s o f two somewhat independent s e t s o f d a t a may be o b s e r v e d . At SRRC the square r o o t s o f the o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s are s c a l e d t o each o t h e r w i t h a l e a s t s q u a r e s f i t t h a t i n c o r p o r a t e s an e x p o n e n t i a l term (9). T h i s s i m u l t a n e o u s l y produces a low magnitude o f R", a s c a l e f a c t o r and a temperature f a c t o r As shown i n F i g u r e 2, c o a r s p o s i t i o n c o u l d be a c c o m p l i s h e values are c a l c u l a t e d at f i n e increments o f r o t a t i o n ( F i g u r e 3 ) , however, the c h o i c e o f type o f R f a c t o r d e t e r m i n e s which c h a i n r o t a t i o n s a r e chosen t o r e p r e s e n t the b e s t s t r u c t u r e . M

W e i g h t i n g Schemes. R"wt i s p r e f e r r e d by s t a t i s t i c i a n s because i t g i v e s e x t r a p e n a l t y t o proposed s t r u c t u r e s t h a t have a few l a r g e d i s c r e p a n c i e s between o b s e r v e d and c a l c u l a t e d i n t e n s i t i e s i n s t e a d of a normal d i s t r i b u t i o n o f e r r o r s . T h i s e f f e c t a r i s e s from the use o f the square o f the d i f f e r e n c e s . A l s o , the w e i g h t i n g i n c r e a s e s R"wt h e a v i l y when the d i s c r e p a n c y p e r t a i n s t o the more a c c u r a t e l y d e t e r m i n e d s p o t s and p e n a l i z e s l i g h t l y when the s p o t s are p o o r l y r e s o l v e d . Only the RMF d a t a s e t i n c l u d e s s t a n d a r d d e v i a t i o n s f o r the i n t e n s i t y measurements. These v a l u e s , d i v i d e d by t h e square r o o t of the o b s e r v e d i n t e n s i t y and i n v e r t e d , c a n be used t o w e i g h t the R value c a l c u l a t i o n . F o r c e l l u l o s e I , which has a few dominant s p o t s , t h i s i n e v i t a b l y r e s u l t s i n a l a r g e range o f w e i g h t s . Other w e i g h t i n g schemes can be a d o p t e d . F o r the Mann, G o n z a l e z and W e l l a r d (20) (MGW) and the WS d a t a s e t s , the 4 t h r o o t o f the i n t e n s i t y was u s e d t o g i v e a s m a l l range i n w e i g h t s t h a t would p a r t i a l l y r e f l e c t the e r r o r s due t o c o u n t i n g s t a t i s t i c s . The scheme f i n a l l y adopted f o r the RMF d a t a u s e d the r e c i p r o c a l s o f the f r a c t i o n a l e r r o r , e x c e p t t h a t a c e i l i n g was s e t a t 0.20 o f the weight o t h e r w i s e c a l c u l a t e d f o r the s t r o n g e s t r e f l e c t i o n . Such w e i g h t i n g s t y p i c a l l y d e c r e a s e the s e n s i t i v i t y o f the R f a c t o r t o v a r i a t i o n s i n the model. A l s o , the w e i g h t e d R" i s u s u a l l y n u m e r i c a l l y s m a l l e r t h a n the u n i t - w e i g h t e d R" which, i n t u r n , i s s m a l l e r than R t o t . Comparison o f Programs. The purpose o f the comparison o f the r e s u l t s d e r i v e d t h r o u g h these 4 d i f f e r e n t computer programs i s t o l e a r n t o what e x t e n t r e s u l t s a r e independent o f the computer program. There a r e a c t u a l l y two s i m i l a r q u e s t i o n s . The f i r s t i s whether s i m i l a r R f a c t o r s w i l l r e s u l t from i d e n t i c a l i n p u t structures. The second c o n s i d e r s the s i m i l a r i t y o f s t r u c t u r e s s e l e c t e d as the f i n a l b e s t model i n each program. A t p r e s e n t , we can comment b e s t on the f i r s t q u e s t i o n .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE Table

II,

R FACTORS CALCULATED BY

1

i

1

(

I

. * unobs

R"

unwt

/

" 1/2 Z I obs

+ R

, unobs

calc

*

obs

Z I

R"

wt

2

4 (10,11). In a d d i t i o n , c o n c l u s i o n s were drawn regarding the asymmetric unit in the crystalline region. Nevertheless, s e v e r a l p o i n t s are s t i l l obscure. The major aim o f the p r e s e n t work was t o c h a r a c t e r i z e c r y s t a l l i n e c e l l o d e x t r i n s u s i n g s e v e r a l methods i n o r d e r t o r e a c h a c o n s i s t e n t description o f the structural f e a t u r e s and to e s t a b l i s h the workability of each approach to the more complex case of cellulose. M a t e r i a l s and

Methods

Nomenclature. The numbering o f t h e atoms and t o r s i o n a n g l e s o f i n t e r e s t f o r c e l l o b i o s e i s shown i n F i g . 1. Numbering p r o c e e d s from the n o n r e d u c i n g end (unprimed) t o the r e d u c i n g end ( p r i m e d ) . The s i g n s o f the t o r s i o n a n g l e s agree w i t h t h e r u l e s o f the IUPAC-IUB Commission o f B i o c h e m i c a l Nomenclature ( 2 0 ) . The t o r s i o n a n g l e s o f i n t e r e s t a r e d e f i n e d as f o l l o w s : $ : 0 {05 - CI - 01 - C4'} V : 0 {CI - 01 - C4'- C5'} The c o n f o r m a t i o n o f t h e p r i m a r y h y d r o x y l group a t C6 (y) is r e f e r r e d t o as e i t h e r g a u c h e - t r a n s , gauche-gauche o r t r a n s - g a u c h e (21). In t h i s t e r m i n o l o g y , the t o r s i o n a n g l e : Q {05 - C5 - C6}06 i s s t a t e d f i r s t , t h e n t h e t o r s i o n a n g l e 0{C4 - C5 - C6 - 06}. The u n i t c e l l d i m e n s i o n s o f c e l l u l o s e I I , as d e t e r m i n e d from X-ray and electron diffraction s t u d i e s (22-25) on Rayon, Fortisan,

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE

F i g u r e 1. Schematic the atom l a b e l i n g and

representation of c e l l o b i o s e , along the t o r s i o n a l a n g l e s o f i n t e r e s t .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

with

3. HENRISSAT ET AL.

Model Compounds for Cellulose II

41

m e r c e r i z e d c o t t o n and s i n g l e c r y s t a l s a r e : a = 8.01 , b = 9.04 , c = 10.36 R and y = 117.1°. F o l l o w i n g the most accepted c r y s t a l l o g r a p h i c models (22, 24), the u n i t c e l l requires two c e l l u l o s e c h a i n s packed w i t h a n t i p a r a l l e l p o l a r i t y . Sample P r e p a r a t i o n . C e l l o b i o s e was p u r c h a s e d from F l u k a Company. Samples were d i s s o l v e d i n water and c r y s t a l l i z a t i o n r e s u l t e d from slow d i f f u s i o n o f e t h a n o l . Methyl 3 - D - c e l l o b i o s i d e , a g i f t from Dr. M. V i n c e n d o n (Grenoble, F r a n c e ) , was d i s s o l v e d i n h o t methanol and c r y s t a l l i z e d by s l o w l y c o o l i n g the s o l u t i o n . H e n d e c a - O - a c e t y l c e l l o t r i o s e and t e t r a d e c a - O - a c e t y l c e l l o t e t r a o s e were o b t a i n e d by acetolysis of cotton c e l l u l o s e according to Wolfrom e t a l . (26) and were p u r i f i e d by p r e p a r a t i v e HPLC. M e t h y l deca-O-acetyl3 - D - c e l l o t r i o s i d e was prepared a c c o r d i n g t o the method d e s c r i b e d by Takeo e t a l . (27). D e a c e t y l a t i o n o f the a c e t y l a t e d s u g a r s was a c h i e v e d i n methanol c o n t a i n i n g c a t a l y t i c amounts o f sodium m e t h y l a t e t h e i r anomeric c o n f i g u r a t i o solution C NMR. S o l i d S t a t e NMR. The samples under i n v e s t i g a t i o n were o b t a i n e d from c r y s t a l s which were ground i n t o homogeneous powder. In each c a s e , wide a n g l e X-ray s c a t t e r i n g diagrams were r e c o r d e d i n o r d e r to c o n f i r m the u n i t c e l l d i m e n s i o n s p r e v i o u s l y f o u n d . Solid state C NMR s p e c t r a were r e c o r d e d a t 50.3 MHz with a B r u k e r CXP-200 s p e c t r o m e t e r e q u i p p e d w i t h a Doty p r o b e . P r o t o n 90° p u l s e w i d t h s were 4 us, and c r o s s - p o l a r i z a t i o n times were 1 ms. M a t c h i n g c o n d i t i o n s were checked w i t h an adamantane s t a n d a r d . The magic a n g l e was s e t by m o n i t o r i n g the Br NMR spectrum o f KBr (28) . Sample c o n t a i n e r s were made o f aluminum o x i d e w i t h K e l - F caps and were spun a t 2.5-3.5 KHz. C h a r a c t e r i s t i c a l l y , 200-10000 a c q u i s i t i o n s were o b t a i n e d p e r spectrum, w i t h r e c y c l e time o f 4 s. A l l peaks i n the s p e c t r a were r e f e r e n c e d t o the peak o f l i n e a r p o l y e t h y l e n e (33.6 ppm). A s m a l l amount o f p o l y e t h y l e n e was added t o each sample (29). 13

7 9

C o n f o r m a t i o n a l A n a l y s i s C a l c u l a t i o n s . PCILP method : The s t a n d a r d v e r s i o n o f the s e m i e m p i r i c a l quantum-chemical method (30, 31) adopted for the optimization of the geometry according to Powell-Zangwill s (32, 33) a l g o r i t h m t h r o u g h bond l e n g t h s , bond angles and t o r s i o n a n g l e s was a p p l i e d . (PCILO : P e r t u r b a t i v e Configuration I n t e r a c t i o n with L o c a l i z e d O r b i t a l s ) . F o r c e F i e l d method : MM2CARB i s the MM2 ( M o l e c u l a r Mechanics) F o r c e F i e l d program (34) m o d i f i e d w i t h the J e f f r e y - T a y l o r a c e t a l segment p a r a m e t e r s (35) and i n t r a m o l e c u l a r hydrogen b o n d i n g (36) . I t was u t i l i z e d , f o l l o w i n g the s t r a t e g y o u t l i n e d by T v a r o s k a and P e r e z (37). To o b t a i n b e t t e r agreement between c a l c u l a t e d C-0 bond lengths and those observed by X-ray or neutron diffraction methods, the following equilibrium bond lengths for bond s t r e t c h i n g energy were used : Cl-05 from 1.396 t o 1.422 S, C l - 0 1 from 1.380 t o 1.388 8, C5-05 from 1.412 t o 1.420 ft , and C4-01 from 1.388 t o 1.415 &. " E m p i r i c a l " method; (PFOS : P o t e n t i a l F u n c t i o n s Oligosaccharide S t r u c t u r e s ) . The p o t e n t i a l energy i s c a l c u l a t e d by i n c l u d i n g the 1

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE

42

partitioned contributions arising from the van der Waals, t o r s i o n a l and hydrogen bond c o n t r i b u t i o n s . The van d e r Waals i n t e r a c t i o n s a r e e v a l u a t e d by u s i n g 6-12 p o t e n t i a l f u n c t i o n s w i t h t h e p a r a m e t e r s p r o p o s e d by S c o t t and Scheraga ( 3 8 ) . A t h r e e - f o l d p o t e n t i a l i s used f o r r o t a t i o n about t h e a g l y c o n bond 01-C4* w i t h a b a r r i e r o f 1.0 k c a l / m o l . F o r r o t a t i o n about g l y c o s i d i c bond C l - 0 1 , t h e i n t r a m o l e c u l a r mechanism r e s p o n s i b l e f o r exo-anomeric effect, i s taken into account u s i n g the p o t e n t i a l function p r o p o s e d by T v a r o s k a (39); a t h r e e - f o l d r o t a t i o n a l b a r r i e r o f 1.0 k c a l / m o l i s a l s o i n c l u d e d . Hydrogen bond e n e r g y i s computed by an empirical expression : V = 33.14 (R - 2.55) (R - 3.05) where R i s t h e d i s t a n c e between oxygen atoms which s h o u l d l i e between 2.55 and 3.05 A. No e l e c t r o s t a t i c i n t e r a c t i o n i s t a k e n i n t o a c c o u n t . The energy map i s c a l c u l a t e d as a f u n c t i o n o f $ and y a t i n t e r v a l s o f 5 ° . With respect t o the r e l a t i v e energy minimum, i s o - e n e r g y c o n t o u r s a r e drawn by interpolation at 1 kcal/mol increments d e s c r i b e d i n terms o f a the number o f r e s i d u e s p e r t u r n o f t h e h e l i x and h b e i n g t h e t r a n s l a t i o n a l o n g t h e h e l i x a x i s . These p a r a m e t e r s a r e c a l c u l a t e d f o l l o w i n g o u r a l g o r i t h m r e p o r t e d p r e v i o u s l y (40) . H B

D i f f r a c t i o n and r e l a t e d Methods. Methyl 8-D-cellotrioside was d i s s o l v e d i n water (10 m g . m l ) . C r y s t a l l i z a t i o n i n t o hexagonal p l a t e l e t s was a c h i e v e d by a slow d i f f u s i o n o f e t h a n o l . A c r y s t a l o f d i m e n s i o n s c a . 0.5 x 0.5 x 0.1 mm was mounted on a Nonius CAD4 d i f f r a c t o m e t e r . The r a d i a t i o n was n i c k e l - f i l t e r e d Cu Ka. The unit-cell d i m e n s i o n s were o b t a i n e d a s p a r t o f t h e a l i g n m e n t p r o c e s s on t h e d i f f r a c t o m e t e r by a l e a s t - s q u a r e s f i t t o t h e s e t t i n g o f 20 w e l l - c e n t e r e d r e f l e x i o n s . The m o n o c l i n i c space group P 2 was a s s i g n e d , b a s e d upon t h e s y s t e m a t i c absences : 0 k 0, k ^ 2n. 431C i n d e p e n d e n t r e f l e x i o n s were measured, o f which 616 were a s s i g n e d z e r o w e i g h t as t h e n e t c o u n t o f each was l e s s than 1.99 (I) where a (I) i s t h e s t a n d a r d d e v i a t i o n e s t i m a t e d from c o u n t i n g s t a t i s t i c s . Because o f t h e low v a l u e o f t h e a b s o r p t i o n coefficient, no a b s o r p t i o n c o r r e c t i o n was a p p l i e d . Scattering factors were obtained from International Tables f o r X-ray c r y s t a l l o g r a p h y ( 4 1 ) . The MULTAN computer program (42) was used f o r n o r m a l i z a t i o n and a t t e m p t s t o s o l v e t h e s t r u c t u r e t h r o u g h d i r e c t methods. _ 1

1

C r y s t a l l i z a t i o n o f c e l l o t e t r a o s e from aqueous s o l u t i o n was done by slow d i f f f u s i o n o f e t h a n o l . C r y s t a l s t h i n enough f o r e l e c t r o n diffraction were prepared by epitaxial crystallization of c e l l o t e t r a o s e onto VaIonia m i c r o f i b r i l s i n a s o l u t i o n c o n t a i n i n g 1% o f c e l l o t e t r a o s e i n a 3:2 m i x t u r e o f e t h a n o l i n water. Crystallization took place within a few weeks, at room temperature. A shish-kebab type o f s t r u c t u r e (43) was o b t a i n e d from which l a m e l l a r fragments o f c e l l o t e t r a o s e c r y s t a l s c o u l d be detached by m i l d sonication. The fragments were mounted on c a r b o n - c o a t e d g r i d s and were o b s e r v e d on a P h i l i p s EM 400T E l e c t r o n M i c r o s c o p e o p e r a t i n g a t 120 kV. A f u l l d e t e r m i n a t i o n o f the u n i t - c e l l p a r a m e t e r s was a c h i e v e d t h r o u g h a s e r i e s o f t i l t e x p e r i m e n t s and subsequent a n a l y s i s o f t h e r e s u l t i n g electron d i f f r a c t o g r a m s (Roche, E . ; u n p u b l i s h e d d a t a ) .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3. HENRISSAT ET AL.

43

Model Compounds for Cellulose II

About 1 cm o f h i g h l y c r y s t a l l i n e powdered c e l l o t e t r a o s e was used t o o b t a i n a n e u t r o n d i f f r a c t i o n p a t t e r n from t h e h i g h r e s o l u t i o n D1A machine a t t h e I n s t i t u t e Laue L a n g e v i n i n G r e n o b l e . S i n c e t h e m a t e r i a l c o u l d n o t be d e u t e r a t e d , t h e e f f e c t i v e a b s o r p t i o n was important, as was t h e " i n c o h e r e n t " background s c a t t e r i n g . In o r d e r t o o b t a i n good s t a t i s t i c s i n t h e background r e g i o n , o n l y a s i n g l e powder p a t t e r n was c o l l e c t e d i n t h e whole o f o u r a l l o c a t e d t h r e e d a y s . We had hoped t o be a b l e t o e x t r a c t a t l e a s t 50 Bragg intensities from such a p a t t e r n by u s i n g a r e l a t i v e l y long wavelength o f 3 A t o s p r e a d o u t t h e lower o r d e r s o v e r t h e whole a n g u l a r r a n g e . However, most o f t h e Bragg i n t e n s i t y goes i n t o s i x s t r o n g peaks, w i t h t h e r e m a i n d e r a l l v e r y s m a l l and b a r e l y above the background l e v e l . F u r t h e r m o r e , i t appears t h a t t h e w i d t h s o f the l i n e s may depend on t h e (h k 1) d i r e c t i o n i n t h e s t r u c t u r e : n e a r 2 0= 8 5 ° , n e i g h b o r i n g l i n e s have v e r y d i f f e r e n t w i d t h s . T h i s e f f e c t , presumably due t o d i r e c t i o n a l d i f f e r e n c e s i n s h o r t range crystalline order, complicates the a p p l i c a t i o n of profile r e f i n e m e n t methods t o Linked-Atom-Least-Squares Procedure. Molecular models having s t a n d a r d bond l e n g t h s , bond a n g l e s and r i n g c o n f o r m a t i o n s (43) were generated using the linked-atom-least-squares method p r e v i o u s l y d e s c r i b e d ( 4 4 ) . In t h e p r e s e n t a p p l i c a t i o n o f t h i s method f o r non-helical structures, the v a r i a b l e parameters include a l l the t o r s i o n angles and valence angles about g l y c o s i d i c l i n k a g e s , and t h o s e d e f i n i n g t h e orientations of r o t a t a b l e s u b s t i t u e n t s , n o t a b l y the primary h y d r o x y l groups. F o r each m o l e c u l e , two v a r i a b l e p a r a m e t e r s , u and v d e f i n e t h e p o i n t where the molecular axis intersects the orthogonal c r y s t a l l o g r a p h i c p l a n e o f t h e u n i t c e l l ; a d d i t i o n a l p a r a m e t e r s |i and define the o r i e n t a t i o n about, and r e l a t i v e translation p a r a l l e l t o t h a t a x i s . Any o r a l l o f t h e s e p a r a m e t e r s may be v a r i e d so as t o m i n i m i z e t h e f o l l o w i n g f u n c t i o n :

Q = I Wm ( oFm - Fm ) + S Z e. The f i r s t summation i s used t o o p t i m i z e agreement between o b s e r v e d s t r u c t u r e a m p l i t u d e s , oFm, w i t h t h o s e c a l c u l a t e d from t h e model, Fm a f t e r s c a l i n g by a f a c t o r 1/k. In c a l c u l a t i n g Fm, a l l atoms were assumed t o v i b r a t e i s o t r o p i c a l l y and 4 A was a s s i g n e d a s the v a l u e o f B i n t h e a t t e n u a t i o n f a c t o r , e x p ( - B s i n OA ) . U n i t w e i g h t s , Wm, were a s s i g n e d t o a l l measurable r e f l e c t i o n s w i t h i n a g i v e n r e s o l u t i o n s p h e r e . The second term i s used t o ensure t h e s t e r e o c h e m i c a l a c c e p t a b i l i t y o f t h e model by m i n i m i z i n g a q u a n t i t y , Z et which a p p r o x i m a t e s t h e non-bonded r e p u l s i o n e n e r g y . T h i s term i s a l s o used t o o p t i m i z e weak attractive i n t e r a c t i o n s such as hydrogen bonds. The p r e c i s e f o r m u l a t i o n o f e . and e m p i r i c a l c o n s t a n t s employed i n i t s e v a l u a t i o n have been d e s c r i b e d e l s e w h e r e ( 4 4 ) . The c a l c u l a t i o n s were p e r f o r m e d on an EC-1045.01 machine, a t t h e Computer C e n t r e o f the Slovak Academy o f S c i e n c e s , B r a t i s l a v a , and on a H o n e y w e l l - B u l l CII-HB68 mainframe a t t h e U n i v e r s i t y o f G r e n o b l e . The m o l e c u l a r drawings shown t h r o u g h o u t t h i s a r t i c l e were o b t a i n e d w i t h t h e a i d o f t h e PITMOS program ( 4 5 ) . The p e r s p e c t i v e drawing o f the three dimensional shape o f t h e m i r r o r image o f t h e c o n f o r m a t i o n a l energy w e l l o f c e l l o b i o s e ( F i g . 2) was o b t a i n e d w i t h t h e CARTOLAB program ( 4 6 ) . 2

2

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987. f

P

F i g u r e 2. P e r s p e c t i v e d r a w i n g o f the t h r e e - d i m e n s i o n a l shape o f the m i r r o r image o f the c o n f o r m a t i o n a l energy w e l l f o r the f u l l a n g u l a r range f o r $ and Y. The volume was c o n s t r u c t e d u s i n g t h e f o l l o w i n g scheme: V( $ ¥ ) > 15 k c a l / m o l V ( $ , ¥) = 0 ; V( $ , < 15 k c a l / m o l : V ( $ , ¥ ) = - ( V ( $, ¥ ) - 1 5 ) . V b e i n g the energy e x p r e s s e d ^ r e l a t i v e t o t h e minimum. P r o c e e d i n g from t o p t o bottom o f t h e t h r e e - d i m e n s i o n a l shape, we note : t h e v e r y low e n e r g y r e g i o n (the arrows p o i n t towards the c o n f o r m a t i o n s o b s e r v e d f o r c r y s t a l l i n e c e l l o b i o s e and m e t h y l 3 - D - c e l l o b i c s i d e ) . The 5 k c a l / m o l t o the 10 k c a l / m o l e n e r g y c o n t o u r s c o r r e s p o n d t o t h e l i g h t g r e y r e g i o n o f t h e volume.

o3 § n r

^ H C H

3. HENRISSAT ET AL.

Model Compounds for Cellulose II

45

R e s u l t s and D i s c u s s i o n Conformational Analysis. Comparison o f r e s u l t s shows t h a t t h e l o c a t i o n s o f t h e energy minima on t h e ( $ , ¥ ) map a r e a l m o s t i n d e p e n d e n t o f t h e f u n c t i o n used f o r c a l c u l a t i n g energy, whereas the d i f f e r e n c e s i n energy levels o f t h e v a r i o u s minima a r e s t r o n g l y dependent on t h i s c h o i c e . The d i f f e r e n c e s a r e most pronounced i n t h e low-energy r e g i o n s . T h i s s u g g e s t s t h a t energy s u r f a c e s a r e v a l u a b l e i n r e f i n i n g t h e p e r t i n e n t p o r t i o n o f t h e map embodying " a l l o w e d " c o n f o r m a t i o n s , b u t r e l a t i v e energy values s h o u l d be t r e a t e d w i t h c a u t i o n . F i g u r e 2 shows t h e energy map f o r c e l l o b i o s e , as c a l c u l a t e d from PFOS method; t h e two d e e p e s t minima a r e i n d i c a t e d by a r r o w s . These two minima c o r r e s p o n d t o c r y s t a l l i n e conformations found f o r c e l l o b i o s e (4) and m e t h y l - 6 - D - c e l l o b i o s i d e (5) . F o r t h e s e models o f t h e two s t a b l e c o n f o r m a t i o n s , an i n t r a m o l e c u a l r hydrogen bond o f t h e t y p e 03*...05 i s p r e s e n t w i t h 03 ...05 d i s t a n c e o f about 2.60 A, compared t o 2.7 We n e x t s t u d i e d t h e i n f l u e n c groups a t C6 on t h e c o n f o r m a t i o n s a t t h e g l y c o s i d i c junction. Because t h e t r a n s - g a u c h e ( t g ) p o s i t i o n o f 06 seldom o c c u r s , p r o b a b l y as t h e r e s u l t o f r e p u l s i v e i n t e r a c t i o n between 04 and 06 (21), we c o n s i d e r e d o n l y t h e gauche-gauche ( gg ) and g a u c h e - t r a n s ( g t ) c o n f o r m e r s . The r o l e o f t h e p r i m a r y h y d r o x y l group d i f f e r s depending on whether i t i s on t h e r e d u c i n g o r t h e n o n - r e d u c i n g r e s i d u e . F o r v a l u e s o f $ and V c o r r e s p o n d i n g t o t h e c r y s t a l l i n e conformations of either cellobiose (4) o r methyl 8-D-cellob i o s i d e ( 5 ) , a g t o r i e n t a t i o n o f t h e p r i m a r y h y d r o x y l group o f t h e non-reducing residue brings an 02-H...06 hydrogen bond. C r y s t a l l i n e m e t h y l c e l l o b i o s i d e (5) has t h i s bond; c e l l o b i o s e does not. P r e v i o u s work r e v e a l e d t h a t c e l l o b i o s e c a n e x i s t i n s e v e r a l s t a b l e c o n f o r m a t i o n s . Conformers c o r r e s p o n d i n g t o t h e e i g h t l o w e s t minima (Fig. 2) were used as s t a r t i n g p o i n t s f o r c a l c u l a t i o n s with MM2CARB. Seven conformers r e s u l t e d ; t h e i r r e l a t i v e e n e r g i e s a r e given i n Table I . In Table I I are s e l e c t e d c h a r a c t e r i s t i c s of ,

T a b l e I . R e l a t i v e e n e r g i e s (kcal/mol) o f s t a b l e c o n f o r m e r s f o r c e l l o b i o s e and m e t h y l B - D - c e l l o b i o s i d e c a l c u l a t e d by MM2CARB method w i t h and w i t h o u t hydrogen bond energy ( V ) .

Conformer

cellobiose w i t h o u t V, HB

CI C2 C3 C4 C5 C6 C7

1.33 0.00 3.42 1.52 4.24 6.39 2.88

methyl B - D - c e l l o b i o s i d e w i t h V, HB 0.93 0.50 0.00 1.75 2.11 2.94 2.24

w i t h o u t V, HB

with V

1.45 0.00 1.60 2.20 4.48 2.69 3.34

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

HB

0.84 0.36 0.00 1.68 2.09 3.04 1.67

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

II.

,

$

|i

H

; ^

4.3

112.7 107.2 117.1

= H1-C1-01-C4'

C(5)-0(5)-C(l) 0(5)-C(l)-0(l) C(l)-0(l)-C(4 )

1.4315 1.4264 1.3923 1.4289

63.6 62.9

X X'

,

= Cl-01-C4 -H4'

3.5

113.1 108.1 114.9

1.4289 1.4237 1.3997 1.4308

58.2 -64.1

49.9 16.7

32.8 -66.1

Y

31.58

C2

-66.3 -120.4

32.06

CI

4.3

112.9 108.8 114.9

1.4286 1.4240 1.3989 1.4283

61.0 61.2

161.4 5.8

48.2 -114.9

31.22

£3

4.9

112.3 109.4 117.3

1.4305 1.4221 1.3973 1.4328

64.3 68.0

44.5 171.7

-74.4 55.1

32.89

CA

5.9

112.6 102.6 115.6

1.4296 1.4273 1.3956 1.4269

64.2 63.7

-35.9 -26.9

-147.9 -145.0

33.31

C5

2.2

113.2 101.2 118.7

1.4299 1.4314 1.3963 1.4281

62.6 -67.2

-62.1 -60.6

-171.9 -176.0

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5.2

112.8 108.6 116.2

1.4280 1.4188 1.3944 1.4318

63.7 60.4

70.1 -164.3

-46.6 -83.0

32.89

C7

MM2CARB c a l c u l a t e d e n e r g i e s E ( k c a l / m o l ) , d i p o l e moments u (D), and s e l e c t e d g e o m e t r i c a l p a r a m e t e r s : bond l e n g t h s ( A ) , bond a n g l e s ( d e g ) , f o r m e t h y l 8 - D - c e l l o b i o s i d e c o n f o r m e r s .

-87.6 178.9

C(5)-0(5) C(5)-C(l) C(l)-0(1) 0(1)-C(4')

* y

E

Conformers

Table

m p C

o n

pa n ^ 73 rn

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H

ON

3.

HENRISSAT ET AL.

47

Model Compounds for Cellulose II

m e t h y l 6 - D - c e l l o b i o s i d e . Note (Table 1) t h a t t h e i n c l u s i o n o f hydrogen b o n d i n g energy (V ) changes t h e r e l a t i v e s t a b i l i t y o f the c o n f o r m e r s . Because a l l possible intramolecular hydrogen bonds were c o n s i d e r e d , i t i s o f i n t e r e s t t o compare t h e v a r i o u s p a t t e r n s t h a t r e s u l t e d from t h e t h r e e b e s t c o n f o r m a t i o n s . The hydrogen b o n d i n g i n C2 c o r r e s p o n d s t o t h a t o f c r y s t a l l i n e o f m e t h y l 6 - D - c e l l o bioside, with 03'H...05 and 03'H...06 hydrogen bonds with i n t e r o x y g e n d i s t a n c e s o f about 2.77 and 2.80 A, r e s p e c t i v e l y . On t h e o t h e r hand, t h e C I c o n f o r m a t i o n does n o t have an 03'H...05 hydrogen bond. The i n t e r o x y g e n d i s t a n c e i s 3.0 A b u t t h e hydrogen atom on 03* i s n o t p o s i t i o n e d f a v o r a b l y . I n s t e a d i t forms a bond t o 06, t h e d i s t a n c e b e i n g 2.90 A. In t h e C3 conformer, 03'H...05 and 06'H...06 were formed. The parameters i n T a b l e I I demonstrate an i n t e r a c t i o n between t h e r i n g geometry and t h e c o n f o r m a t i o n around t h e g l y c o s i d i c and a g l y c o n bonds. The c a l c u l a t e d bond l e n g t h s agree w i t h t h e v a l u e s observed i n the c r y s t a for C5-05-C1 and C1-01-C4• o b s e r v e d v a l u e s and v a r y w i t h $ and y . A t t h e same t i m e , t h e 05-C1-01 bond a n g l e s v a r y from 101.2 t o 109.5° and g l y c o s i d i c bond a n g l e s from 113 t o 1 2 0 ° . These v a r i a t i o n s o f i n t e r n a l geometry owing t o changes i n l i n k a g e t o r s i o n a n g l e a r e c o n s i s t e n t w i t h s t a t i s t i c a l a n a l y s i s o f carbohydrate c r y s t a l s t r u c t u r e s (47,48). D i p o l e moments range from 3.4 t o 6.0 D. When t h e t o r s i o n a n g l e s $ , y a r e i d e n t i c a l f o r a l l t h e p a i r s o f g l u c o s e r e s i d u e s a l o n g t h e c h a i n , t h e polymer c h a i n assumes a r e g u l a r h e l i c a l shape. From t h e g e o m e t r i c a l parameters defining the r e p e a t u n i t , i t i s p o s s i b l e t o c a l c u l a t e , f o r each $ , y t h e v a l u e s o f t h e h e l i c a l p a r a m e t e r s n, h . I n t u r n , t h e s e v a l u e s may be compared t o t h o s e d e r i v e d from X-ray d i f f r a c t i o n o f o r i e n t e d f i b r i l l a r m a t e r i a l which, i n t h e c a s e o f c e l l u l o s e c h a i n s a r e n = 2 and h = 5.15 A. None o f t h e s t a b l e c e l l o b i o s e c o n f o r m e r s as derived from e i t h e r e n e r g y m i n i m i z a t i o n o r c r y s t a l structure e l u c i d a t i o n s c a n g e n e r a t e such a h e l i c a l s t r u c t u r e . 1 3

Charge D e n s i t i e s - CP/MAS NMR S p e c t r o s c o p y . Recent C NMR s t u d i e s of s o l i d carbohydrates u s i n g the c r o s s - p o l a r i z a t i o n / m a g i c angle spinning (CP/MAS) technique (15) d e m o n s t r a t e s that NMR c h e m i c a l s h i f t s o f the C I , C4 and C6 c a r b o n atoms a r e c o n s i d e r a b l y d i s p l a c e d , up t o 10 ppm, depending on t h e p a r t i c u l a r s t r u c t u r e . A r e l a t i o n o f t h e s e s h i f t s t o c o n f o r m a t i o n would be v a l u a b l e i n l e a r n i n g whether a c o n f o r m a t i o n i s r e t a i n e d i n n o n - c r y s t a l l i n e o r solution states. A simple linear relationship h a s been p r o p o s e d between t h e chemical shift f o r C6 and t h e t o r s i o n angle X about t h e e x o - c y c l i c C5-C6 bond. The c h e m i c a l s h i f t s f e l l i n t o 3 groups o f 60-62 ppm, 62.5 ppm and 65.5-66.6 ppm f o r t h e gg, g t a n f t g c o n f o r m a t i o n s ( 1 8 ) . I n a more s o p h i s t i c a t e d e m p i r i c a l o b s e r v a t i o n , i t was p r o p o s e d that chemical s h i f t s i n a series of closely r e l a t e d m o l e c u l e s a r e r e l a t e d t o charge d e n s i t y (50,51). As a f i r s t s t e p i n u n d e r s t a n d i n g t h e r e l a t i o n s h i p o f c h e m i c a l shift t o c o n f o r m a t i o n , we c a l c u l a t e d charge densities fora and 6-D-glucose, c e l l o b i o s e , and m e t h y l 6 - D - c e l l o b i o s i d e , u s i n g the PCILO program. Charge d e n s i t i e s f o r C6 o f a - and 6-D-glucose

American Chemical Society. Library 1155 16th St., N.w. In The Structures of Cellulose; Atalla, R.;

O.C.Society: 20036 ACS Symposium Series;Washington. American Chemical Washington, DC, 1987.

48

THE STRUCTURES OF CELLULOSE were, r e s p e c t i v e l y , 0.1195 and 0.1173 ( gg ) , 0.1189 and 0.1169 ( g t ) , and 0.1182 and 0.1164 ( t g ) . These v a l u e s c o r r e l a t e w e l l w i t h the o b s e r v e d c h e m i c a l s h i f t s , above. C a l c u l a t e d e l e c t r o n d e n s i t i e s on s e l e c t e d carbon and oxygen atoms of m e t h y l B - D - c e l l o b i o s i d e ( i n the c r y s t a l l i n e c o n f o r m a t i o n ) as a f u n c t i o n o f c o n f o r m a t i o n o f the p r i m a r y a l c o h o l groups a r e g i v e n i n T a b l e I I I . The results for cellobiose are s i m i l a r . While c o n f o r m a t i o n a l change results i n changed d e n s i t y w i t h i n the g l u c o s e r e s i d u e , t h e r e i s no e f f e c t on the o t h e r r e s i d u e . However, the c o r r e l a t i o n o f charge d e n s i t y and c h e m i c a l s h i f t observed above f o r g l u c o s e i s n o t o b s e r v e d i n t h i s c a s e . There a r e s e v e r a l p o s s i b l e e x p l a n a t i o n s . Owing t o the r a r i t y o f tc[ c o n f o r m a t i o n s , the c h e m i c a l s h i f t d a t a i s i n s u f f i c i e n t . (The v a l u e s r e p o r t e d a r e from c e l l u l o s e I , d e t e r m i n e d by f i b e r d i f f r a c t i o n , and may n o t be r e l i a b l e ) . A l s o , c h e m i c a l s h i f t s h o u l d be a c o n t i n u o u s f u n c t i o n o f torsional angle. Therefore, the proposed linear relationship cannot be c o r r e c t . Our u n d e r s t a n d i n g o f a r e l a t i o n s h i p between e l e c t r o n d e n s i t y and c h e m i c a more c a l c u l a t i o n s a r e needed r o t a t i o n . Such attempts w i l l a l s o p r o v i d e i n f o r m a t i o n on c h e m i c a l s h i f t s o f u n s t a b l e conformers t h a t cannot be i s o l a t e d . P a c k i n g f e a t u r e s o f known o l i g o m e r s . X-ray d a t a on crystalline o l i g o m e r s p r o v i d e s i n f o r m a t i o n about some f e a t u r e s o f the p a c k i n g habits as well as on molecular conformation. For a full u n d e r s t a n d i n g o f any p a c k i n g arrangement, t h e energy between a reference molecule and a l l i t s neighbors i n the crystal is evaluated, taking into account the hydrogen bonds and the non-bonded i n t e r a c t i o n s ( 5 2 ) . These c a l c u l a t i o n s use the same p a r a m e t e r s as i n the c o n f o r m a t i o n a l a n a l y s i s o f s i n g l e m o l e c u l e s u s i n g the PFOS method. C o n t r i b u t i o n s t o the energy were b r o k e n down i n t o the pure, non-bonded p a r t and the i n t e r m o l e c u l a r hydrogen b o n d i n g . The p a c k i n g i n c r y s t a l s o f c e l l o b i o s e i s v e r y dense, each m o l e c u l e i s s u r r o u n d e d by 10 n e i g h b o r s t h a t o c c u r i n p a i r s ( F i g . 3a) . The s t r o n g e s t i n t e r a c t i o n s a r e c a l c u l a t e d f o r m o l e c u l e s r e l a t e d by p u r e t r a n s l a t i o n a l symmetry a l o n g t h e c r y s t a l l o g r a p h i c c a x i s (c = 5.091 A ) . The energy o f p a i r i n g o f p a r a l l e l m o l e c u l e s i s due o n l y t o van d e r Waals i n t e r a c t i o n s . The r e m a i n i n g p a c k i n g energy a r i s e s from m o l e c u l e s r e l a t e d by symmetry o p e r a t i o n s about t h e 2^ a x i s , w i t h a p p r o p r i a t e t r a n s l a t i o n s a l o n g the a and/or c axes. In these i n s t a n c e s , the i n t e r m o l e c u l a r a s s o c i a t i o n s a r e obtained through van d e r Waals and hydrogen b o n d i n g c o n t r i b u t i o n s . Some o f t h e s e same f e a t u r e s a r e a l s o found i n m e t h y l 6 - D - c e l l o b i o s i d e , which a l s o c r y s t a l l i z e s i n t h e P2 space group, each molecule has 10 p a i r e d neighbors (Fig. 3b). The strongest i n t e r m o l e c u l a r p a i r i n g i s a l s o obtained f o r molecules r e l a t e d to each o t h e r by p u r e t r a n s l a t i o n a l o n g the c r y s t a l l o g r a p h i c c a x i s (c = 4.496 £) . The p a i r i n g energy a g a i n a r i s e s from van d e r Waals f o r c e s . O t h e r pure t r a n s l a t i o n a l o p e r a t i o n s , such as - a o r - a c result i n strong cohesive energy, provided by comparable magnitudes o f van d e r Waals and hydrogen b o n d i n g . Here, the o n l y i n t e r a c t i o n s a l o n g the 2^ a x i s ( b = 25.532 K) o c c u r v i a the methanol molecule through hydrogen bonding with subsequent b r i d g i n g i n the d i s a c c h a r i d e m o l e c u l e s .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.8842 3.8837 3.8798 3.8839 3.8834 3.8795 3.8840 3.8835 3.8795

*0(6")

"0(6)

6.1672 6.1674 6.1670 6.1666 6.1668 6.1666 6.1680 6.1678 6.1683

*C(6)

3.8857 3.8857 3.8856 3.8846 3.8846 3.8846 3.8852 3.8852 3.8852

*C(5)

3.8979 3.8981 3.8980 3.8979 3.8981 3.8980 3.8940 3.8942 3.8941

(see t e x t f o r e x p l a n a t i o n ) .

6.1692 6.158C 6.1657 6.1691 6.1578 6.1658 6.1691 6.1574 6.1661

respectively

3.8851 3.8837 3.8847 3.8851 3.8837 3.8847 3.8850 3.8837 3.8846

*C(6')

g and t s t a n d f o r gauche and t r a n s ,

f

gt, tg tg gt gg*gt gt,tg tg,tg ggrtg gt,gg tg,gg gg^gg

*C(5»)

T a b l e I I I . PCILO c a l c u l a t e d e l e c t r o n d e n s i t i e s f o r s e l e c t e d c a r b o n and oxygen atoms as a f u n c t i o n o f h y d r o x y m e t h y l g r o u p s o r i e n t a t i o n s o f m e t h y l β-D-cellobioside i n c r y s t a l c o n f o r m a t i o n .

Ι

ε-

9 I

70

m ζ

THE STRUCTURES OF CELLULOSE

F i g u r e 3. Stereoscopic representations of s u r r o u n d i n g found i n t h e c r y s t a l l i n e s t a t e o f : a C e l l o b i o s e (4) b M e t h y l β-D-cellobioside ( 5 ) .

the

molecular

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

HENRISSAT ET AL.

51

Model Compounds for Cellulose II

M e t h y l β-D-cellotrioside. Fig. 4 shows a typical hexagonal crystalline platelet o f m e t h y l β-D-cellotrioside, which upon r e d u c t i o n t o powder g i v e s r i s e t o t h e d i f f r a c t i o n p a t t e r n o f F i g u r e 5. The u n i t c e l l d a t a a r e i n T a b l e IV. From t h e o b s e r v e d

T a b l e IV.

C r y s t a l d a t a f o r m e t h y l β-D

C

a =

8.005 (2) A

β = 116.39 (5) °

19

H

34

°16

; monoclinic ;

= 518.46

Mr

; b = 76.403 (9) Κ Ρ2

χ

cellotrioside.

;

c =

;

Ζ = 8

8.995 (2) A

d e n s i t y o f 1.50 Mg.m independent molecules. S o l v i n g a non-centrosymmetrical s t r u c t u r e w i t h 140 non-hydrogen atoms from o n l y 3694 r e f l e c t i o n s i s a formidable challenge. Preliminary attempts to resolve the s t r u c t u r e w i t h c o n v e n t i o n a l d i r e c t methods were u n s u c c e s s f u l ; the work i s s t i l l b e i n g p u r s u e d i n our l a b o r a t o r y . The c r y s t a l morphology ( F i g . 4) and the d i m e n s i o n o f the u n i t c e l l b parameter o f 76.403 A l e a d t o the c o n c l u s i o n s t h a t : -the m o l e c u l e s a r e i n extended c o n f o r m a t i o n s and a r e p l a c e d p a r a l l e l t o the b a x e s . In some r e s p e c t , t h e y form a pseudochain structure. -the base p l a n e p a r a m e t e r s ( a , c and β ) resemble those o f c e l l u l o s e I I and o f c e l l o t e t r a o s e . Fig. 6 summarizes t h e p o s s i b l e m o l e c u l a r f e a t u r e s i n terms o f p a r a l l e l and a n t i p a r a l l e l arrangements c o m p a t i b l e w i t h t h e u n i t cell d a t a f o r m e t h y l β-D-cellotrioside. By analogy with the a n t i p a r a l l e l t y p e o f p a c k i n g p r o p o s e d f o r c e l l u l o s e I I and f o r c e l l o t e t r a o s e , one would f a v o r t y p e d i n F i g . 6. Owing t o i t s h i g h c r y s t a l l i n i t y and the s i m i l a r i t y o f i t s X-ray d i f f r a c t o g r a m w i t h t h a t o f c e l l o t e t r a o s e , m e t h y l β-D-cellotrioside was used as a model f o r c e l l u l o s e I I i n a s t u d y u s i n g C NMR s p e c t r o s c o p y . I t was thought t h a t the a g l y c o n i c m e t h y l group would a l s o be u s e f u l i n p r o v i d i n g i n f o r m a t i o n about t h e v a r i e t y o f l i n k a g e c o n f o r m a t i o n s i n t h e s t r u c t u r e . F i g . 7 shows t h e s p e c t r a , a l o n g w i t h some r e l a t e d o l i g o m e r s . The spectra are similar, e s p e c i a l l y the c h e m i c a l s h i f t s f o r C4, which o c c u r a t lower f i e l d than i n the lower o l i g o m e r s . The r e s o n a n c e a t t r i b u t e d t o the C6 n u c l e i g i v e s r i s e t o a s i n g l e b r o a d peak c e n t e r e d a t 64.2 ppm; t h r e e s i g n a l s appear a t 108.3, 106.0 and 104.0 ppm f o r t h e 3 CI atoms. At h i g h f i e l d , t h e m e t h y l groups produce two peaks, a t 58.4 and 56.7 ppm. T h i s i m p o r t a n t o b s e r v a t i o n s i g n i f i e s t h a t t h e r e a r e two main magnetic e n v i r o n m e n t s , and t h e r e f o r e , c o n f o r m a t i o n s , f o r the a g l y c o n i c m o i e t i e s . Moreover, t h e s e two peaks are themselves composites o f two o r t h r e e peaks, c o n s i s t e n t w i t h t h e l a r g e u n i t c e l l . T h i s s u p p o r t s model d, which has b o t h "head-to-head" and " h e a d - t o - t a i l " environments f o r the m e t h y l g r o u p s . By comparing

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE

F i g u r e 4. Typical β-D-cellotrioside.

Figure 5.

X-ray

hexagonal

powder

crystalline

diffraction

platelet

of

pattern

of

β-D-cellotrioside.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

methyl

methyl

HENRISSAT ET AL.

Model Compounds for Cellulose II

Φ © ©

©

©

© © Φ ©

©

©

Φ © Φ

Θ

©

© ©

©

© © © Φ

©

©

Φ © ©

©

F i g u r e 6. Schematic drawings of the possible molecular arrangements f o r c r y s t a l l i n e m e t h y l 0 - D - c e l i o t r i o s i d e c o n s i s t e n t w i t h a P2^ space group symmetry. The 4 i n d e p e n d e n t m o l e c u l e s a r e numbered from 1 t o 4; t h e arrows p o i n t towards t h e m e t h y l g r o u p s . F o r o b v i o u s c r y s t a l l o g r a p h i c r e a s o n s t h e 2^ a x i s o f symmetry cannot c o i n c i d a t e w i t h any o f t h e m o l e c u l a r a x i s .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

54

THE STRUCTURES OF CELLULOSE

£(ppm) 1 3

F i g u r e 7. CP/MAS C NMR s p e c t r a o f some c e l l u l o s e o l i g o m e r s r e c o r d e d a t 50.36 MHz : a / c e l l c b i o s e , b / m e t h y l β-D-cello­ b i o s i d e , c / c e l l o t e t r a o s e and d/ m e t h y l β-D-cellotrioside.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

55

Model Compounds for Cellulose II

HENRISSAT E T A L .

this spectrum with t h a t o f methyl β-D-cellobioside we could a t t r i b u t e t h e s i g n a l a t 58.1 ppm t o the a g l y c o n i c m e t h y l group; the s i g n a l a t 51.6 ppm t h e n a r i s e s from t h e methanol m o l e c u l e t h a t c o - c r y s t a l l i z e s w i t h m e t h y l β-D-cellobioside. Cellotetraose. D e s p i t e s e v e r a l y e a r s o f s t e a d y e f f o r t , no s i n g l e c r y s t a l adequate f o r c o n v e n t i o n a l X-ray c r y s t a l l o g r a p h y c o u l d be grown. F i g 8. shows a powder d i f f r a c t i o n p h o t o g r a p h ; t h e wide a n g l e n e u t r o n d i f f r a c t o g r a m i s i n F i g . 9. C r y s t a l d a t a a r e i n T a b l e V. F i g . 10 shows b o t h a l a m e l l a r fragment o f c e l l o t e t r a o s e

T a b l e V.

C r y s t a l data f o r c e l l o t e t r a o s e .

C

a =

8.963

(3) A

α = 94.98

(10)

triclinic

;

° PI

24

H

41

°21

Μ

'

Γ

=

6

6

5

·

5

7

; b ; β = 89.34

(10)

°

; γ =116.13

(10)

0

; Ζ = 2

and the corresponding electron d i f f r a c t o g r a m . The base plane p a t t e r n has t w o - d i m e n s i o n a l p i symmetry which i s c o n s i s t e n t w i t h both triclinic and monoclinic space groups. The fact that r e c o r d i n g d i f f r a c t i o n d a t a from t h e base p l a n e r e q u i r e d t i l t i n g the c r y s t a l by 9-11° i n d i c a t e d t h a t the c o r r e c t c h o i c e was the t r i c l i n i c space g r o u p . F o r a c h i r a l m o l e c u l e t h i s c o u l d o n l y be P I . The u n i t c e l l p a r a m e t e r s agreed w i t h p r e v i o u s v a l u e s ( 8 ) . We a l s o confirmed t h a t t h i s t r i c l i n i c c e l l f i t s the neutron d a t a . Observed s t r u c t u r e f a c t o r s were k i n d l y p r o v i d e d by P o p p l e t o n . We found that only 869 of 2951 measured reflections could be c o n s i d e r e d as " o b s e r v e d " (1/ σ (I) > 1.99). S i n c e t h e r e a r e two tetramers per cell, determination of the structure requires location of 90 non-hydrogen atoms with only 869 observed reflections. Preliminary calculations produced a negative temperature f a c t o r , i n d i c a t i n g an u n r e l i a b l e d a t a s e t , as i f from a damaged c r y s t a l . The c r y s t a l used by P o p p l e t o n was o n l y 0.01 mm t h i c k , and i t r e q u i r e d 900 hours o f e x p o s u r e t o a h i g h - i n t e n s i t y beam t o r e c o r d t h e d a t a . Because the d a t a were n o t l i k e l y t o be a c c u r a t e enough f o r c o n v e n t i o n a l methods, we d e c i d e d t o a t t t e m p t to s o l v e the structure with one of the "Real-Space Crystal Structure Resolution" procedures with the 70 reflections corresponding to 3 A r e s o l u t i o n . The Linked-Atom L e a s t - S q u a r e s (LALS) p r o c e d u r e (44) was used t o g e n e r a t e m o l e c u l a r models o f c e l l o t e t r a o s e . The g l u c o s e r e s i d u e s were k e p t i n the s t a n d a r d C c o n f o r m a t i o n ; a l l bond a n g l e s and bond l e n g t h s were f i x e d a t s t a n d a r d v a l u e s ( 4 3 ) . The c o n s t r a i n e d model o f the c r y s t a l s t r u c t u r e was o p t i m i z e d a g a i n s t b o t h X-ray d a t a and n o n - c o v a l e n t i n t e r a t o m i c i n t e r a c t i o n s , as d e s c r i b e d by Smith and A r n o t t (44). h

1

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

56

THE STRUCTURES OF CELLULOSE

F i g u r e 8.

X-ray powder d i f f r a c t i o n p a t t e r n o f c e l l o t e t r a o s e .

COUNT

1

1

1

1

1

I

!

2200 L-

1

1700

1+

Li

ι 50

.

.

.

.

ι 100

.

—•

F i g u r e 9. Wide cellotetraose.

angle

neutron

powder

.

.

.

ι 150

2 THETA

diffractogram

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

of

HENRISSAT ET AL.

Model Compounds for Cellulose II

F i g u r e 10. Electron micrograph of microcrystals of cellotetraose. Insert : corresponding electron diffraction diagram p r o p e r l y o r i e n t e d .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE

58

The p r e l i m i n a r y m o l e c u l a r models were t r e a t e d as r i g i d b o d i e s and the r e l a t i v e o r i e n t a t i o n s about and t r a n s l a t i o n a l o n g the l o n g crystal axis were v a r i e d i n increments. At each step, the magnitude o f s t e r i c i n t e r f e r e n c e was c a l c u l a t e d . The v a l u e s o f packing parameters c o r r e s p o n d i n g t o l o c a l minima were used as s t a r t i n g p o i n t s f o r r e f i n e m e n t t h a t v a r i e d some o f the m o l e c u l a r and p a c k i n g parameters a l o n g w i t h t h e X-ray s c a l e f a c t o r . We c o n c l u d e d a t t h i s s t a g e t h a t the s t r u c t u r e c o n t a i n s a n t i p a r a l l e l molecules. F u r t h e r refinement v a r i e d m o l e c u l a r parameters such as t o r s i o n a n g l e s of the p r i m a r y h y d r o x y l groups and the t o r s i o n and v a l e n c e a n g l e s a t the g l y c o s i d i c l i n k a g e s . The f i n a l R was 0.21; the o b s e r v e d and c a l c u l a t e d s t r u c t u r e f a c t o r s a r e i n T a b l e V I . Atomic c o o r d i n a t e s a r e i n T a b l e V I I ; l a b e l i n g o f the atoms p r o c e e d s from the n o n - r e d u c i n g r e s i d u e ( a and e i n t h e f i r s t and second molecules, r e s p e c t i v e l y ) t o the r e d u c i n g r e s i d u e ( d and h respectively). O n l y the main f e a t u r e r e s o l u t i o n . The c h i e f on two independent molecules i n the unit cell. F i g . 11 is a stereoscopic drawing of the structure, Fig. 12 shows the p r o j e c t i o n o n t o the a, b base p l a n e . T h i s p r o j e c t i o n i s c o n s i s t e n t w i t h the p l a t e l e t morphology o f the c r y s t a l s shown i n F i g . 10. Because a l l g l u c o s e r e s i d u e s were t r e a t e d as r i g i d b o d i e s i n the r e f i n e m e n t , t h e v a r i a b l e l i n k a g e t o r s i o n a n g l e s and glycosidic bond a n g l e s had t o accomodate a l l v a r i a t i o n s r e q u i r e d t o produce t h e b e s t s t r u c t u r e . They may therefore suffer some l o s s of a c c u r a c y . A l l t h e ( Φ, Ψ) a n g l e s , however, f a l l i n t o t h e v e r y low-energy r e g i o n o f the map (see F i g . 1 3 ) . G o i n g from r e s i d u e t o residue, φ and ψ were not similar enough to suggest local pseudo-symmetry. The glycosidic angles ranged from 117.2 to 121.8 . D i s t a n c e s between 03 and 05 on t h e p r e c e d i n g u n i t ranged from 2.47 to 3.02 A, i n d i c a t i n g hydrogen bonding. P r o c e e d i n g from the n o n - r e d u c i n g end, d i s t a n c e s s p a n n i n g d i s a c c h a r i d e r e s i d u e s a r e 10.43, 10.55 and 10.38 A ( m o l e c u l e 1) and 10.36, 10.48, and 10.41 A (molecule 2 ) . While t h e r e i s s c a t t e r i n the d i s t r i b u t i o n of φ and ψ, p a r t l y due t o t h e low a c c u r a c y , some o f the conformations a r e near the C l conformer (methyl β-D-cellobioside) and some a r e near C2 ( c e l l o b i o s e ) . C

Conclusion The m u l t i d i s c i p l i n a r y approaches d e s c r i b e d i n t h e p r e s e n t work a r e all aimed a t describing, and understanding the structural features of c e l l u l o s e oligomers a t a molecular l e v e l . Intimate details are indeed taken i n t o account i n the conformational a n a l y s i s c a l c u l a t i o n s . The utilization of h i g h l y parametrized molecular mechanics programs, including the contribution of i n t r a m o l e c u l a r hydrogen bonding t o the t o t a l energy, produced c o n f o r m a t i o n s i n agreement w i t h o b s e r v e d c r y s t a l s t r u c t u r e s . The v e r y same method was a l s o very powerful i n demonstrating the interactions between ring geometry and rotations about the glycosidic and aglycon linkages. When the hydrogen bond i s c o n s i d e r e d , i t appears t h a t t h e p y r a n o s e r i n g undergoes s m a l l

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

HENRISSAT ET AL.

Table VI.

59

Model Compounds for Cellulose II

Structure factor table resolution.

f o r cellotetraose

at 3 A

h

k

1

F-calc

F-obs

h

k

1

F-calc

F-obs

0 1 -1 0 0 1 0 0 -1 0 1 0 0 -1 1 0 -1 0 1 -1 2 2 2 1 -1 -1 0 2 -2 1 2 -1 1 -2 0

0 0 0 0 1 -1 -1 1 0 -1 -1 1 0 0 0 1 0 -1 0 -1 -1 -1 1 1 1 -1 1 -1 1 1 0 2 -2 0 -1

2 0 1 3 0 0 1 1 2 2 2 2 4 3 3 3 4 4 4 1 0 1 1 1 4 2 4 2 2 2 0 0 1 1 5

41.03 12.12 22.48 28.67 127.23 22.69 45.58 29.38 49.74 48.73 30.88 19.88 30.80 33.77 21.08 19.42 75.26 30.20 91.85 52.42 243.12 83.26 31.31 31.12 15.41 56.27 31.47 62.66 51.92 39.63 256.80 20.21 21.00 38.89 21.25

19.56 14.28 14.62 20.42 101.05 22.91 48.63 16.32

1 -1 2 1 1 1 -2 2

-1 0 0 -1 -2 0 0 1

3 5 1 5 2 5 2 3

23.45 35.84 33.76 27.01 12.17 15.93 23.48 34.10

38.39 28.27 25.37 39.77 28.07 25.74 23.81 27.95

17.84 27.46 34.60 40.05 25.08 23.04 101.92 32.20 97.10 43.09 224.51 78.40 43.04 36.83 31.30 54.58 34.21 50.98 54.34 51.39 245.83 20.66 25.12 34.66 34.86

0 -2 2 2 0 0 -2 -1 1 1 1 0 -2 0 2 -1 -1 0 0 -1 0 1 -2 -2 -1

2 0 -2 -2 -2 2 1 0 -1 -2 1 _2

0 3 0 1 2 1 4 6 6 4 4 3 2 2 3 5 6 6 4 4 3 5 5 5 7

17.05 18.07 45.81 42.48 21.73 22.37 14.20 25.14 33.34 16.96 78.75 24.16 34.27 41.33 29.25 59.45 68.13 40.29 14.84 52.79 42.20 57.56 28.68 20.49 36.51

22.46 23.20 34.62 32.61 31.18 23.81 22.05 26.39 35.35 20.34 112.36 42.68 40.22 50.74 28.65 87.40 81.51 45.66 30.40 64.98 58.31 56.06 28.89 21.60 34.90

2 2 -2 -1 1 1 -2 2 2 -2 1 0 0

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

60

THE STRUCTURES OF CELLULOSE

Table VII.

a-Cl a-01 a-C2 a-02 a-C3 a-03 a-C4 a-04 a-C5 a-05 a-C6 a-06 b-Cl b-01 b-C2 b-02 b-C3 b-03 b-C4 b-C5 b-05 b-C6 b-06 c-Cl c-01 c-C2 c-02 c-C3 c-03 c-C4 c-C5 c-05 c-C6 c-06 d-Cl d-01 d-C2 d-02 d-C3 d-03 d-C4 d-C5 d-05 d-C6 d-06

0.0624 -0.0243 -0.0468 -0.0767 0.0350 -0.0765 0.0815 0.1785 0.1832 0.0932 0.2214 0.0741 0.0717 0.1568 0.1687 0.1738 0.0906 0.1931 0.0699 -0.0206 0.0658 -0.0319 -0.0047 0.0701 -0.0125 -0.0286 -0.0320 0.0462 -0.0580 0.0653 0.1579 0.0743 0.1678 0.2222 0.0209 0.0931 0.0242 -0.0755 -0.0366 -0.0159 0.0596 0.0569 0.1204 0.1630 0.2370

F r a c t i o n a l c o o r d i n a t e s o f C and Ο atoms f o r ο c e l l o t e t r a o s e model o b t a i n e d a t 3 A resolution.

0.4084 0.3739 0.2672 0.0851 0.3112 0.1891 0.5126 0.5616 0.6429 0.5914 0.8440 0.8651 0.4925 0.5298 0.6525 0.8202 0.6126 0.7544 0.4234 0.2739 0.3239 0.0856 0.0078 0.4546 0.4329 0.2848 0.1235 0.3099 0.1586 0.4931 6535 6168 8368 9753 6793 7968 7979 8914 6784 0.7945 0.5642 0.4580 0.5846 0.3547 0.3757

-0.9037 -0.8510 -0.9534 -0.9403 -1.0134 -1.0600 -1.0241 -1.0764 -0.9709 -0.9170 -0.9766

e-Cl e-01 e-C2 e-02 e-C3 e-03 e-C4 e-04 e-C5 e-05 e-C6

0.5370 0.5944 0.5992 0.5292 0.5548 0.6289 0.6160 0.5549 0.5550 0.6065 0.6234

0.4817 0.4398 0.4067 0.2092 0.4662 0.4108 0.6764 0.7298 0.7400 0.6800 0.9497

-0.4833 -0.5375 -0.4342 -0.4440 -0.3731 -0.3275 -0.3655 -0.3117 -0.4181 -0.4733 -0.4157

-0.6134 -0.7057 -0.6771 -0.7683 -0.8049 -0.7960 -0.7537 -0.6967 -0.7766 -0.7298 -0.4334 -0.3798 -0.4767 -0.4538 -0.5383 -0.5798 -0.5596 -0.5122 -0.4565 -0.5287 -0.4791 -0.2070 -0.1557 -0.2568 -0.2411 -0.3159 -0.3630 -0.3290 -0.2756 -0.2228 -0.2840 -0.3412

f-Ol f-C2 f-02 f-C3 f-03 f-C4 f-C5 f-05 f-C6 f-06 g-Cl g-01 g-C2 g-02 g-C3 g-03 g-C4 g-C5 g-05 g-C6 g-06 h-Cl h-01 h-C2 h-02 h-C3 h-03 h-C4 h-C5 h-05 h-C6 h-06

0.5000 0.5307 0.6074 0.5898 0.5368 0.5207 0.5576 0.4931 0.4791 0.3077 0.5378 .6127 .5783 .5071 .5148 .5694 0.5780 0.5395 0.6085 0.6124 0.5230 0.5775 0.5081 0.6044 0.7217 0.6632 0.6705 0.5456 0.5194 0.4604 0.3921 0.2414

0.5000 0.6391 0.8210 0.6400 0.7490 0.4420 0.3177 0.3243 0.1155 0.0227 0.3450 0.3303 0.2381 .0459 .2663 .1817 .4732 0.5702 0.5379 0.7792 0.8576 0.3976 0.4129 0.5657 0.7304 0.5464 0.6950 0.3604 0.2014 0.2320 0.0144 0.0203

-0.7735 -0.6748 -0.6945 -0.6115 -0.5710 -0.5933 -0.6404 -0.6979 -0.6273 -0.6455 -0.9522 -1.0055 -0.9073 -0.9282 -0.8459 -0.8032 -0.8269 -0.8757 -0.9311 -0.8615 -0.8918 -1.1852 -1.2382 -1.1419 -1.1656 -1.0808 -1.0392 -1.0587 -1.1061 -1.1613 -1.0888 -1.0704

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

HENRISSAT ET AL.

Model Compounds for Cellulose II

F i g u r e 11. Stereoscopi of c r y s t a l l i n e cellotetraose.

F i g u r e 12. Projection of a,b base p l a n e .

the

c e l l o t e t r a o s e s t r u c t u r e onto

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

the

THE STRUCTURES OF CELLULOSE

F i g u r e 13. Calculated linear cellodextrins.

and

observed

(Φ,Ψ)

conformations

of

A : c a l c u l a t e d c o n f o r m a t i o n s from M o l e c u l a r Mechanics l a b e l e d from 1 t o 7 and r e f e r t o c o n f o r m e r s CI t o C7 l i s t e d i n Table I I . Observed

( Φ , ψ) conformations :

m : cellobiose m e t h y l β-D-cellobioside Ο : cellotetraose, unit a to unit b to unit c to unit e to unit f to unit g to

b; c; d; f; g; h;

1 2 3 4 5 6

-73.3 -88.9 -90 -103 -68 -80 -94 -83

-132.3 -160.7 -141 -138 -129 -156 -150 -131

The e x t e r n a l c o n t o u r c o r r e s p o n d s t o 10 k c a l / m o l e x p r e s s e d r e l a t i v e t o t h e minimum.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

3.

HENRISSAT ET AL.

Model Compounds for Cellulose II

63

d i s t o r s i o n s . The p r e s e n t r e s u l t s o f f e r a p i c t u r e which i s a compromise between the extreme d i s t o r s i o n s mentioned by Melberg and Rasmussen (53) on t h e one hand, and the lack of such d i s t o r s i o n s r e p o r t e d by P i z z i and E a t o n (54) on t h e o t h e r hand. C o n v e n t i o n a l X-ray c r y s t a l l o g r a p h y on o l i g o m e r s i s i n f a c t t h e u l t i m a t e method s i n c e , upon c o m p l e t i o n o f a t h r e e - d i m e n s i o n a l s t r u c t u r e , t h e c o o r d i n a t e s o f a l l t h e atoms i n a m o l e c u l e are d e t e r m i n e d w i t h a h i g h a c c u r a c y . T h i s , s i m p l y , r e s u l t s from t h e f a c t t h a t t h e r e i s u s u a l l y a f a r g r e a t e r number o f o b s e r v a b l e s , i. e. d i f f r a c t i o n intensities, t h a n p a r a m e t e r s t o be found. U n f o r t u n a t e l y , such a s i t u a t i o n i s n o t found i n the c a s e o f cellulose o l i g o m e r s w i t h a DP h i g h e r t h a n two. N e i t h e r t h e t h r e e d i m e n s i o n a l s t r u c t u r e o f c e l l o t e t r a o s e , nor t h a t o f m e t h y l B-Dcellotrioside have been s o l v e d and refined t o an acceptable a c c u r a c y . N e v e r t h e l e s s , c r u c i a l s t r u c t u r a l i n f o r m a t i o n has been gained since the antiparallel orientation of cellotetraose m o l e c u l e s has been d e t e r m i n e d Also our r e s u l t s s t r o n g l y suggest that s e v e r a l d i s t i n c t conformation found i n the c e l l o t e t r a o s 1 3 b e h a v i o u r may e x p l a i n t h e s p l i t t i n g o f t h e C NMR resonance o f the CI atoms i s s t i l l n o t w e l l e s t a b l i s h e d . Whereas i t was w e l l known t h a t c e l l o t e t r a o s e was a good model f o r c e l l u l o s e I I , we have c l e a r l y shown t h a t m e t h y l β-D-cellotrioside i s a l s o q u i t e an adequate model. Such an adequacy i s based on t h e similarity between the h i g h r e s o l u t i o n s o l i d s t a t e NMR s p e c t r a t o g e t h e r w i t h the dimensions of the crystal unit cell. Crystal structure elucidation of methyl β-D-cellotrioside represents quite a c h a l l e n g e , b u t a t l e a s t , s i n g l e c r y s t a l s good enough f o r X-ray a n a l y s i s can be grown i n a r e p r o d u c i b l e manner, which i s n o t t h e case y e t f o r c e l l o t e t r a o s e . The high resolution solid state spectrum o f m e t h y l β-D-cellotrioside e x h i b i t s a d i s t i n c t s p l i t t i n g of t h e CI atoms r e s o n a n c e s and t h i s may r e f l e c t t h e o c c u r e n c e o f d i s t i n c t c o n f o r m a t i o n s a t t h e g l y c o s i d i c l i n k a g e s . T h i s v e r y same spectrum a l s o shows how the m e t h y l groups o f m e t h y l g l y c o s i d e s may be used as c o n v e n i e n t p r o b e s i n e s t a b l i s h i n g gross structural f e a t u r e s . N e v e r t h e l e s s , drawing d e f i n i t e c o n c l u s i o n s about any direct correlation between an observed splitting and known s t r u c t u r a l f e a t u r e s as d e r i v e d from X-ray i n v e s t i g a t i o n s h o u l d be h a n d l e d w i t h c a u t i o n . Whereas i t appears t h a t t h e o c c u r r e n c e o f s p l i t r e s o n a n c e s r e f l e c t s c o n f o r m a t i o n a l inhomogeneity (55), the converse i s not n e c e s s a r i l y t r u e . T h i s statement is illustrated by t h e m e t h y l β-D-cellobioside c a s e , f o r which two distinct c o n f o r m a t i o n s o f the C6 c a r b o n atoms a r e o b s e r v e d i n the c r y s t a l s t r u c t u r e whereas o n l y one resonance a t 63.1 ppm can be a s s i g n e d to the c o r r e s p o n d i n g atoms i n t h e . C CP /MAS NMR spectrum. Our c a l c u l a t i o n s , a l t h o u g h s t i l l p r i m i t i v e , appear t o e x p l a i n such a behavior. D e s p i t e the l a c k o f h i g h a c c u r a c y , some c o n s i s t e n t f e a t u r e s can be drawn from the s t u d y o f c e l l u l o s e o l i g o m e r s t h r o u g h our a p p r o a c h . The r e s u l t s i n d i c a t e c l e a r l y t h a t the g l y c o s i d i c l i n k a g e s i n c e l l u l o s e o l i g o m e r s have d i f f e r e n t c o n f o r m a t i o n s . I t i s a l s o c l e a r t h a t t h e g l y c o s i d i c l i n k a g e does n o t e x i s t i n a conformation c o n s i s t e n t w i t h a " t w o - f o l d " h e l i x symmetry. These c o n c l u s i o n s which a r e e s s e n t i a l l y r e a c h e d through conformational analysis c a l c u l a t i o n s a r e s u p p o r t e d by t h e appearence o f t h e s o l i d s t a t e 1 3

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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C NMR spectra o f both cellotetraose and m e t h y l β-D-cello­ t r i o s i d e , and c e l l u l o s e I I as w e l l . D e s p i t e a l a r g e d i f f e r e n c e between t h e u n i t c e l l c o n t e n t s o f m e t h y l β-D-cellotrioside and c e l l o t e t r a o s e , t h e i r r e s p e c t i v e base p l a n e d i m e n s i o n s , i . e . normal to the d i r e c t i o n p a r a l l e l t o the molecular a x i s , a r e s t r i k i n g l y comparable ; t h i s must r e f l e c t s t r o n g p a c k i n g h a b i t s , o f n o t o n l y l o n g c e l l u l o s e c h a i n s b u t c e l l u l o s e - l i k e c h a i n s as c e l l o t e t r a o s e or m e t h y l β-D-cellotrioside (at the exclusion o f metastable arrangements resulting from biosynthetic pathways and organizations). There has been c o n s i d e r a b l e d i s c u s s i o n r e g a r d i n g t h e c r y s t a l s t r u c t u r e o f c e l l u l o s e I I and two models have been p r o p o s e d ; t h e y b o t h a r e b a s e d upon a m o n o c l i n i c P2^ space g r o u p . I n one, t h e u n i t c e l l i s p o s t u l a t e d t o c o n t a i n two i n d e p e n d e n t c h a i n s (22-24) (model A ) , w h i l e i n t h e o t h e r , t h e c h a i n c o n f o r m a t i o n i s thought to be such t h a t a c e l l o b i o s y l u n i t does i n d e e d c o n s t i t u t e t h e r e p e a t i n g u n i t (17) (model B) In model A, t h e t w o - f o l d h e l i c a l conformation i s assumed atoms) a r e c o n f o r m a t i o n a l l a x i s . S i n c e t h e c h a i n s a r e l o c a t e d on t h e c r y s t a l l o g r a p h i c axes o f symmetry t h e y may have d i f f e r e n t c o n f o r m a t i o n s . Model A h a s an antiparallel arrangement, i n agreement with our finding on c e l l o t e t r a o s e . T h e r e f o r e , e a c h atom i n a c h a i n would have a d i f f e r e n t p a c k i n g environment than t h e c o r r e s p o n d i n g atom i n t h e n e i g h b o r i n g c h a i n s . Can such f e a t u r e s , i n v o l v i n g d i f f e r e n c e s i n d i s t a n c e s g r e a t e r than 3 A, e x p l a i n t h e s i g n i f i c a n t s p l i t t i n g o f the C I r e s o n a n c e s on t h e NMR s p e c t r a ? Why i s s i m i l a r s p l i t t i n g not observed f o r the other carbon atoms ? In model B, a c e l l o b i o s y l u n i t c o n s t i t u t e s t h e r e p e a t i n g u n i t (11), i n agreement w i t h t h e r e s u l t s o f c o n f o r m a t i o n a l a n a l y s i s p e r f o r m e d on i s o l a t e d molecules. Therefore, two p a i r s o f g l y c o s i d i c torsion angles a l t e r n a t e a l o n g t h e polymer c h a i n , t h e r e b y p r o v i d i n g a r a t i o n a l e x p l a n a t i o n f o r t h e o b s e r v e d s p l i t t i n g o f t h e C I r e s o n a n c e s . No l o n g e r c a n t h e m a c r o m o l e c u l a r c h a i n have a t w o - f o l d helical symmetry and hence t h e c o i n c i d e n c e between polymer a x i s and c r y s t a l l o g r a p h i c t w o - f o l d a x i s i s f o r b i d d e n . To keep m o n o c l i n i c symmetry, t h e c e l l u l o s e c h a i n s would have t o be l o c a t e d between the c r y s t a l l o g r a p h i c axes; t h e c h a i n s a r e no l o n g e r i n d e p e n d e n t and have t o be i n a p a r a l l e l r e g i s t e r which i s n o t c o n s i s t e n t w i t h the arrangement found f o r cellotetraose and p o s t u l a t e d f o r m e t h y l - β-D c e l l o t r i o s i d e . The o n l y way would be t o c o n s i d e r t h e triclinic space group, w i t h two i n d e p e n d e n t , antiparallel c h a i n s t r a v e r s i n g t h e u n i t c e l l . T h i s would be c o n s i s t e n t w i t h a l l the f e a t u r e s d i s p l a y e d by t h e c e l l u l o s e o l i g o m e r s . T h i s would a l s o e x p l a i n why t h e (0 0 1) r e f l e c t i o n i s n o t a b s e n t from t h e X-ray f i b e r d i f f r a c t i o n p a t t e r n o f c e l l u l o s e I I as i t s h o u l d r e a l l y be i n a m o n o c l i n i c P2^ space g r o u p . Addendum : Subsequent t o t h e p r e s e n t a t i o n o f t h i s work, a r e p o r t by D.L. V a n d e r H a r t ( J . Chem. Phys. (1986) 84, 1196) e s t a b l i s h e d a f i e l d dependency f o r C NMR c h e m i c a l s h i f t s o f p o l y e t h y l e n e i n t h e s o l i d s t a t e . A t t h e 50.3 MHz f r e q u e n c y u s e d i n t h e p r e s e n t work, the c o r r e c t v a l u e i s 32.9 ppm n o t 33.6 and hence a l l c h e m i c a l s h i f t s s h o u l d be d e c r e a s e d by 0.7 ppm from t h e v a l u e s r e p o r t e d above. 1 3

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Acknowledgments The a u t h o r s a r e much i n d e b t e d t o D r . H. Chanzy o f t h i s I n s t i t u t e f o r i n i t i a t i n g some p a r t s o f t h e work r e p o r t e d h e r e and h i s u n t i r i n g s u p p o r t . A p p r e c i a t i o n i s e x t e n d e d t o D r . E. O h l e y e r f o r h i s e x p e r t i s e i n the p r e p a r a t i o n o f c o n s i d e r a b l e amounts o f the cellodextrins, and t o D r . E. Roche who made a v a i l a b l e some u n p u b l i s h e d r e s u l t s on e l e c t r o n d i f f r a c t i o n o f c e l l o t e t r a o s e . D r . B. P o p p l e t o n , Commonwealth S c i e n t i f i c and I n d u s t r i a l R e s e a r c h O r g a n i z a t i o n , Melbourne, A u s t r a l i a , k i n d l y s u p p l i e d a l i s t o f measured i n t e n s i t i e s on c e l l o t e t r a o s e . The h e l p o f D r . A. Hewatt, Institut Laue L a n g e v i n , G r e n o b l e , F r a n c e , was invaluable f o r r e c o r d i n g t h e n e u t r o n powder d i f f r a c t i o n p a t t e r n o f c e l l o t e t r a o s e . Dr. R. H. Marchessault, Xerox R e s e a r c h C e n t r e , Mississauga, Canada, gave us a c c e s s t o t h e high resolution s o l i d state NMR spectrometer. G r a n t s f o r s u p p o r t i n g t h e s a b b a t i c a l s t a y s o f two o f us ( I T and W. T. W.) were s u p p l i e Scientifique.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Wellard, H. J., J. Polym. Sci. (1954) 13, 471. Marrinan, H. J. and Mann, J. J., J. Appl. Chem. (1954) 4, 204. Pérez, S. and Brisse, F., Biopolymers (1978) 17, 2083. Chu, S. S. C. and Jeffrey, G. Α., Acta Crystallogr. (1968) B24, 830. Ham, J. T. and Williams, D. G., Acta Crystallogr. (1970) B26, 1373. Poppleton, B. J. and Mathieson, A. McL., Nature (1968) 219, 1946. Williams, D. G., J. Polym. Sci., A-2 (1972) 8, 637. Poppleton, B. J. and Gatehouse, J. Polym. Sci., A-2 (1972) 10, 375. Schaefer, J., Stejskal, Ε. O. and Buchdahl, R., Macromolecules (1977) 10, 384. Fyfe, C. Α., Dudley, R. L., Stephenson, P. J., Deslandes, Y., Hamer, G. Κ. and Marchessault, R. H., J. Am. Chem. Soc. (1983) 105, 2469. Atalla, R. H., Gast, J. C., Sindorf, D. W., Bartuska, F. J. and Maciel, G. E., J. Am. Chem. Soc. (1980) 102, 3249. Earl, W. L. and VanderHart, D. L., J. Am. Chem. Soc. (1980) 102, 3251. Earl, W. L. and VanderHart, D. L., Macromolecules (1981) 14, 570. Atalla, R. H. and VanderHart, D. L., Science (1984) 223, 283. Horii, F., Hirai, A. and Kitamaru, R., Adv. Chem. Ser. (1984) 260, 27. Cael, J. J., Kwoh,, D. L. W., Bhattacharjee, S. S. and Patt, S. L., Macromolecules (1985) 18, 821. Teeäär, R. and Lippmaa, E., Polym. Bull. (1984) 12, 315. Horii, F., Hirai, A. and Kitamaru, R., Polym. Bull. (1983) 10, 357.

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THE STRUCTURES OF CELLULOSE 19. Fyfe, C. Α., Stephenson, P. J., Veregin, R. P., Hamer, G. K. and Marchessault, R. H., J. Carbohydr. Chem. (1984) 3, 663. 20. IUPAC-IUB Commission on Biochemical Nomenclature, Arch. Biochem. Biophys. (1971) 145, 405. 21. Marchessault, R. H. and Pérez, S., Biopolymers (1979) 18, 2369. 22. Kolpak, K. J. and Blackwell, J., Macromolecules (1976) 9, 273. 23. Kolpak, K. J., Weih, M. and Blackwell, J., Polymer (1978) 19, 123. 24. Stipanovic, A. J. and Sarko, Α., Macromolecules (1976) 9, 851. 25. Buleon, A. and Chanzy, H., J. Polym. Sci., Polym. Phys. Ed. (1978) 16, 833. 26. Wolfrom, M. L., Dacons, J. C. and Fields, D. L., TAPPI (1956) 39, 803. 27. Takeo, Κ., Okushio Κ. Fukuyama K and Kuge T. Carbohydr Res. (1983) 121, 28. Frye, J. S. and Maciel 615.29. Earl, W. L. and VanderHart, D. L., J. Magn. Reson. (1982) 48, 35. 30. Diner, S., Malrieu, J.P. & Claverie, P., Theoret. Chim. Acta, (Berl.) (1969), 13, 1. 31. Malrieu, J.P., Claverie, P. & Diner, S., Theoret. Chim. Acta (Berl.) (1969), 13, 18. 32. Powell, M.J.D., Computer J. (1964), 7, 155. 33. Zangwill, V.I., Computer J. (1965), 8, 293. 34. Allinger, N. L., J. Am. Chem. Soc. (1977) 99, 8127. 35. Jeffrey, G. A. and Taylor, R., J. Comput. Chem. (1980) 1, 99. 36. Taylor, R., J. Molec. Struct. (1981), 71, 311. 37. Tvaroska, I. and Pérez, S., Carbohydr. Res. (1986) 149, 389. 38. Scott, R. A. and Scheraga, Η. Α., J. Chem. Phys. (1965) 42, 2209. 39. Tvaroska, I., Carbohydr. Res. (1984) 125, 155. 40. Gagnaire, D., Pérez, S. and Tran, V., Carbohydr. Res. (1980) 78, 89. 41. International Tables for X-ray Crystallography (1962) Vol. III, Birmingham, U.K., Kynoch Press. 42. Main, P., Lessinger, L., Woolfson, M. M., Germain, G. and Declercq, J. P. (1977) MULTAN 77. A System of Computer Programs for the Automatic Solution of Crystal Structures from X-ray Diffraction Data. University of York, U.K. and Louvain, Belgium. 43. Arnott, S. and Scott, W. E., J. Chem. Soc., Perkin Trans. II (1972) 324. 44. Smith, P. J. C. and Arnott, S., Acta Crystallogr. (1978) A34, 3. 45. Dheu, M. L. and Pérez, S. (1980) Programmes Interactifs de Tracés de Molécules et de Structures, C.E.R.M.A.V., Grenoble, France. Pérez, S. and Scaringe, R., J. Appl. Cryst. (1986) 19, 65. 46. Mallet, J. L. (1976) Programmes de Cartographie Automatique. Présentation de la Bibliothèque CARTOLAB, Sciences de la Terre, Série Informatique Géologique, N°7, Nancy, France. 47. Tvaroska, I. and Kozar, T., Chemicke Zvesti (1981) 35, 425.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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48. Fuchs, B., Schleifer, L. and Tartakovski, E., Nouv. J. Chim. (1984) 8, 275. 49. Ebaheem, Κ. A. K. and Webb, G. Α., Progr. Nucl. Magn. Reson. Spectrosc. (1977) 11, 149. 50. Cyr, Ν., Perlin, A. S. and Whitehead, Μ. Α., Can. J. Chem. (1972) 50, 814. 51. Korpi-Tommola, S. L. and Lindberg, J. J., Comm. Phys. Math. (1973) 43, 167. 52. Mackie, W., Sheldrick, B., Akrigg, D. and Pérez, S., Int. J. Biol. Macromol. (1986) 8, 43. 53. Melberg, S. and Rasmussen, Κ., Carbohydr. Res. (1979) 71, 25. 54. Pizzi, A. and Eaton, N., J. Macromol. Sci.- Chem. (1984) A21, 1443. 55. Taylor, M. G., Marchessault, R. H., Pérez, S., Stephenson, P. J. and Fyfe, C. Α., Can. J. Chem. (1985) 63, 270. RECEIVED March 17,1987

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 4

Application of the Rietveld Crystal Structure Refinement Method to Cellotetraose 1

2

3,4

3

A. Sakthivel , A. D. French , B. Eckhardt , and R. A. Young 1

School of Textile Engineering, Georgia Institute of Technology, Atlanta, GA 30332 Southern Regional Research Center, U.S. Department of Agriculture, P.O. Box 19687, New Orleans, LA 70179 School of Physics, Georgia Institute of Technology, Atlanta, GA 30332

2

3

The Rietveld method of structure refinement from x-ray powder d i f f r a c t i o cellotetraose, whic similar to that of cellulose II. Unit cell dimensions were consistent with earlier work. The space group was shown to be P1 rather than P2 . Models, r i g i d except for the positions of the 0(6) atoms, were used to test the effects on the calculated diffraction patterns of parallel and antiparallel packing modes, 0(6) positions, and the presence of hydrogen atoms. The hydrogen atoms had a negligible effect on the c a l c u l a t e d x-ray d i f f r a c t i o n pattern. The intensities of specific individual Bragg reflections were s u f f i c i e n t l y affected by the 0(6) positions so that they may help to indicate those positions, even though the net effect on the overall pattern-fitting was s m a l l . Results from refinements of an antiparallel model against intensities calculated from a parallel model indicated that differentiation may be d i f f i c u l t on the basis of x-ray powder diffraction patterns. Preliminary results based on simple models with tetramer symmetry close to 2 suggested that the two tetramers in the unit cell are slightly inclined to the c axis. 1

1

I t h a s l o n g b e e n known t h a t c e l l o t e t r a o s e , t h e β 1 , ^ - l i n k e d t e t r a m e r o f D - g l u c o s e , y i e l d s a powder d i f f r a c t i o n pattern e x h i b i t i n g more c r y s t a l l i n i t y b u t o t h e r w i s e s i m i l a r t o t h a t o f c e l l u l o s e I I , the a l l o m o r p h r e s u l t i n g from treatment of c e l l u l o s e I i n s t r o n g l y a l k a l i n e s o l u t i o n or from r e g e n e r a t i o n from s o l u t i o n . I n the quest t o o b t a i n s t r u c t u r a l i n f o r m a t i o n a p p l i c a b l e t o c e l l u l o s e I I , p r e v i o u s workers, using t i n y s i n g l e c r y s t a l s , e.g., 0.2 χ 0.1 χ 0.01 mm " u s u a l l y o f p o o r q u a l i t y b u t 3

4

Current address: Fachbereich Physik, Universität Bremen, Bibliotekstrasse, Postfach 330440, 2800 Bremen 33, Federal Republic of Germany 0097-6156/87/0340-0068$06.00/0 © 1987 American Chemical Society

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Rietveid Crystal Structure Refinement Method

SAKTHIVEL ET AL.

69

w i t h w e l l developed (001) f a c e s " (1_), s u g g e s t e d (2) t h a t the space g r o u p o f c e l l o t e t r a o s e was P1 and d e t e r m i n e d t h e u n i t c e l l o f cellotetraose accordingly. Further single-crystal diffraction work has, however, been t h w a r t e d by x - r a d i a t i o n damage o c c a s i o n e d by t h e l o n g e x p o s u r e s r e q u i r e d b e c a u s e o f t h e s m a l l s i z e o f t h e crystals. R i e t v e i d r e f i n e m e n t (3>4) p e r m i t s s t r u c t u r a l study with powder d i f f r a c t i o n d a t a . The e n t i r e d i f f r a c t i o n p a t t e r n i s c a l c u l a t e d from a model c o n s i s t i n g of the u n i t c e l l p a r a m e t e r s , c r y s t a l l i t e s i z e ( l i n e w i d t h ) , p r o p o s e d a t o m i c p o s i t i o n s and thermal parameters. The v a l i d i t y o f t h e p r o p o s e d m o d e l i s measured by c o m p a r i s o n of the o b s e r v e d and c a l c u l a t e d i n t e n s i t y a t each s t e p - s c a n i n c r e m e n t , i n c l u d i n g background c o r r e c t i o n , and the model p a r a m e t e r s a r e s y s t e m a t i c a l l y r e f i n e d i n o r d e r to p r o v i d e the b e s t agreement i n a l e a s t squares sense. I n I m m i r z i s v e r s i o n (5), the m o l e c u l e i s d e s c r i b e d i n terms of g e n e r a l i z e d c o o r d i n a t e s so t h a t the model can be r e f i n e d when the d a t a a r e l i m i t e d an t o the a n a l y s e s of f i b e are kept r i g i d e x c e p t f o r r o t a t i n g s i d e groups. However, powder d i f f r a c t i o n d a t a a r e l e s s l i k e l y t o be a f f e c t e d by e r r o r s i n c o l l e c t i o n , c o r r e c t i o n and r e d u c t i o n t h a n f i b e r data. E r r o r s from p r e f e r r e d o r i e n t a t i o n produced i n sample p r e p a r a t i o n a r e p o s s i b l e , but can be t e s t e d f o r . A l t h o u g h the R i e t v e i d method has been used s u c c e s s f u l l y w i t h hundreds of m a t e r i a l s , i n c l u d i n g some p o l y m e r s s u c h as p o l y p r o p y l e n e (6), i t i s b e l i e v e d t h a t t h i s i s the f i r s t a t t e m p t to p e r f o r m a thorough s t r u c t u r a l s t u d y of an o l i g o s a c c h a r i d e w i t h powder x-ray d i f f r a c t i o n data. In t h i s paper, we d e s c r i b e some of t h e d e t a i l s o f t h e R i e t v e i d m e t h o d , as m o d i f i e d by I m m i r z i f o r polymers and f u r t h e r m o d i f i e d l o c a l l y , and d i s c u s s our assessments of v a r i o u s a s s u m p t i o n s used i n s u c h a s t u d y . A preliminary s t r u c t u r a l r e s u l t i s a l s o presented. ?

EXPERIMENTAL PROCEDURE The s a m p l e s o f c e l l o t e t r a o s e s t u d i e d h e r e i n w e r e p r o v i d e d by Dr. F r e d P a r r i s h , S o u t h e r n R e g i o n a l R e s e a r c h C e n t e r , Dr. R o s s Brown, U n i v e r s i t y o f F l o r i d a , and Dr. J o h n V e r c e l l o t t i , V - L a b s , C o v i n g t o n , LA. T h e y w e r e made by h y d r o l y s i s o f c e l l u l o s e i n h y d r o c h l o r i c a c i d and s e p a r a t e d from the o t h e r r e s u l t i n g o l i g o m e r s through column chromatography. The s t e p - s c a n n e d , w i d e - a n g l e d i f f r a c t i o n d a t a were o b t a i n e d ( r e f l e c t i o n mode) w i t h a s t a n d a r d Θ-2Θ x - r a y powder d i f f r a c t o m e t e r u s i n g a d i f f r a c t e d beam monochromator and CuKa r a d i a t i o n from a s t a n d a r d s e a l e d - o f f x - r a y tube. The s c a n n i n g range was from 7° t o 43.40° 2Θ; no c l e a r l y r e c o g n i z a b l e p e a k s w e r e o b s e r v e d b e y o n d 43.40°. The s t e p w i d t h was 0.04° and t h e c o u n t i n g t i m e was 250 seconds per step. S i n c e l i t t l e improvement i n the d a t a r e s u l t e d from m a i n t a i n i n g the specimen near l i q u i d n i t r o g e n t e m p e r a t u r e s , a l l r e s u l t s r e p o r t e d h e r e w e r e o b t a i n e d f r o m room t e m p e r a t u r e data. The a b s o r p t i o n f a c t o r exp(-ut) (equal to ïtransmitted^incident ? n o r m a l l y i n c i d e n t beam) was measured as 76.4$. The w i d e - a n g l e d i f f r a c t i o n p a t t e r n i s shown i n F i g u r e 1. o r

a

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70

THE STRUCTURES OF CELLULOSE

S m a l l - a n g l e d a t a were c o l l e c t e d w i t h a s m a l l a n g l e x - r a y d i f f r a c t o m e t e r equipped w i t h a p o s i t i o n s e n s i t i v e d e t e c t o r p l a c e d 25.0 cm away from the sample ( t r a n s m i s s i o n mode). The r e s u l t i s shown i n F i g u r e 2. RIETVELD REFINEMENT METHOD Given the a t o m i c p o s i t i o n s i step i s t

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where t

n

^ = background at the i step Η ' = M i l l e r i n d i c e s h , k , l f o r a Bragg r e f l e c t i o n G ( e i ~ 6 ) = Bragg functio f o r more d e t a i l s ) Q± = s c a t t e r i n g a n g l e at the i step F = structure factor P = m u l t i p l i c i t y of Η (LP)j = L o r e n t z and p o l a r i z a t i o n f a c t o r at s t e p i T = exp(-P(a ) ), preferred orientation function Ρ = p r e f e r r e d o r i e n t a t i o n parameter ot = a c u t e a n g l e between the p r e f e r r e d o r i e n t a t i o n d i r e c t i o n and the r e c i p r o c a l l a t t i c e v e c t o r f o r Η H

t

n

H

H

2

H

H

H

The Y

background = b

i f b

i n t e n s i t y (y^

) i s calculated

+ (b -b )(29 -2e )/

1

2

1

1

i

as

fe

(2θ -2θ ) + b 1

2

3

*

G(2e -28 ) s

1

where 20! and 2 θ are the l i m i t s of the s c a n n i n g range, G ( 2 6 - 2 0 ) i s a Pearson VII f u n c t i o n w i t h 3 r e f i n a b l e parameters, and b , b , b , 2 0 are a l s o parameters t h a t are r e f i n e d . The e q u a t i o n g i v e n above f o r y i s e m p i r i c a l and a g r a p h i c a l r e p r e s e n t a t i o n of i t can be found i n r e f e r e n c e 6. The s t r u c t u r e f a c t o r F , i s c a l c u l a t e d as f o l l o w s : 2

3

x

2

i

3

3

i b

H

F

H

j Mj fj Xj,yj,Zj B^ d λ H

= I

Mjfjexp{-2πi(hXj+kyj + l Z j ) } e x p { - B j ( d

9 t H

2

/2) }

= r a n g e s from 1 to no. of atoms i n the u n i t = s i t e occupancy (of the j atom s i t e ) = atomic s c a t t e r i n g f a c t o r f o r the j atom = f r a c t i o n a l c o o r d i n a t e s of the atom j = temperature f a c t o r =2 sin0/X = wavelength o f the r a d i a t i o n used

The d i f f e r e n c e between the i s measured by the r e s i d u a l ,

t

t

observed

cell

n

and

n

calculated

patterns

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4.

SAKTHIVEL ET AL.

β

1

9

ΐΌ

11

12

A A Α

71

Rietveid Crystal Structure Refinement Method

ΐ'β

Λ A ΐ!» À>

2!

22

1

23

2*4

Α

2(5 2^ 2^ 29 &

31 32 33 34

A A

1

3?

Α

39

Α

41 42

SCATTERING ANGLE

F i g u r e 1. Wide a n g l e x - r a y powder d i f f r a c t i o n p a t t e r n o f c e l l o t e t r a o s e a t room t e m p e r a t u r e . CuK radiation.

1.00 0.96

0.24 0.20 016 0.12 0.08 0.04 0.00

1

1

2

1

3

1

4

5*

6*

7*

β'

9*

ΐΌ l'l

12 13 1*4

A

SCATTERING ANGLE

F i g u r e 2. Small-angle p o w d e r - d i f f r a c t i o n p a t t e r n of c e l l o ­ t e t r a o s e a t room temperature ( t r a n s m i s s i o n mode). CuK radiation and a p o s i t i o n s e n s i t i v e d e t e c t o r a t 25.0 cm from the sample were used.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Α

72

THE STRUCTURES OF CELLULOSE

χ

2

=

w

Σ

i(yi,obs-yi,calc

) 2

( 1 )

w h e r e t h e sum e x t e n d s o v e r a l l s c a t t e r i n g a n g l e s 2Θ a t w h i c h a measurement of the d i f f r a c t e d i n t e n s i t y y^ , was made. Wj i s a weight f a c t o r , u s u a l l y chosen t o be 1/y ' . A minimum i n χ is sought. A n e c e s s a r y c o n d i t i o n f o r t e r m i n a t i o n of r e f i n e m e n t i s t h a t the g r a d i e n t of χ w i t h r e s p e c t t o each o f the M r e f i n e d parameters, x , has vanished. The r e s u l t i n g equations are n o n l i n e a r i n t h e x s and c a n n o t be s o l v e d a n a l y t i c a l l y f o r t h e parameter s h i f t s dx which w i l l minimize χ . Therefore, the c a l c u l a t e d i n t e n s i t i e s a r e e x p a n d e d i n t h e x * s and o n l y l i n e a r t e r m s a r e k e p t . L e t t i n g b r a c e s d e n o t e m a t r i c e s and p a r e n t h e s e s denote v e c t o r s , one ends up wih an e q u a t i o n of the form o b s

2

i

0

bs

2

k

f

k

2

k

k

{A}

(dx)

= b

(dx)

= {A}

(2)

from which,

The

various

A

quantities w

= I

j k

i

are: 5 y

i,calc

6 v

i,calc

6xj

w

i

6 v

i,calc

6 X

dx

k

= x

o

l

d

k

6x

( v

... (4)

k

v

i,obs~ i,calc

)

J

- x

n

e

w

k

(6)

T h i s c l e a r l y shows s e v e r a l i m p o r t a n t f e a t u r e s of t h e R i e t v e i d r e f i n e m e n t p r o c e d u r e . F i r s t l y , i t i n v o l v e s i n v e r s i o n o f an M*M matrix. S e c o n d l y , i t works o n l y i f the s t a r t i n g v a l u e s are a l r e a d y c l o s e t o a minimum, because of the l i n e a r a p p r o x i m a t i o n i n the d e r i v a t i o n of e q u a t i o n (2). T h i r d l y , w h a t e v e r minimum i s o b t a i n e d , i t i s not n e c e s s a r i l y the g l o b a l minimum one i s l o o k i n g f o r b u t may be a l o c a l m i n i m u m , s i n c e t h e m e t h o d i s a l o c a l one. Finally, because of f i n i t e w i d t h of every r e f l e c t i o n , the c a l c u l a t e d i n t e n s i t y at each s t e p c o n t a i n s c o n t r i b u t i o n s from several neighboring r e f l e c t i o n s . I n our c a s e up t o 45 p o s s i b l e r e f l e c t i o n s can c o n t r i b u t e t o the i n t e n s i t y c a l c u l a t e d at a s i n g l e p o i n t i n the d i f f r a c t i o n p a t t e r n . The program used f o r R i e t v e i d r e f i n e m e n t i n t h i s work i s an e x t e n s i v e l y m o d i f i e d v e r s i o n o f REFIN (FORTRAN V), w r i t t e n by A. I m m i r z i (5). I n o r d e r to a s s e s s the agreement between the c a l c u l a t e d and o b s e r v e d p a t t e r n , s e v e r a l n u m b e r s can be c a l c u l a t e d . Most commonly used a r e R and R. w p

p

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4.

SAKTHIVEL ET AL.

Rietveid Crystal Structure Refinement Method

73

1/2

(7)

wp obs

and

^ Rp

=

y

y

I i,obs~ y

i,calcl (8)

i,obs

On a s t r i c t l y s t a t i s t i c a l b a s i s R i s the p r e f e r r e d i n d i c a t o r b e c a u s e i t s numerator i s the f u n c t i o n t h a t i s b e i n g m i n i m i z e d ( e q u a t i o n 1). Note t h a t t h e s e R v a l u e s are n o r m a l l y l a r g e r than t h o s e r e p o r t e d f o r f i b e r and s i n g l e - c r y s t a l s t u d i e s because they a r e b a s e d on t h e i n t e n s i t i e s a t a l l s t e p s i n t h e p a t t e r n and n o t j u s t on i n d i v i d u a l l y o b s e r v e Another i n d i c a t o r c o n v e n t i o n a l R v a l u e w i t h w h i c h some r e a d e r s w i l l be f a m i l i a r from t h e l i t e r a t u r e on s i n g l e c r y s t a l s t r u c t u r e r e f i n e m e n t : w p

f

Σ R

B

( I

i,"obs"

"

I

i,calc

)

-

(9) ί

I

i,"obs"

In i t s f o r m u l a t i o n , R i s comparable to the c o n v e n t i o n a l R v a l u e based on i n t e n s i t i e s which, f o r a normal d i s t r i b u t i o n o f e r r o r s , i s l a r g e r t h a n t h e most quoted R v a l u e based on s t r u c t u r e f a c t o r s by a f a c t o r of /2. In e q u a t i o n 9, I i i b t i i s w r i t t e n w i t h q u o t a t i o n marks because i t i s not a c t u a l l y o b s e r v e d d i r e c t l y . Rather, t h e t o t a l i n t e n s i t y observed f o r a s e t of o v e r l a p p i n g r e f l e c t i o n s i s a p p o r t i o n e d among the r e f l e c t i o n s i n the r a t i o s of the c a l c u l a t e d i n t e n s i t i e s (3fJ0. R i s , t h e r e f o r e , b i a s e d i n f a v o r of the model b e i n g used. However, i t i s u s e f u l because i t i s r e l a t i v e l y i n s e n s i t i v e to f e a t u r e s , such as p r o f i l e shape e r r o r s , which produce an i n f l a t i o n of Rp and R without c r y s t a l s t r u c t u r a l s i g n i f i c a n c e . B

0

S

B

w p

COMPUTER MODEL BUILDING The m o l e c u l a r f o r m u l a f o r c e l l o t e t r a o s e , the Β 1 , 4 - l i n k e d t e t r a m e r of D - g l u c o s e , i s ^2^4^42^21 · S* the c o n t r i b u t i o n s of the h y d r o g e n a t o m s t o t h e x - r a y p a t t e r n may be s a f e l y d i s r e g a r d e d ( F i g u r e 3), s t r u c t u r a l d e t e r m i n a t i o n c o n s i s t s of d e f i n i n g the x, y, and ζ c o o r d i n a t e s ( p l u s any t h e r m a l p a r a m e t e r s ) f o r each o f the r e m a i n i n g 45 atoms i n each t e t r a m e r . Even though 910 data p o i n t s were taken, no more t h a n a b o u t 30 i n t e n s i t y " p e a k s " c a n be observed, most of w h i c h are c o m p o s i t e s of s e v e r a l B r a g g reflections. T h e r e f o r e , o n l y a v e r y l i m i t e d number o f p a r a m e t e r s i n t h e model can be m e a n i n g f u l l y r e f i n e d . n c e

The c o o r d i n a t e s of the atoms i n the monomer were assumed t o be known from s i n g l e c r y s t a l s t u d i e s of g l u c o s e (7). Each monomer was t r e a t e d as a r i g i d body w i t h 3 d e g r e e s , e a c h , o f r o t a t i o n a l and t r a n s l a t i o n a l freedom.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

74

THE STRUCTURES OF CELLULOSE

The o r i g i n o f the c o o r d i n a t e system ( f o r example, c o i n c i d i n g w i t h t h e c e n t e r o f t h e f i r s t monomer) c a n be f r e e l y c h o s e n . The f i r s t monomer then has o n l y 3 r o t a t i o n a l degrees of freedom. The o t h e r t h r e e monomers a r e d e s c r i b e d by 3 t r a n s l a t i o n a l and 3 a n g u l a r p a r a m e t e r s , each. One parameter f o r each monomer d e s c r i b e s the o r i e n t a t i o n o f the oxygen of the p r i m a r y h y d r o x y l group (0(6)), w i t h r e s p e c t t o t h e C(4) - C(5) bond. W i t h t h i s s e l e c t i o n o f p a r a m e t e r s , the t e t r a m e r i s d e s c r i b e d i n terms of 25 parameters. S p e c i f i c a t i o n of i t s o r i e n t a t i o n w i t h r e s p e c t t o the u n i t c e l l r e q u i r e s another t h r e e a n g l e s . ( I n a s t r i c t l y m a t h e m a t i c a l sense, t h e s e t h r e e p a r a m e t e r s a r e redundant. Hence, t h e y a r e never r e f i n e d s i m u l t a n e o u s l y w i t h the o r i e n t a t i o n p a r a m e t e r s of the monomers). Thus, t h e t o t a l number of p a r a m e t e r s f o r t h e f i r s t t e t r a m e r i s 28. I t i s t h e n e a s y t o d e s c r i b e many t e t r a m e r s per u n i t c e l l ; a l l o n e h a s t o do i s t o t a k e 28 p a r a m e t e r s f o r the f i r s t t e t r a m e r and 28 p a r a m e t e r s p l u s t h r e e f o r the a d d i t i o n a l t r a n s l a t i o n a additional tetramer parameters. I n most of our r e f i n e m e n t s , t h e number of p a r a m e t e r s a c t u a l l y r e f i n e d i n any one c y c l e was l i m i t e d t o about 20. O n l y β - c e l l o t e t r a o s e has been m o d e l e d i n t h i s s t u d y . The c e l l o t e t r a o s e sample c o u l d a l s o be a m i x t u r e o f a - and Bc e l l o t e t r a o s e s o r e v e n a l l a. In the case of m i x t u r e s , the a n o m e r s may o c c u r i n t h e same c r y s t a l s as i n t h e c a s e o f a-B m a l t o s e (8), or the powder sample may c o n t a i n two t y p e s of s i m i l a r c r y s t a l s t h a t d i f f e r a t t h e a n o m e r i c c a r b o n . T h i s i s among t h e t o p i c s t h a t w i l l be e x a m i n e d d u r i n g t h e s e c o n d p h a s e o f t h i s study. RESULTS AND

DISCUSSION

Space Group and U n i t c e l l p a r a m e t e r s . A c c o r d i n g t o P o p p l e t o n and M a t h i e s o n (]_), the space group f o r c e l l o t e t r a o s e i s e i t h e r P1 and The f a i r l y s t r o n g 001 r e f l e c t i o n o b s e r v e d a t 3.90° ( F i g u r e 2) e l i m i n a t e s t h e P 2 space group. S t a r t i n g from those i n the l i t e r a t u r e (2), the l a t t i c e parameters r e f i n e d to 1

a = 8.98 α

A

= 94.31°

b = 8.01

A,

Β = 89.27°

c = 22.34A Ύ =

116.45°

These p a r a m e t e r s agree w e l l w i t h t h e r e p o r t e d l a t t i c e p a r a m e t e r s (2), e x c e p t f o r c, w h i c h i s s m a l l e r by 0.25A i n o u r s t u d y . The d e n s i t y c a l c u l a t e d f r o m o u r p a r a m e t e r s i s 1.55 g / c m , t h a t f r o m (2) i s 1.52 g / c m and t h e m e a s u r e d d e n s i t y i s r e p o r t e d (1_) t o be 1.49 g / c m . The l a r g e , a s y m m e t r i c (P1) u n i t c e l l p e r m i t s 338 p o s s i b l e r e f l e c t i o n s i n the a n g u l a r range s t u d i e d . Most peaks are composed of many u n r e s o l v a b l e r e f l e c t i o n s . For i n s t a n c e , the major peaks a t 12.3, 19.9, 22.0, and 4 0 . 8 ° ( t o w h i c h 1T0, 1 10, 200, and 310, r e s p e c t i v e l y , a r e t h e major c o n t r i b u t o r s ) c o n t a i n 6, 9, 10 and 40 reflections, respectively. I n the R i e t v e i d method, the i n t e n s i t y at any p o i n t i n the c a l c u l a t e d d i f f r a c t i o n p a t t e r n i s the sum of c o n t r i b u t i o n s from n e i g h b o r i n g r e f l e c t i o n s . In the d i f f r a c t i o n 3

3

3

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4.

SAKTHIVEL ET AL.

Rietveid Crystal Structure Refinement Method

75

p a t t e r n s c a l c u l a t e d i n t h i s work, t h e r a n g e o f i n f l u e n c e o f any r e f l e c t i o n was l i m i t e d t o t w i c e t h e w i d t h of the p r o f i l e at h a l f the maximum h e i g h t . Number o f T e t r a m e r s

per U n i t

Cell.

A: T r i a l s w i t h one t e t r a m e r p e r c e l l . We f i r s t t r i e d s e v e r a l m o d e l s c o n t a i n i n g one t e t r a m e r p e r u n i t c e l l ( o f h a l f t h e c e l l volume f i n a l l y used), as proposed by P o p p l e t o n and M a t h i e s o n (1_). However, we c o u l d not produce t h e r o u g h l y e q u a l i n t e n s i t i e s of the two v e r y s t r o n g p e a k s a t 20° and 22° a l o n g w i t h a s t r o n g one a t 12°. We t h e r e f o r e c o n c l u d e d t h a t the number of t e t r a m e r s per u n i t c e l l i s g r e a t e r than one. B: T r i a l s w i t h two t e t r a m e r s p e r c e l l . As d e s c r i b e d by G a r d n e r and B l a c k w e l l f o r c e l l u l o s e ( 9 ) , two t e t r a m e r s c a n be p a c k e d i n the u n i t c e l l i n 3 ways: P a r a l l e l - u p antiparallel and p a r a l l e l down. I n p a r a l l e l - u p r e d u c i n g ends of both th n o n r e d u c i n g e n d s , and c o n v e r s e l y f o r t h e p a r a l l e l - d o w n m o d e l s . A n t i p a r a l l e l models c o n t a i n one "up" and one "down" t e t r a m e r . In t h e i r s t u d i e s , Sarko and M u g g l i (1_0) have used a and b axes w h i c h a r e i n t e r c h a n g e d compared t o ours, t h o s e i n the B l a c k w e l l - G a r d n e r system, and t h o s e i n r e f e r e n c e s (1,2). T h i s r e s u l t s i n the c a x i s p o i n t i n g i n a d i r e c t i o n o p p o s i t e t o t h a t i n the B l a c k w e l l - G a r d n e r system. T h i s has c a u s e d some c o n f u s i o n i n t h a t a p a r a l l e l - u p m o d e l i n one s y s t e m comes t o be c a l l e d a p a r a l l e l - d o w n m o d e l i n the o t h e r system and v i c e v e r s a . We have r e f i n e d s e v e r a l p a r a l l e l - u p and a n t i p a r a l l e l models a g a i n s t the d i f f r a c t i o n d a t a and the b e s t agreements between t h e c a l c u l a t e d and the e x p e r i m e n t a l d a t a are shown i n F i g u r e s 4 and 5. The ab and ac p r o j e c t i o n s of the c o r r e s p o n d i n g models are shown i n F i g u r e s 6 and 7. The R v a l u e s f o r the b e s t models a r e g i v e n i n T a b l e I. Since

a v a i l a b l e models

T a b l e I . R v a l u e s f o r the b e s t

Model

R

P

R

wp

R

Bragg

Parallel-up

0.1 41

0.190

0.185

Antiparallel

0.171

0.226

0.235

the R v a l u e s a r e f a i r l y low ( f o r x - r a y R i e t v e i d r e f i n e m e n t ) , e s p e c i a l l y f o r the p a r a l l e l up model, we b e l i e v e t h a t the g e n e r a l f e a t u r e s of the m o d e l s a r e c o r r e c t . B e f o r e a f i n a l m o d e l can be p r o p o s e d , h o w e v e r , f a c t o r s s u c h as t h e p a r a l l e l down m o d e l , t h e 0(6) p o s i t i o n , t h e p o s s i b i l i t y o f t h e α a n o m e r i c f o r m , and t h e c o n f o r m a t i o n a n g l e s between monomers i n t h e t e t r a m e r s must be more t h o r o u g h l y examined. The c a r t e s i a n and the f r a c t i o n a l c o o r d i n a t e s

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

76

THE STRUCTURES OF CELLULOSE

l't

Α A

1*4

Α Λ Λ A A A

2*1

A A

A A A Ai

24 SCATTERMO ANGLE

F i g u r e 3. C a l c u l a t e d x - r a y powder d i f f r a c t i o n p a t t e r n s f o r models w i t h o r w i t h o u t the hydrogen atoms. The p a t t e r n f o r the model not i n c l u d i n g hydrogen i s shown i n the same f i e l d , and t h e i r d i f f e r e n c e i s shown i n the lower f i e l d .

lO l'l

12

A

14

A

16

Λ A A

2(3 2*1 22 23 2*4 2^ 26 2*7 26^ 29 SCATTERING ANGLE

A

31 32

A

34

A A jV A

39

A

41 42

A

F i g u r e 4. C a l c u l a t e d and o b s e r v e d p a t t e r n s f o r the b e s t p a r a l l e l model. The o b s e r v e d p a t t e r n i s shown by d o t s w i t h v e r t i c a l e r r o r b a r s based on c o u n t i n g s t a t i s t i c s . The c a l c u l a t e d p a t t e r n i s shown by a c o n t i n u o u s c u r v e i n the same f i e l d and the d i f f e r e n c e between the o b s e r v e d and the c a l c u l a t e d p a t t e r n s i s shown i n the lower f i e l d . R f o r t h i s model i s 0.19. wp

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4.

Rietveid Crystal Structure Refinement Method

SAKTHIVEL ET AL.



ι A

. _ β'

9

u

1

ΐΌ l'i

12 13 14

hA Α

16

Λ

IB 1*9

Α

21 22 23 24 2^ 26 2*7 28 29 3(5 31 3*2 33 3*4 SCATTERING ANGLE

Α

1

36

Z?

38 3*9 *!θ 4*1 42 43

F i g u r e 5. C a l c u l a t e d and o b s e r v e d p a t t e r n s f o r the b e s t a n t i p a r a l l e l model. The o b s e r v e d p a t t e r n i s shown by d o t s w i t h v e r t i c a l b a r s based on c o u n t i n g s t a t i s t i c s . The c a l c u l a t e d p a t t e r n i s shown by a c o n t i n u o u s c u r v e i n the same f i e l d and the d i f f e r e n c e between the o b s e r v e d and the c a l c u l a t e d p a t t e r n s i s shown i n the lower f i e l d . R f o r t h i s model i s 0.226. wp

F i g u r e 6. Best p a r a l l e l model o b t a i n e d from R i e t v e i d r e f i n e m e n t a g a i n s t o b s e r v e d p a t t e r n (R = 0.19). a) be p r o j e c t i o n and b) ab p r o j e c t i o n . A l l the cErbon and the oxygen atoms a r e shown f o r one c o r n e r and one c e n t r a l m o l e c u l e o n l y , i n the u n i t cell. Only the oxygen atoms a r e shown f o r o t h e r m o l e c u l e s .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

77

THE STRUCTURES OF CELLULOSE

78

of the best a v a i l a b l e p a r a l l e l - u p and a n t i p a r a l l e l models a r e g i v e n i n T a b l e s I I and I I I , r e s p e c t i v e l y . I n a l l t h e models, t h e m o l e c u l a r axes o f the t e t r a m e r s are s l i g h t l y i n c l i n e d w i t h r e s p e c t to the c a x i s . 0(6)

position.

The p o s i t i o n o f t h e 0(6) i s d e s c r i b e d by t h e c o n f o r m a t i o n o f t h e C(6)-0(6) bond w i t h r e s p e c t t o t h e C ( 5 ) - 0 ( 5 ) a n d t h e C ( 4 ) - C ( 5 ) bonds. F o r e x a m p l e , i f t h e C(6)-0(6) bond i s t r a n s t o C ( 5 ) 0(5)and gauche t o C(4)-C(5), t h e n t h e p o s i t i o n o f 0(6) i s d e s c r i b e d as " t g " . The p o s i t i o n o f 0(6) atoms i n c e l l u l o s e s t r u c t u r e s i s a t t h e center of s e v e r a l c o n t r o v e r s i e s . I n an e x p l o r a t o r y way, we have e x p l o i t e d t h e c o n t i n u o u s dependence o f the f e a t u r e s of the powder d i f f r a c t i o n p a t t e r n on c r y s t a l s t r u c t u r a l d e t a i l s t o i n v e s t i g a t e t h e s e n s i t i v i t y o f t h e d i f f r a c t i o n p a t t e r n t o c h a n g e s i n 0(6) position. I n t h e wor R p values are for compariso o b s e r v e d p a t t e r n . The p o s i t i o n o f 0(6) was f i x e d as g g , t g , g t by s p e c i f y i n g t h e t o r s i o n a n g l e d e s c r i b i n g i t s p o s i t i o n as 0, -120, and 120°, r e s p e c t i v e l y . A l s o , t h e i n d i v i d u a l t e t r a m e r s were c l o s e t o h a v i n g a 2 - f o l d screw symmetry. W

A: " t g " vs. "gt" models. A powder d i f f r a c t i o n p a t t e r n f o r a model w i t h a l l 0(6) a t o m s l o c a t e d i n t h e " t g " p o s i t i o n was c a l c u l a t e d . R was 0.20. T h i s v a l u e i s s l i g h t l y h i g h e r than t h a t o f the b e s t p a r a l l e l m o d e l b e c a u s e i n t h e b e s t p a r a l l e l m o d e l t h e 0(6)'s a r e a l l p o s i t i o n e d s l i g h t l y away from " t g " . W i t h a l l o t h e r p a r a m e t e r s i n the model kept unchanged, a l l t h e 0(6) p o s i t i o n s w e r e c h a n g e d t o " g t " . R was 0.22, w h i c h i n d i c a t e s t h a t the o v e r a l l change i n t h e p a t t e r n was s m a l l . The c a l c u l a t e d p a t t e r n s f o r both t h e " t g " and the "gt" models are g i v e n i n F i g u r e 8. The p a t t e r n s a r e s i m i l a r but a n o t i c e a b l e d i f f e r e n c e o c c u r s i n t h e p e a k s a r i s i n g f r o m t h e 004, 10*4 a n d 104 r e f l e c t i o n s ( a t 15.9°, 19.1° a n d a t 1 9 . 6 ° , r e s p e c t i v e l y ) . The s e n s i t i v i t y of t h e c a l c u l a t e d p a t t e r n t o changes i n i n t e r n a l c o n f o r m a t i o n , s u c h a s t h e 0(6) p o s i t i o n , makes i t seem p r o b a b l e t h a t a s y s t e m a t i c s t u d y o f t h e v a r i o u s 0(6) p o s i t o n s and o t h e r i n t e r n a l changes w i l l l e a d t o lower R values. w p

w p

w p

n g ^ v s " t g , g t , t g , g t " m o d e l s . I n t h e n e x t m o d e l , t h e 0(6) c o n f o r m a t i o n s were t g , g t , t g , g t ( f r o m the n o n - r e d u c i n g t o t h e r e d u c i n g end). A g a i n , the o v e r a l l e f f e c t on t h e p a t t e r n was s m a l l ( R p = 0.22) b u t t h e r e w e r e p e r c e p t i b l e c h a n g e s i n n o n - z e r o " £ " r e f l e c t i o n s ( F i g u r e 9), e.g., 002 h a s v a n i s h e d . I t i ssmall but s i g n i f i c a n t changes such as t h i s t h a t may u l t i m a t e l y l e a d t o t h e c o r r e c t model. W

C. " t g " v s " t g , gg, t g , gg" m o d e l s . F i g u r e 10 shows t h a t t h e v e r y s t r o n g 110 and 200 r e f l e c t i o n s have changed c o n s i d e r a b l y from t h e " t g " model. R f o r t h i s model was 0.25. These c o m p u t a t i o n a l e x p e r i m e n t s show t h a t changes i n t h e 0(6) p o s i t i o n produce changes i n the r e f l e c t i o n s a t s m a l l 2Θ which a r e w p

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

4.

SAKTHIVEL ET AL.

Rietveid Crystal Structure Refinement Method

F i g u r e 7. Best a n t i p a r a l l e ment a g a i n s t o b s e r v e d p a t t e r b) ab p r o j e c t i o n . A l l the carbon^and the oxygen atoms a r e shown f o r one c o r n e r and one c e n t r a l m o l e c u l e o n l y , i n the u n i t cell. Only the oxygen atoms a r e shown f o r o t h e r m o l e c u l e s . i.oo 0.96 0.92 0.88 0.84 0.80 0.76 0.72 0.68 0.64 0.60 0.56 0.52 0.48 X

0.44 0.40 0.36 0.32 0.28 0.24 0.20 0.16 0.12 0.08 0.04 0.00

lO l'l

7 7 , 12 13 14 15 16 17

, ,

19 2*0 2*1 22 23

> 26 27 28 29 30 3*1 32 33 34 3i> 36 !

3V 38

39 40

SCATTERING ANGLE

F i g u r e 8. Calculated d i f f r a c t i o n patterns with d i f f e r e n t C(6)0(6) c o n f o r m a t i o n s . The p a t t e r n f o r the " t g " model i s g i v e n by d o t s . The p a t t e r n f o r the " g t " model i s g i v e n by the continuous curve. The d i f f e r e n c e i s shown i n the lower f i e l d .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE

T a b l e I I . F r a c t i o n a l and c a r t e s i a n c o o r d i n a t e s o f t h e b e s t p a r a l l e l model. S p e c i e s 1 i s oxygen and s p e c i e s 2 i s carbon. The f i r s t 45 atoms a r e i n t h e c o r n e r m o l e c u l e and the r e s t a r e in t h e c e n t e r molecule. I n each t e t r a m e r t h e f i r s t three monomers c o n s i s t o f 6 c a r b o n atoms and 5 oxygen atoms and t h e monomer a t t h e r e d u c i n g end has 6 carbons and 6oxygens No. S p e c i e s 1 2 3

4 5 6

I

9 10 11

12 1

?

14 15 16 1

I 18

19

20

21 22 ?

2

24 25 26 2

Z 28 29

30 31 32 33 34 35 36 37 38 39

40 41 42 43 44 45

Fractional

1 2 2 2 2 2 2

-.121

1 1 1 1 2 2 2 2 2 2 1 1 1

.032 .035 .139

\ 2

2 2 2 2 2 1 1 1 1 1 2 2 2 2 2 2

1

-.001

-.095 -.024 .074 .029 .039 -.060

.297 .164

.278 .020 -.087

-.170 -.024

.127 .013

-.246 -.291 -.153

.029

-.014 .093

.108

-.068 -.016

-.019 .079 -.032 -.035

-.125 -.095 .025 -.069 .002 .100

.054

.065 -.035

.057

.061 .165

.152 .039 .133

.055 -.042 .010

.006 .104

1

-.006

i

-.009 -.099

1 1

Coordinates

.083

-.266

.182

.041 -.047 -.116 .237 .258 .320 .187 .301

.043

-.064

-.147

-.001

.087

.139 -.213

-.223 -.268 -.130 -.243 .009 .116

.205 .064 -.024 -.093 .260

.281 .162

-.076 -.008 -.014 -.060 -.048 .013 .063

.028 .138 .215 .201 .169 .189 .252 .295 .271 .113 .149 .274 .386 .454 .447 .401 .413 .474 .524 .506 .348 .368 .489 .599 .676 .662 .630 .650 .714 .757 .733 .574 .611 .735 .810

Cartesian

Coordinates

-2.142 -.587 -1.841 -.282 .974

2.123 1.170 1.987 .142 -.621 -1.213 -.172

-1 .879 .273 .500 -1 ,362 -1 .009 .385 1.417

2.123

-1.757 -2.082 -1.092 -1.905 -.102 .662

.780 3.268 4.892 4.652 3.783 4.152 5.521 6.572 6.098 2.580 3.186 5.940 8.415 10.022 9.795 8.933 9.285 10.682 11.712 11.257 7.702 8.347 11.077 13.565 15.189 14.949 14.077 14.447 15.817 16.867 16.393 12.874 13.481 16.237 18.012

2.175 .664 1.916 .314 -.938 -.787 -.321 .874 .130 -1.157 -2.040 -1.993 -.440 -1.693 -.134 1.122 1 .010 .586 -.619 .017 1.302

2.270 2.322 .811 2.064 .462 -.790 -.640 -.174 1.022 .277 -1.010 -1.892 .165

1.300

.296 -.337 -.832 1.697 1.845 2.287 1.334 2.151 .306 -.457 -1.050 -.009 .621 .995 -1.524 -1.592 -1.918 -.927 -1.740 .062 .826 1.464 .460 -.172 -.667 1.861 2.008 1.160

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

SAKTHIVEL ET AL.

Rietveid Crystal Structure Refinement Method

Table I I . Continued

No.

Species

46

1 2 2 2 2 2 2 1 1 ί 1 i 2 2 2 2 2 2 1 ί 1 1

^Z

48 49 50 51 52 53

54 55 56 57 58 59 60 61 62 6

3

64 65 66 6

Z

68 69 70 71 72 73 74 75 76 78 79 80 81 82 8

3

84 85 86 87 88 89 90

2 2 2 2 2 2 1 i 1

\

2 2 2 2 2 2 1 i ί 1

Fractional

.576 .555 .656 .566 .462 .511 .512 .616 .509 .490 .396 .528 .539 .444 .524 .622 .570 .572 .474 .585 .590 .679 .563 .541 .643 .553 .449 .498 .499 .603 .496 .477 .383 .515 .526 .431 .511 .609 .557 .559 .461 .572 .577 .666 .483

Coordinates

-.003 .266 .158

.389 .492 .598 .473

.671 .900 .657 .765 .503 .404 • 337

.492 •5Z

1

.586 .245 .269 .136 .406 .298 .529 .632 .737 .613 .526 .412 .763 .810 1.040 .797 .904 .642 .543 .476 .631 .710 .725 .385 .408 .552

Cartesian

Coordinates

.107 .156 .148 .105 .120 .181 .229

5.183 4.031 5.325 3.695 2.398 2.456 2.912

-.023 1.905 1 .130 2.783 3.518 4.274 3.383

2.458 3.390 3.275

.198 .327 .383 .368 .335 .355 .418 .462 .437 .280 .314 .440 .569 .618 .609 .567 .582 .643 .691 .672

1.165 1.539 2.500 1.261 2.916 4.145 3.917 3.387 2.220 3.168 4.424 5.143 4.569 3.417 4.711 3.082 1.785 1.843 2.299 3.540 2.985 1.570 .551 926 887 647 303 532 . 3J-CH -OH 2

ι

HO-C-C-OH

The hydroxymethyl g r o u p s i n DMDHEU w e r e m e t h y l a t e d t o f o r m V a l r e z ULF. F i b e r s w i t h a c e l l u l o s e t r i a c e t a t e c o r e and a c e l l u l o s e s k i n w e r e p r e p a r e d by p a r t i a l l y s a p o n i f y i n g c e l l u l o s e t r i a c e t a t e f i b e r s f o l l o w i n g a c o n v e n t i o n a l p r o c e d u r e by the use o f a hot, s t r o n g sodium hydroxide s o l u t i o n [28]. To d e t e r m i n e the t h i c k n e s s e s o f the s k i n and the c o r e , t h e f i b e r s were d y e d w i t h C. I . D i r e c t G r e e n 26 so c e l l u l o s e was d y e d more d e e p l y t h a n t r i a c e t a t e . T h i c k n e s s o f the c e l l u l o s e s k i n was measured by o p t i c a l m i c r o s c o p y a f t e r f i b e r c r o s s

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

13.

YANG ET AL.

Cellulose Textile Materials and FTIR Spectroscopy

219

s e c t i o n s were p r e p a r e d from the dyed f i b e r s . The f i b e r s were found t o h a v e d i a m e t e r o f a p p r o x i m a t e l y 18 ym and s k i n t h i c k n e s s e s o f a p p r o x i m a t e l y 2 ym. R e s u l t s and D i s c u s s i o n Near S u r f a c e A n a l y s i s . A c e l l u l o s e f i b e r , a c e l l u l o s e t r i a c e t a t e f i b e r and a f i b e r w i t h a c e l l u l o s e s k i n and a t r i a c e t a t e c o r e w e r e s t u d i e d u s i n g FT-IR/PAS ( F i g u r e 3). The s p e c t r a l c h a r a c t e r i s t i c s o f b o t h the c e l l u l o s e and the c e l l u l o s e t r i a c e t a t e f i b e r s ( F i g u r e s 3A and 3 B ) a r e r e v e a l e d i n the spectrum of the c e l l u l o s e s k i n - c e l l u l o s e t r i a c e t a t e core f i b e r ( F i g u r e 3 θ where the s t r o n g h y d r o g e n - b o n d e d OH s t r e t c h i n g peak around 3300 cm i s due t o the c e l l u l o s e s k i n , and t h e s t r o n g c a r b o n y l s t r e t c h i n g peak a t 1730 cm i s due t o t h e c e l l u l o s e t r i a c e t a t e underneath the s k i n . S i n c e the thermal d i f f u s i o n l e n g t h o f c e l l u l o s e at scan v e l o c i t y 0 i s a p p r o x i m a t e l y 412 urn i n t h e mid i n f r a r e d f r e q u e n c y range (4000-400 cm ) which i s l a r g e r than the s k i n t h i c k n e s the c e l l u l o s e t r i a c e t a t e the i n t e n s i t y o f t h e c a r b o n y l peak a t 173.9 r e l a t i v e t o the i n t e n s i t y o f t h e s t r o n g e s t peak a t 1045 cm i n the spectrum o f the s k i n - c o r e sample ( F i g u r e 3C) i s lower t h a n t h a t i n t h e s p e c t r u m of p u r e c e l l u l o s e t r i a c e t a t e ( F i g u r e 3 B ) , because l e s s c e l l u l o s e t r i a c e t a t e was d e t e c t e d by PAS i n the s k i n - c o r e sample. I n t h e t e x t i l e i n d u s t r y , s i z i n g agents are g e n e r a l l y a p p l i e d t o warp yarns t o i n c r e a s e t h e i r a b r a s i o n r e s i s t a n c e d u r i n g w e a v i n g . A f t e r weaving i s completed, s i z i n g agents a r e removed t h r o u g h a d e s i z i n g p r o c e s s t o o b t a i n d e s i r e d y a r n p r o p e r t i e s . In our r e s e a r c h , a p u r e c o t t o n y a r n and a c o t t o n y a r n s i z e d w i t h a p o l y u r e t h a n e were examined by^PAS a t v e l o c i t y 0 ( F i g u r e s 4A and 4 B ) . An i n t e n s e peak a t 1730 cm o b s e r v e d i n F i g u r e 4B was due t o the c a r b o n y l s t r e t c h i n g o f the p o l y u r e t h a n e s i z i n g a g e n t . The s i z e d c o t t o n y a r n was then g r o u n d i n t o a powder t o pass a 40-mesh s c r e e n and re-examined by FTIR/PAS a t v e l o c i t y 0 ( F i g u r e 4 C ) . I t can be seen t h a t the i n t e n s i t y o f t h e c a r b o n y l p e a k o f t h e powder s a m p l e ( F i g u r e 4C) was g r e a t l y r e d u c e d compared w i t h t h a t o f t h e w h o l e y a r n s a m p l e ( F i g u r e 4 B ) . S i n c e p h o t o a c o u s t i c s i g n a l s a r e g e n e r a t e d o n l y from the s u b s t a n c e s w i t h i n one thermal d i f f u s i o n l e n g t h t h i c k n e s s , the i n f r a r e d spectrum o f t h e s i z e d y a r n ( F i g u r e 4B) p r o v i d e s i n f o r m a t i o n o f the c h e m i c a l c o m p o s i t i o n o f the s u b s t a n c e s w i t h i n a few m i c r o n s s u r f a c e l a y e r . Upon g r i n d i n g t h e s a m p l e , h o w e v e r , t h e s u r f a c e l a y e r and the bulk were mixed and averaged. Because the diameter o f the c o t t o n y a r n i s a p p r o x i m a t e l y 350 ym, the amount o f s u b s t a n c e s w i t h i n a few microns s u r f a c e l a y e r i s v e r y s m a l l compared t o the amount o f s u b s t a n c e s i n t h e b u l k ; t h e r e f o r e , t h e i n f r a r e d s p e c t r u m o f t h e powder sample ( F i g u r e 4C) r e p r e s e n t s m a i n l y the c h e m i c a l c o m p o s i t i o n o f t h e b u l k . The o b s e r v a t i o n t h a t the c a r b o n y l peak f o r the y a r n sample i n F i g u r e 4B was much more i n t e n s e than the same peak f o r the powder s a m p l e i n F i g u r e 4C d e m o n s t r a t e s a h i g h e r c o n c e n t r a t i o n o f the p o l y u r e t h a n e s i z i n g agent i n the s u r f a c e l a y e r o f the y a r n than i n the b u l k . This suggests t h a t the s i z i n g p r o c e s s d i d not r e s u l t i n uniform d i s t r i b u t i o n o f the p o l y u r e t h a n e s i z i n g a g e n t i n t o t h e y a r n b u l k . S i n c e t h e w e a v i n g p e r f o r m a n c e o f s i z e d yarns i s g r e a t l y a f f e c t e d by s i z e p e n e t r a t i o n , F T - I R / P A S a p p e a r s t o be a v a l u a b l e t o o l f o r studying s i z i n g processes. Λ

c

m

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

220

THE STRUCTURES OF CELLULOSE

cc < CQ CC


H

r m m

Η

ο m ζ

280

THE STRUCTURES OF CELLULOSE

' 1 TIO

1 1 1

' ι ' ' 100

1

ι , 90

80

70

60

PPM

Figure

7.

S o l i d - s t a t e *^C-NMR s p e c t r a o f the amorphous h y d r o ­ c e l l u l o s e d u r i n g d e g r a d a t i o n i n 1.0M NaOH a t 80°C.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

GENTILE ET AL.

Alkaline Degradation of Hydrocellulose

281

Resonance l o c a t i o n s and m u l t i p l i c i t i e s a r e c h a r a c t e r i s t i c o f the c e l l u l o s e I I a l l o m o r p h , c o n f i r m i n g the r e s u l t s o f x - r a y d i f f r a c t i o n and Raman s p e c t r o s c o p y . The absence o f change i n the p h y s i c a l s t r u c t u r e o f the f i b r o u s h y d r o c e l l u l o s e d u r i n g d e g r a d a t i o n suggests three a l t e r n a t i v e hypotheses. F i r s t , s e l e c t i v e d e g r a d a t i o n o f amorphous c e l l u l o s e c o u l d have o c c u r r e d but t o an e x t e n t not d e t e c t a b l e by the methods applied. Second, removal o f amorphous m a t e r i a l c o u l d have been accompanied by a comparable amount o f d e c r y s t a l l i z a t i o n o f c e l l u l o s e I domains. F i n a l l y , c e l l u l o s e removed d u r i n g d e g r a d a t i o n may have displayed p a r t i a l c e l l u l o s e I character. T h i s would i n v o l v e molec u l e s i n s l i g h t l y d i s t o r t e d c e l l u l o s e I domains ( t i l t e d o r t w i s t e d segments o f elementary f i b r i l ) and a t c r y s t a l l i t e s u r f a c e s ( 2 0 ) , s i n c e removal o f e i t h e r would not r e s u l t i n d e t e c t a b l e changes i n p h y s i c a l s t r u c t u r e . A l k a l i n e r e a c t i o n d a t a p r e s e n t e d i n the f o l l o w i n g s e c t i o n t e n d t o support the l a t t e r h y p o t h e s i s . The i n c r e a s e i n c e l l u l o s II characte d decreas i accessi b i l i t y o f the amorphous medium and d u r i n g the r e a c t i o recrystallization. However, s e l e c t i v e removal o f amorphous c e l l u l o s e may have o c c u r r e d s i m u l t a n e o u s l y . This additional p o s s i b i l i t y i s c o n s i s t e n t w i t h the a l k a l i n e r e a c t i o n d a t a . P e e l i n g and S t o p p i n g R e a c t i o n s . The y i e l d l o s s d u r i n g a l k a l i n e d e g r a d a t i o n was more r a p i d and e x t e n s i v e f o r the amorphous h y d r o c e l l u l o s e than f o r the f i b r o u s h y d r o c e l l u l o s e , a t b o t h 60 and 80°C ( F i g u r e 8). However, the e v o l u t i o n o f y i e l d l o s s w i t h time was d i f f e r e n t at 60 and 80°C. While at 60°C y i e l d l o s s o c c u r r e d t h r o u g h o u t the time i n t e r v a l s t u d i e d (168 h r ) , the y i e l d o f b o t h s u b s t r a t e s l e v e l e d o f f a f t e r ca. 48 h o u r s a t 80°C. While s m a l l amounts o f p e c t i c m a t e r i a l a r e p r o b a b l y l o s t d u r i n g the d e g r a d a t i o n s , such l o s s e s are i n s i g n i f i c a n t r e l a t i v e t o y i e l d l o s s e s due t o p e e l i n g (10). D i r e c t c o m p a r i s o n o f y i e l d d a t a f o r the two s u b s t r a t e s i s not p o s s i b l e due t o the d i f f e r e n c e s i n i n i t i a l a c c e s s i b l e r e d u c i n g endgroup c o n t e n t s ( T a b l e I ) . The k i n e t i c model used by Haas, e t a l . (50 was t h e r e f o r e employed t o p r o v i d e a b a s i s f o r comparison o f r e a c t i o n r a t e s o f m o l e c u l e s w i t h i n the two s u b s t r a t e s . T h i s model i n corporates pseudo-first-order rate expressions for peeling (Equation 1), c h e m i c a l s t o p p i n g ( E q u a t i o n 2), and p h y s i c a l s t o p p i n g ( E q u a t i o n 4 ) ; our n o t a t i o n d i f f e r s from t h a t o f Haas, e t a l . In a l l t h r e e r a t e e x p r e s s i o n s , the r e a c t i o n r a t e s a r e r e l a t e d t o the number o f a c c e s s i b l e r e d u c i n g endgroups by p s e u d o - f i r s t - o r d e r r a t e c o e f f i cients. Thus, the r a t e c o e f f i c i e n t s r e f l e c t the r e a c t i v i t i e s o f a c c e s s i b l e r e d u c i n g endgroups o c c u p y i n g d i f f e r e n t s t r u c t u r a l environments. S i n c e the y i e l d l o s s e s were p r e d o m i n a n t l y due t o p e e l i n g ( 1 0 ) , the p s e u d o - f i r s t - o r d e r r a t e e x p r e s s i o n f o r p e e l i n g can be written: a[Y ]/dt x

where

= k [ARE ] p

t

[ Y \ ] = Y i e l d l o s s , as mole f r a c t i o n o f t o t a l monomer u n i t s at zero-time

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

(1)

282

THE STRUCTURES OF CELLULOSE

Figure

8.

Hydrocellulose y i e l d

during degradation

i n 1.0M

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

NaOH.

16.

GENTILE ET AL.

283

Alkaline Degradation of Hydrocellulose

t = Time, hr kp = Rate c o e f f i c i e n t f o r p e e l i n g , h r " * [ARE J = A c c e s s i b l e r e d u c i n g endgroup c o n t e n t at time " t , " as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e t

The d e r i v a t i v e i n E q u a t i o n 1 was e v a l u a t e d at s e l e c t e d r e a c t i o n t i m e s from the s l o p e s o f p l o t s o f y i e l d l o s s v e r s u s r e a c t i o n time. V a l u e s o f kp, c a l c u l a t e d from E q u a t i o n 1, are l i s t e d i n T a b l e I I .

Table I I . Reaction Time, h r

Peeling

3

60° C Fibrous

0 2 4 48 96 a

Rate C o e f f i c i e n t s f o r

4.13 3.28 2.74 0.48 0.38

80°C Amorphous

Fibrous

Amorphous

6.16 5.52

27.9 8.91

40.4 13.6

0.53

0.60

0.14

kp,hr"l.

In a l l c a s e s , kp d e c r e a s e d w i t h r e a c t i o n time. Thus, the a c c e s s i b l e r e d u c i n g endgroups i n b o t h h y d r o c e l l u l o s e s were more r e a c t i v e i n i t i a l l y , a p p a r e n t l y due to t h e i r l o c a t i o n i n l e s s o r d e r e d r e g i o n s o f the r e s p e c t i v e p h y s i c a l s t r u c t u r e s . As the l e s s o r d e r e d m a t e r i a l was removed, the a c c e s s i b l e r e d u c i n g endgroups o c c u p i e d i n c r e a s i n g l y o r d e r e d r e g i o n s o f the s t r u c t u r e s and were t h e r e f o r e l e s s r e a c t i v e . The h i g h e r kp v a l u e s f o r the amorphous h y d r o c e l l u l o s e t h r o u g h o u t the 60°C r e a c t i o n and d u r i n g the i n i t i a l p e r i o d o f the 80°C r e a c t i o n i n d i c a t e t h a t the a c c e s s i b l e r e d u c i n g endgroups were more r e a c t i v e t h a n t h o s e i n the f i b r o u s h y d r o c e l l u l o s e . T h i s c o i n c i d e s w i t h the p e r i o d s d u r i n g which the a c c e s s i b i l i t y d e c r e a s e d ( F i g u r e 1), s u g g e s t i n g t h a t s e l e c t i v e removal ( p e e l i n g ) o f amorphous m a t e r i a l did occur. Thus, the l e s s o r d e r e d environment o c c u p i e d by the d e g r a d i n g m o l e c u l e s i n the amorphous h y d r o c e l l u l o s e c l e a r l y r e n d e r e d them more s u s c e p t i b l e t o p e e l i n g . D u r i n g the l a t e r p e r i o d o f the 80°C r e a c t i o n , the amorphous h y d r o c e l l u l o s e e x h i b i t e d a lower v a l u e o f kp t h a n the f i b r o u s h y d r o cellulose. S i n c e t h i s c o r r e s p o n d s t o the p e r i o d d u r i n g which the h y d r o x y l a c c e s s i b i l i t y o f the amorphous h y d r o c e l l u l o s e l e v e l e d - o f f ( F i g u r e 1), i t appears t h a t the p o p u l a t i o n o f d e g r a d a b l e c h a i n s w i t h a c c e s s i b l e r e d u c i n g endgroups had been d e p l e t e d . Consequently, p e e l i n g was p r o b a b l y o c c u r r i n g c l o s e t o c e l l u l o s e I I domains where i t was s i g n i f i c a n t l y i n h i b i t e d . In c o n t r a s t , p e e l i n g p r o g r e s s e d more s l o w l y toward the c e l l u l o s e I domains o f the f i b r o u s h y d r o c e l l u l o s e , f o r example, i n s l i g h t l y d i s t o r t e d c e l l u l o s e I domains, but was a l s o s t r o n g l y i n h i b i t e d a t the f a c e s of more p e r f e c t c e l l u lose I c r y s t a l l i t e s . The degree o f i n h i b i t i o n o f p e e l i n g i s e v i denced by the convergence of 60 and 80°C kp v a l u e s f o r both s u b s t r a t e s at l o n g e r r e a c t i o n t i m e s .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

284

THE STRUCTURES OF CELLULOSE

I n t h e case o f c h e m i c a l s t o p p i n g , t h e r a t e o f f o r m a t i o n o f c a r b o x y l i c a c i d endgroups i s a l s o p r o p o r t i o n a l t o the number o f a c c e s s i b l e r e d u c i n g endgroups. The p s e u d o - f i r s t - o r d e r r a t e e x p r e s s i o n i s g i v e n by: [AE]/dt = k

c s

[ARE ]

(2)

t

where [AE] = C a r b o x y l i c a c i d endgroup c o n t e n t , as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e k = Rate c o e f f i c i e n t f o r c h e m i c a l s t o p p i n g , h r " * c s

C a r b o x y l i c a c i d endgroup c o n t e n t s were f i r s t c o r r e c t e d f o r l o s s e s o f c a r b o x y l i c a c i d groups a s s o c i a t e d w i t h p e c t i c m a t e r i a l l o s t d u r i n g the r e a c t i o n s ( 1 0 ) . Values o f k were then d e t e r mined a t s p e c i f i c time i n t e r v a l s by t h e same g r a p h i c a l p r o c e d u r e as o u t l i n e d f o r k ; the values o f k are given i n Table I I I . c s

p

c s

Table I I I .

Rate C o e f f i c i e n t

Reaction time, h r

Fibrous

Amorphous

Fibrous

0.0142 0.0112 0.0094 0.0015 0.0010

0.0302 0.0271 0.0204 0.0057 0.0055

0.106 0.0338 0.0305 0.0119 0.0073

0 2 4 48 96 *k

c s

60°C

80°C Amorphous 0.262 0.107 0.0808 0.0225 0.0205

,hr-l.

The r a t e c o e f f i c i e n t s f o r c h e m i c a l s t o p p i n g d e c r e a s e d w i t h time f o r both s u b s t r a t e s i n a p a t t e r n s i m i l a r t o t h a t f o r p e e l i n g . Thus, as t h e a c c e s s i b l e r e d u c i n g endgroups o c c u p i e d p r o g r e s s i v e l y more o r d e r e d r e g i o n s o f the s t r u c t u r e s , t h e i r r e a c t i v i t y toward c h e m i c a l stopping also decreased. The amorphous h y d r o c e l l u l o s e e x h i b i t e d h i g h e r v a l u e s o f k throughout b o t h the 60 and 80°C r e a c t i o n s . D u r i n g t h e e a r l y p e r i o d o f t h e 80°C r e a c t i o n and throughout t h e 60°C r e a c t i o n , when t h e r a t e c o e f f i c i e n t f o r p e e l i n g was h i g h e r f o r t h e amorphous s u b s t r a t e (Table I I ) , i t s higher k v a l u e c a n be p r i m a r i l y a t t r i b u t e d t o t h e r e a c t i o n o c c u r r i n g i n l e s s o r d e r e d r e g i o n s o f the s t r u c t u r e . Howe v e r , t h i s c o u l d not account f o r t h e b e h a v i o r i n t h e l a t e r p e r i o d o f the 80°C r e a c t i o n s , where p e e l i n g was a p p a r e n t l y h i n d e r e d t o s i m i l a r e x t e n t s by t h e c r y s t a l l i n e r e g i o n s o f b o t h s t r u c t u r e s . Therefore, i t i s c o n c l u d e d t h a t the c e l l u l o s e I I domains i n the amorphous subs t r a t e d i d not i n h i b i t c h e m i c a l s t o p p i n g as d r a s t i c a l l y as the c e l l u l o s e I domains i n t h e f i b r o u s s u b s t r a t e . c s

c s

F u r t h e r c l a r i f i c a t i o n o f t h e s e d i f f e r e n c e s i s p r o v i d e d by comp a r i n g t h e r e l a t i v e r a t e s o f p e e l i n g and c h e m i c a l s t o p p i n g f o r t h e two s u b s t r a t e s . Average v a l u e s o f k p / k ( T a b l e IV) were c a l c u l a t e d u s i n g E q u a t i o n 3, d e r i v e d by d i v i d i n g E q u a t i o n 1 by E q u a t i o n 2. c s

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

GENTILE ET AL.

285

Alkaline Degradation of Hydrocellulose d[Yi]/d[AE]

= k /k p

( )

c s

3

The r e l a t i v e r a t e s o f p e e l i n g and c h e m i c a l s t o p p i n g were h i g h e r f o r t h e f i b r o u s h y d r o c e l l u l o s e t h r o u g h o u t the r e a c t i o n s . F o r t h e amorphous s u b s t r a t e , t h e v a l u e s o f k p / k were lower a t t h e o u t s e t , and d e c r e a s e d s u b s t a n t i a l l y , a t l o n g e r r e a c t i o n t i m e s . Since k / k remained c o n s t a n t throughout t h e f i b r o u s h y d r o c e l l u l o s e r e a c t i o n s , the r e l a t i v e r e a c t i v i t y o f i t s a c c e s s i b l e r e d u c i n g endgroups toward b o t h r e a c t i o n s appears not t o change as t h e r e a c t i o n s p r o g r e s s from r e g i o n s o f lower o r d e r t o r e g i o n s o f h i g h e r o r d e r . This i s consist e n t w i t h the l e s s o r d e r e d r e g i o n s e x h i b i t i n g some c e l l u l o s e I c h a r a c t e r ; reference here i s to s l i g h t l y d i s t o r t e d c e l l u l o s e I domains and c r y s t a l l i t e s u r f a c e s ( 2 0 ) . In t h e l a t e r p e r i o d s o f the amorphous h y d r o c e l l u l o s e r e a c t i o n s , however, p e e l i n g was i n h i b i t e d much more than c h e m i c a l s t o p p i n g . T h i s i s c o n s i s t e n t w i t h the e a r l i e r p r o p o s a l t h a t c e l l u l o s e I I domains do not h i n d e r c h e m i c a l s t o p p i n g as e f f e c t i v e l y as c e l l u l o s e I domains w h i l e b o t h c r y s t a l l i n e forms a r e h i g h l y r e s i s t a n p r e s e n c e o f the c r y s t a l l i n o f p e e l i n g a l o n g a c e l l u l o s e m o l e c u l e , w h i l e b o t h the degree o f s t r u c t u r a l o r d e r and the p a r t i c u l a r m o l e c u l a r c o n f o r m a t i o n d i c t a t e the r e a c t i v i t y o f an a c c e s s i b l e r e d u c i n g endgroup toward c h e m i c a l stopping. c s

p

Table

IV.

R e l a t i v e Rates o f P e e l i n g and C h e m i c a l

c s

Stopping

Reaction

kp/k

F i b r o u s 60°C (0-168 h r ) F i b r o u s 80°C (0-96 h r ) Amorphous 60°C (0-48 h r ) (48-168 h r ) Amorphous 80°C (0-4 h r ) (4-96 h r )

c s

291 264 204 84 154 23

These f i n d i n g s a r e c o n s i s t e n t w i t h r e s u l t s o f c o m p a r a t i v e a l k a l i n e d e g r a d a t i o n s t u d i e s o f n a t i v e ( c e l l u l o s e I ) and m e r c e r i z e d ( c e l l u l o s e I I ) c e l l u l o s e (3^ 6^8). i n a d d i t i o n , decreases i n k p / k f o r b o t h s u b s t r a t e s w i t h i n c r e a s i n g temperature a r e i n agreement w i t h the h i g h e r a c t i v a t i o n energy r e p o r t e d f o r c h e m i c a l s t o p p i n g versus p e e l i n g i n h y d r o c e l l u l o s e ( 5 ) . In a d d i t i o n t o u n d e r g o i n g c h e m i c a l s t o p p i n g r e a c t i o n s , c e l l u l o s e m o l e c u l e s a l s o a r e thought t o t e r m i n a t e i n r e d u c i n g endgroups which a r e p h y s i c a l l y i n c a p a b l e o f r e a c t i n g due t o t h e i r i n a c c e s s b i l i t y t o the a l k a l i n e medium (3_-_5) · T h term " p h y s i c a l s t o p p i n g " has been used t o c h a r a c t e r i z e the f o r m a t i o n o f i n a c c e s s i b l e r e d u c i n g endgroups ( n o n r e a c t i v e ) on m o l e c u l e s which p r e v i o u s l y c o n t a i n e d a c c e s s i b l e ( r e a c t i v e ) r e d u c i n g endgroups. The p s e u d o - f i r s t - o r d e r rate expression f o r p h y s i c a l stopping i s written: c s

e

d[IRE]/dt

= k

p s

[ARE ] t

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

(4)

286

THE STRUCTURES OF CELLULOSE

where

[IRE] = I n a c c e s s i b l e r e d u c i n g endgroup c o n t e n t , as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e kp = Psudo-first-order rate c o e f f i c i e n t f o r physical stopping S

A l t h o u g h p h y s i c a l s t o p p i n g i s not a c h e m i c a l r e a c t i o n , per s e , kp v a l u e s d e t e r m i n e d u s i n g E q u a t i o n 4 may be compared t o k v a l u e s , p r o v i d i n g a measure o f t h e r e l a t i v e importance o f t h e two modes o f s t o p p i n g . F u r t h e r m o r e , comparison o f k p v a l u e s f o r two s u b s t r a t e s g i v e s an i n d i c a t i o n o f t h e r e l a t i v e e x t e n t o f s t r u c t u r a l hindrance to peeling. In b o t h the 60 and 80°C r e a c t i o n s , t h e f i b r o u s h y d r o c e l l u l o s e exhibited higher k v a l u e s t h a n t h e amorphous h y d r o c e l l u l o s e (Table V). T h i s appears t o be due t o t h e i n v o l v e m e n t o f more m o l e c u l e s i n c r y s t a l l i n e domains o f the f i b r o u s s u b s t r a t e . The g r e a t e r i n h i b i t i o n o f c h e m i c a l s t o p p i n g by c e l l u l o s e I than c e l l u l o s e I I domains may a l s o have c o n t r i b u t e d t o t h i s e f f e c t by a l l o w i n g more m o l e c u l e s i n the f i b r o u s h y d r o c e l l u l o s endgroup would be i n a c c e s s i b l e S

c s

S

p s

T a b l e V. Reaction Time, h r 0 2 4 48 96 3

k

p s

Rate C o e f f i c i e n t s f o r P h y s i c a l

Stopping

3

60° C

80° C

Fibrous

Amorphous

0.0410 0.0218 0.0157 0.0032 0.0028

0.0142 0.0137 0.0132 0.0021 0.0019

Fibrous

Amorphous

0.363 0.0812 0.0543 0 0

0.154 0.0700 0.0195 0 0

,hr-l.

At 80°C and f o r l o n g e r r e a c t i o n t i m e s , b o t h h y d r o c e l l u l o s e s ceased p h y s i c a l stopping. T h i s may be an i n d i c a t i o n t h a t each phys i c a l s t r u c t u r e has some maximum number o f p o t e n t i a l p h y s i c a l stopping s i t e s . As a consequence, i n a c c e s s i b l e r e d u c i n g endgroups c o u l d become a c c e s s i b l e as a d j a c e n t m o l e c u l e s a r e removed by p e e l i n g , g i v i n g r i s e t o a steady s t a t e d i s t r i b u t i o n o f a c c e s s i b l e and i n a c e s s i b l e r e d u c i n g endgroups. E x c e p t f o r t h e l a t e r p e r i o d o f t h e 80°C r e a c t i o n s , t h e f i b r o u s h y d r o c e l l u l o s e e x h i b i t e d a h i g h e r v a l u e o f k p ( T a b l e V) t h a n k (Table IV). C o n s e q u e n t l y t h e d e g r a d a t i o n o f a m a j o r i t y o f t h e molec u l e s i n the f i b r o u s h y d r o c e l l u l o s e was t e r m i n a t e d by p h y s i c a l r a t h e r than chemical stopping processes. In c o n t r a s t , c h e m i c a l s t o p p i n g was t h e dominant mechanism o f s t a b i l i z a t i o n i n t h e amorphous h y d r o c e l l u l o s e . S

c s

Random C h a i n C l e a v a g e R e a c t i o n . In a d d i t i o n t o p e e l i n g , c e l l u l o s e i s a l s o r e p o r t e d t o undergo random c l e a v a g e o f g l y c o s i d i c l i n k a g e s i n a l k a l i n e media (J^,2). T h i s r e a c t i o n r e s u l t s i n the f o r m a t i o n o f

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

287

Alkaline Degradation of Hydrocellulose

GENTILE ET AL.

one r e d u c i n g and one n o n r e d u c i n g endgroup. S i n c e r e d u c i n g endgroups can a l s o be i n v o l v e d i n p e e l i n g and s t o p p i n g r e a c t i o n s , i t i s not p o s s i b l e to m o n i t o r d i r e c t l y t h e i r f o r m a t i o n due t o random c h a i n cleavage. However, the r a t e o f c h a i n c l e a v a g e can be c h a r a c t e r i z e d by m o n i t o r i n g i n c r e a s e s i n t h e t o t a l number o f endgroups. Accurate c h a r a c t e r i z a t i o n o f the r e a c t i o n does r e q u i r e t h a t no o t h e r changes i n the t o t a l number o f endgroups o c c u r , as f o r example, from l o s s o f m o l e c u l e s by complete p e e l i n g o r d i s s o l u t i o n . D u r i n g d e g r a d a t i o n o f the f i b r o u s h y d r o c e l l u l o s e , no changes i n t o t a l endgroup c o n t e n t were d e t e c t e d ( T a b l e V I ) . T h i s i s c o n s i s t e n t w i t h r e s u l t s o f p r e v i o u s s t u d i e s (6,7) i n which c h a i n c l e a v a g e was found to be i m p o r t a n t i n n a t i v e c e l l u l o s e o n l y above 100°C.

Table VI.

T o t a l Endgroup C o n t e n t s

Reaction Time, h r

of

Hydrocelluloses 80°C

60°C Fibrous

0 2 4 8 24 48 96 168 3

3

1.94 1.94 1.92 2.04 1.95 1.92 1.86 1.89

E x p r e s s e d as t ime.

10

3

4.13 3.57 3.44 3.52 3.59 3.65 3.76 4.09 χ mole

1.88 1.90 1.96 1.96 1.89 1.89 1.90

4.51 4.28 4.35 4.44 4.96 5.05 5.35





f r a c t i o n o f t o t a l monomer u n i t s at

zero-

In c o n t r a s t , t h e amorphous h y d r o c e l l u l o s e underwent i n i t i a l d e c l i n e i n t o t a l endgroup c o n t e n t ( T a b l e VI) which may be a t t r i b u t e d t o complete p e e l i n g and/or d i s s o l u t i o n o f low DP m o l e c u l e s . After the i n i t i a l p e r i o d s , t o t a l endgroup c o n t e n t s i n c r e a s e d g r a d u a l l y a t b o t h 60 and 80°C, i n d i c a t i n g t h a t random c h a i n c l e a v a g e o c c u r r e d . Random c h a i n c l e a v a g e must a l s o have o c c u r r e d d u r i n g t h e i n i t i a l p e r i o d s but was p r o b a b l y masked by the more s u b s t a n t i a l n e g a t i v e e f f e c t s o f complete p e e l i n g o r d i s s o l u t i o n on the t o t a l endgroup contents. The amorphous s u b s t r a t e s u f f e r e d the most r a p i d d e c l i n e i n h y d r o x y l a c c e s s i b i l i t y ( F i g u r e 1) d u r i n g the same p e r i o d s i n which t o t a l endgroup l o s s e s o c c u r r e d . T h i s i n d i c a t e s t h a t complete p e e l ­ ing or d i s s o l u t i o n p r i m a r i l y involved molecules e x i s t i n g e n t i r e l y w i t h i n amorphous r e g i o n s and became i n s i g n i f i c a n t once the m a j o r i t y o f h i g h l y a c c e s s i b l e c h a i n s had been removed o r c h e m i c a l l y s t a b i ­ lized. F u r t h e r s u p p o r t i s thus p r o v i d e d f o r the h y p o t h e s i s t h a t s e l e c t i v e p e e l i n g o f amorphous m a t e r i a l c o n t r i b u t e s t o the h i g h e r r a t e c o e f f i c i e n t o f p e e l i n g i n the case o f the amorphous h y d r o c e l l u ­ l o s e ( T a b l e I I ) . The c o m p a r a t i v e l a c k o f s i m i l a r l o s s e s from t h e f i b r o u s s u b s t r a t e s u g g e s t s t h a t the l a r g e m a j o r i t y o f m o l e c u l e s were embedded t o some e x t e n t i n c r y s t a l l i n e r e g i o n s .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

THE STRUCTURES OF CELLULOSE

288

Because endgroup l o s s e s o c c u r r e d s i m u l t a n e o u s l y w i t h random c h a i n c l e a v a g e d u r i n g the i n i t i a l p e r i o d s , a n a l y s i s o f the t o t a l endgroup d a t a f o r k i n e t i c s o f c h a i n c l e a v a g e was c o n f i n e d t o the l a t e r reaction periods. S i n c e the t o t a l number o f monomer u n i t s , o r y i e l d , i s e s s e n t i a l l y e q u a l t o the number o f g l y c o s i d i c l i n k a g e s , the p s e u d o - f i r s t - o r d e r r a t e e x p r e s s i o n f o r random c h a i n c l e a v a g e can be w r i t t e n as: d[TE]/dt where [TE] k [Y ] c c

t

= k

c c

[Y ]

(5)

t

= T o t a l endgroup c o n t e n t , as mole f r a c t i o n o f t o t a l monomer u n i t s a t z e r o - t i m e = Rate c o e f f i c i e n t f o r random c h a i n c l e a v a g e , h r " * = Y i e l d , as mole f r a c t i o n of t o t a l monomer u n i t s a t time " t "

Rate c o e f f i c i e n t s f o r random c h a i n c l e a v a g e i n the 60°C amor phous h y d r o c e l l u l o s e r e a c t i o hours ( T a b l e V I I ) . At 80°C r a p i d l y between 2 and 48 h o u r s , w i t h a more g r a d u a l d e c l i n e up t o 96 hours. T h i s r e f l e c t s the more r a p i d d e c l i n e i n a c c e s s i b i l i t y o f the amorphous h y d r o c e l l u l o s e at 80°C ( F i g u r e 1). Thus, random c h a i n c l e a v a g e appears t o be i n h i b i t e d by the l a r g e r c e l l u l o s e I I f r a c t i o n t h a t formed i n the amorphous s u b s t r a t e a t 80°C.

Table

VII.

Reaction Time, h r 0 2 8 48 96 168

3

Rate C o e f f i c i e n t s f o r Random C h a i n C l e a v a g e 60°C Fibrous 0 0 0 0 0 0

80° C Fibrous

Amorphous

8.42 8.38 8.34 7.53

ND ND χ 10" χ 10' χ 10" χ 1(T

6

6

6

6

ak ,hr-l. ND = Rate c o e f f i c i e n t s not d e t e r m i n e d due peeling or d i s s o l u t i o n

0 0 0 0 0

Amorphous

5.78 5.31 0.82 0.49



ND χ χ χ χ

ΙΟ" 10~ ΙΟ" ΙΟ"



c c

to s i m u l t a n e o u s

complete

The absence o f c h a i n c l e a v a g e i n the f i b r o u s h y d r o c e l l u l o s e s u g g e s t s t h a t i t s d i s o r d e r e d r e g i o n s were more h i g h l y s t r u c t u r e d t h a n the c o r r e s p o n d i n g r e g i o n s o f the amorphous h y d r o c e l l u l o s e . T h i s i s c o n s i s t e n t w i t h the r e s u l t s o f a p r e v i o u s study (6^ i n which m e r c e r i z e d c e l l u l o s e was found t o be more s u s c e p t i b l e t o random c h a i n c l e a v a g e t h a n n a t i v e c e l l u l o s e . Another i m p l i c a t i o n i s t h a t the d i s o r d e r e d r e g i o n s a s s o c i a t e d w i t h the two c r y s t a l l i n e polymorphs d i s p l a y d i f f e r e n t d e g r e e s o f s t r u c t u r a l o r d e r , g i v i n g r i s e to d i f f e r e n c e s i n r e a c t i v i t y . Thus, i n a d d i t i o n t o m o l e c u l a r m o b i l i t y and a c c e s s i b i l i t y , the p a r t i c u l a r m o l e c u l a r c o n f o r m a t i o n

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

5

5

5

5

16.

GENTILE ET AL.

289

Alkaline Degradation of Hydrocellulose

appears t o i n f l u e n c e s u s c e p t i b i l i t y reaction.

t o the random c h a i n

cleavage

Conclusions A l k a l i n e p e e l i n g and c h e m i c a l s t o p p i n g o c c u r more r a p i d l y i n t h e amorphous r e g i o n s o f amorphous h y d r o c e l l u l o s e t h a n i n t h e d i s o r d e r e d r e g i o n s o f f i b r o u s h y d r o c e l l u l o s e . In a d d i t i o n , random c h a i n c l e a v a g e a t 60 and 80°C o c c u r s o n l y i n amorphous h y d r o c e l l u l o s e . T h e r e f o r e , i t i s proposed t h a t the d i s o r d e r e d r e g i o n s o f the f i b r o u s hydrocelluose c o n s i s t o f less r e a c t i v e molecules at c r y s t a l l i t e s u r f a c e s and i n s l i g h t l y d i s t o r t e d c r y s t a l l i n e domains, as p r e v i o u s l y suggested ( 2 0 ) . P e e l i n g i s i n h i b i t e d t o s i m i l a r e x t e n t s by the c r y s t a l l i n e o r d e r o f b o t h c e l l u l o s e I and I I a l l o m o r p h s , w h i l e c h e m i c a l s t o p p i n g i s s i g n i f i c a n t l y more i n h i b i t e d i n t h e c e l l u l o s e I a l l o m o r p h . This i s c o n s i s t e n t w i t h the h i g h e r r a t i o o f the r a t e o f c h e m i c a l s t o p p i n g t o that of peeling t y p i c a l l parison to native c e l l u l o s Physical stopping, that i s , formation o f i n a c c e s s i b l e reducing endgroups, o c c u r s when p e e l i n g o f m o l e c u l a r c h a i n s r e a c h e s t h e c r y s t a l l i n e domains i n both c e l l u l o s e I and I I . The r e l a t i v e r a t e s o f p h y s i c a l and c h e m i c a l s t o p p i n g a r e d i c t a t e d by t h e number o f m o l e c u l e s i n v o l v e d i n c r y s t a l l i n e domains. In a p r e v i o u s study (5^), c e l l u l o s e m o l e c u l e s were r e p o r t e d t o m a i n t a i n c o n s t a n t r e a c t i v i t y toward p e e l i n g and c h e m i c a l s t o p p i n g u n l e s s p h y s i c a l s t o p p i n g occurred. However, the r e s u l t s o f t h e p r e s e n t study i n d i c a t e t h a t r e a c t i v i t y d i m i n i s h e s g r a d u a l l y as r e a c t i o n s approach more h i g h l y ordered regions of p h y s i c a l structure. S i m u l t a n e o u s l y , abrupt p h y s i c a l s t o p p i n g can o c c u r . The r a t e o f c h e m i c a l s t o p p i n g i n c r e a s e s w i t h temperature relat i v e t o p e e l i n g i n b o t h f i b r o u s and amorphous h y d r o c e l l u l o s e . T h i s o b s e r v a t i o n i s c o n s i s t e n t w i t h p r e v i o u s f i n d i n g s ( 5_). Experimental Cellulose Substrates. Raw c o t t o n f i b e r c u t i n c a . 0.25 i n c h l e n g t h s was p u r i f i e d by e x t r a c t i o n w i t h c h l o r o f o r m , 95% e t h a n o l , b o i l i n g 1% (w/w) sodium h y d r o x i d e ( o x y g e n f r e e ) , and d i e t h y l e n e t r i a m i n e p e n t a a c e t i c a c i d (0.15% w/v, pH 9) ( 1 0 ) . F i b r o u s h y d r o c e l l u l o s e was p r e p a r e d by t r e a t i n g the p u r i f i e d f i b e r s (60 g) w i t h 0.1M h y d r o c h l o r i c a c i d (6L) a t 40°C f o r 20 h o u r s , washing w i t h d i s t i l l e d water ( u n t i l n e u t r a l ) , and then f r e e z e - d r y i n g . Amorphous h y d r o c e l l u l o s e was p r e p a r e d by d r o p w i s e a d d i t i o n o f a DMSO-PF s o l u t i o n o f the f i b r o u s h y d r o c e l l u l o s e (0.2%, w/v, c e l l u l o s e / D M S O , 3.5L) t o 0.2M sodium m e t h o x i d e - i s o p r o p o x i d e s o l u t i o n (1:1, v / v , m e t h a n o l : i s o p r o p a n o l , 14L) ( 9 , 1 0 ) . The r e s u l t i n g p r e c i p i t a t e was washed w i t h 0.2M sodium m e t h o x i d e - i s o p r o p o x i d e , methanol ( u n t i l n e u t r a l ) , 0.1M h y d r o c h l o r i c a c i d , and d i s t i l l e d water ( u n t i l n e u t r a l ) , and then freeze-dried. Both the f i b r o u s and amorphous h y d r o c e l l u l o s e s were f u r t h e r d r i e d i n v a c u o o v e r phosphorus p e n t o x i d e t o c o n s t a n t weight. Degradation Procedure. A l k a l i n e d e g r a d a t i o n s were conducted i n 316 s t a i n l e s s s t e e l laboratory d i g e s t e r s (10). Hydrocellulose substrate

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

290

THE STRUCTURES OF CELLULOSE

(400 mg) and o x y g e n - f r e e 1.0M sodium h y d r o x i d e (40 mL) were s e a l e d i n t h e r e a c t i o n v e s s e l s under n i t r o g e n , and t h e v e s s e l s were r o t a t e d end-over-end a t c a . 3 rpm i n a c o n s t a n t temperature o i l b a t h . The r e a c t i o n m i x t u r e s were m a i n t a i n e d a t 60 o r 80°C f o r t h e s p e c i f i e d time i n t e r v a l , c o o l e d t o 20°C, and n e u t r a l i z e d w i t h 1.0M h y d r o chloric acid. Z e r o - t i m e samples were p r e p a r e d by l i m i t i n g t h e time a t t h e r e a c t i o n t e m p e r a t u r e t o c a . one minute. Degraded h y d r o c e l l u l o s e was washed w i t h 0.1M h y d r o c h l o r i c a c i d and d i s t i l l e d water ( u n t i l n e u t r a l ) , and then f r e e z e - d r i e d . Y i e l d was d e t e r m i n e d a f t e r f u r t h e r d r y i n g i n v a c u o o v e r phosphorus p e n t o x i d e t o c o n s t a n t weight. A n a l y t i c a l Methods. C a r b o x y l i c a c i d endgroup c o n t e n t s were d e t e r mined by methylene b l u e a b s o r p t i o n u s i n g TAPPI S t a n d a r d Method T237 su-63 w i t h minor m o d i f i c a t i o n s ( 1 0 ) . A c c e s s i b l e r e d u c i n g endgroups were d e t e c t e d by r e d u c t i o n w i t h sodium b o r o h y d r i d e - ^ H , and t o t a l r e d u c i n g endgroups were d e t e r m i n e d s i m i l a r l y a f t e r r e g e n e r a t i n g t h e c e l l u l o s e from t h e DMSO-P endgroup c o n t e n t s were c a l c u l a t e endgroup c o n t e n t s . C e l l u l o s e h y d r o x y l a c c e s s i b i l i t y was measured by t h e d e u t e r a t i o n method o f R o u s e l l e and N e l s o n ( 1 3 ) , b u t t h e d e u t e r a t i o n time ( i n l i q u i d D2O) was extended t o 12 h o u r s ( 1 0 ) . X-ray d i f f r a c t o g r a m s were c o l l e c t e d on a N o r e l c o d i f f r a c t o m e t e r , u s i n g n i c k e l - f i l t e r e d , CuKct r a d i a t i o n . Raman s p e c t r a were a c q u i r e d w i t h a J o b i n Yvon Ramanor S p e c t r o m e t e r , u t i l i z i n g t h e 5145 Â l i n e o f an a r g o n l a s e r o p e r a t e d , a t 100 mw, as t h e e x c i t i n g s o u r c e . S o l i d - s t a t e l^C-NMR s p e c t r a were o b t a i n e d on a G e n e r a l E l e c t r i c S-100 i n s t r u m e n t employing t h e combined t e c h n i q u e s (16,17) o f p r o t o n - c a r b o n c r o s s p o l a r i z a t i o n , h i g h power p r o t o n d e c o u p l i n g , and m a g i c - a n g l e sample spinning. Acknowledgments The a u t h o r s pany, I n c . Woitkovich appreciates during this

w i s h t o thank Dr. T. E a r l y o f GE-NMR Instruments Comf o r o b t a i n i n g t h e s o l i d - s t a t e l^c-NMR s p e c t r a and Mr. C. f o r a c q u i r i n g t h e Raman s p e c t r a . V. M. G e n t i l e s i n c e r e l y f e l l o w s h i p s u p p o r t from The I n s t i t u t e o f Paper C h e m i s t r y work.

References 1.

Meller, A. Holzforschung 1960, 14, 78 and references cited therein. 2. Richards, G. N. In Cellulose and Cellulose Derivatives; Part V, p. 1007 and references cited therein, N. Bikales and L. Segal (Eds.), Wiley-Interscience, New York, 1971. 3. Machell, G.; Richards, G. N. Tappi 1958, 41, 12. 4. Colbran, R. L.; Davidson, G. F. J. Textile Inst. 1961, 52, T73. 5. Haas, D. W.; Hrutfiord, B. F.; Sarkanen, Κ. V. Appl. Polymer Sci. 1967,11,587. 6. Lai, Y.-Z.; Sarkanen, Κ. V. Cellulose Chem. Technol. 1967, 1, 517.

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Alkaline Degradation of Hydrocellulose

291

7. Franzon, O.; Samuelson, O. Svensk Papperstid. 1957, 60, 872. 8. Christofferson, K.; Samuelson, O. Svensk Papperstid. 1962, 65, 571. 9. Gentile, V. M.; Schroeder, L. R.; Atalla, R. H. J. Wood Chem. 1986, 6, 1. 10. Gentile, V. M. Doctoral Dissertation, The Institute of Paper Chemistry, Appleton, Wisconsin (1986). 11. Nicholson, M. D.; Johnson, D. C.; Haigh, F. C. Appl. Polymer Symp. 1976, 28, 931. 12. Baker, T. J.; Schroeder, L. R.; Johnson, D. C. Cellulose Chem. Technol. 1981, 15, 311. 13. Rouselle Μ. Α.; Nelson, M. L. Textile Res. J. 1971, 41, 599. 14. Wadsworth, L.C.; Cuculo, L. C. In Modified Cellulosics, Part III, p. 117, R. M. Rowell and R. A. Young (Eds.), Academic Press, New York, 1978. 15. Atalla, R. H. J. Appl. Polymer Sci. (Appl. Polymer Symp.) 1983, 37, 295. 16. Earl, W. L.; VanderHart 17. VanderHart, D. L.; 1465. 18. Tripp, V. M. In Cellulose and Cellulose Derivatives, Part IV, p. 305, N. Bikales and L. Segal (Eds.), Wiley-Interscience, New York, 1981. 19. Howsmon, J. Α.; Sisson, W. A. In Cellulose, Part I, 2nd Ed., p. 231, E. Ott and H. M. Spurlin (Eds.), Interscience Publishers, New York, 1954. 20. Rowland, S. P. In Modified Cellulosics, Part III, p. 162, and references cited therein, R. M. Rowell and R. A. Young (Eds.), Academic Press, New York, 1978. RECEIVED March 5, 1987

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Chapter 17

Intra- and Intermolecular Hydrogen Bonds in Native, Mercerized, and Regenerated Celluloses Reflection in Patterns of Solubility and Reactivity 1

1

1

2

A. Isogai , A. Ishizu , J. Nakano , and Rajai H. Atalla 1

Department of Forest Products, Faculty of Agriculture, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Institute of Paper Chemistry, Appleton, WI 54912 2

An unusual patter related polysaccharide has been observed, and interpreted in terms of distinc tive hydrogen bonding patterns. In particular, i t was found that only native and mercerized cellulose dissolve in this system, while regenerated celluloses, glucomannan, xylan, starch, pectin and curdlan are insoluble. The pattern for the celluloses has been correlated with relative reactivities of hydroxyl groups in etherification reactions in different environments, and with results of solid state C NMR studies on the celluloses and related oligosaccharides and polysaccharides. They suggest that the solvent system acts at particular sites involving cooperative hydrogen bonding incorpora­ ting, among others, the primary hydroxyl group at C6 and the linkage oxygen. 13

Our proposal has the key implication that regenerated and mercerized celluloses have different patterns of intermolecular hydrogen bonding, even though they may have similar heavy atom lattices. This is analogous to what has been proposed as the key difference between the I and I forms of native cellulose. Taken together these findings suggest that the organization and packing of the heavy atom lattices in celluloses are dominated by the shapes of the molecules in their different conformations, and that more than one stable pattern of intermolecular hydrogen bonding is con­ sistent with each heavy atom lattice. α

ß

The supermolecular structures of cellulose have been investigated extensively by many techniques including x-ray and electron diffractometry, electron microscopy, IR and Raman spectroscopy, broad-line proton NMR and solid-state l^C-NMR. Nevertheless, many questions remain concerning the solid-state structures. 0097-6156/87/0340-0292$06.00/0 © 1987 American Chemical Society

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

17.

ISOGAI ET AL.

Intra- and Intermolecular Hydrogen Bonds in Cellulose 293

D u r i n g our s t u d i e s on c e l l u l o s e c h e m i s t r y U_-_5), we have e n c o u n t e r e d an u n u s u a l p a t t e r n o f s o l u b i l i t i e s o f v a r i o u s c e l l u l o s e s and r e l a t e d p o l y s a c c h a r i d e s i n one o f t h e nonaqueous c e l l u l o s e s o l v e n t systems we i n v e s t i g a t e d , t h e S 0 2 - d i e t h y l a m i n e ( D E A ) - d i m e t h y l s u l f o x i d e ( D M S O ) system. In t h i s paper, we propose an i n t e r p r e t a t i o n o f t h i s p a t t e r n i n terms o f i n t r a - and i n t e r m o l e c u l a r hydrogen bonds i n n a t i v e , m e r c e r i z e d and r e g e n e r a t e d c e l l u l o s e s . We a l s o c o n s i d e r p a r a l l e l s w i t h t h e r e l a t i v e r e a c t i v i t i e s o f h y d r o x y l groups i n g l u cose r e s i d u e s o f c e l l u l o s e toward e t h e r i f i c a t i o n , under b a s i c c o n d i t i o n s , and the d a t a from s o l i d - s t a t e l^C-NMR r e p o r t e d by A t a l l a and o t h e r s ( 6 ^ 9 ) . Experimental Sample P r e p a r a t i o n s . The n a t i v e c e l l u l o s e s used were A v i c e l , c o t t o n l i n t e r s and ramie. Degrees o f p o l y m e r i z a t i o n o f A v i c e l and the c o t t o n l i n t e r s d e t e r m i n e d by t h e c o p p e r e t h y l e n e d i a m i n e v i s c o s i t y method (10) were 250 and 1360, r e s p e c t i v e l y pared from t h e s e c e l l u l o s e t a i n i n g 1% NaBH under a n i t r o g e n atmosphere f o r 20 hrs a t room temperature. The samples were washed o n a 1G2 g l a s s f i l t e r w i t h l a r g e amounts o f water, d i l u t e a c e t i c a c i d , l a r g e amounts o f water a g a i n and, f i n a l l y , acetone. They were d r i e d a t 40°C i n vacuo f o r 1 day. Regenerated samples were p r e p a r e d from n a t i v e c e l l u l o s e s by d i s s o l v i n g i n cadoxene ( 1 1 ) , and r e g e n e r a t i n g by d r o p w i s e a d d i t i o n t o d i l u t e a c e t i c a c i d . The r e g e n e r a t e d c e l l u l o s e was f i l t e r e d and washed w i t h l a r g e amounts o f water. H a l f o f each sample was washed w i t h acetone and d r i e d a t 40°C i n vacuo f o r 1 day, and the o t h e r h a l f was s u b j e c t e d t o l y o p h i l i z a t i o n f o l l o w e d by d r y i n g a t 40°C i n vacuo f o r 1 day. Amylose and s t a r c h , b o t h d e r i v e d from p o t a t o , were commercial samples. Glucomannan and x y l a n were i s o l a t e d from spruce and beech h o l o c e l l u l o s e s , r e s p e c t i v e l y , and p u r i f i e d by t h e u s u a l method ( 1 2 ) . P e c t i n was i s o l a t e d and p u r i f i e d from the m i d r i b o f N i c o t i a n a t a b a cum and k i n d l y p r o v i d e d by Dr. S h i g e r u Eda a t The C e n t r a l Research I n s t i t u t e , Japan Tobacco Inc. (5^)· The s o l u b i l i t y o f the p o l y s a c c h a r i d e s was t e s t e d by d i s p e r s i n g l g o f a d r i e d sample i n 42 ml DMSO, and then adding t h e SO2/DMSO solution c o n t a i n i n g 1.19 g o f SO2 (4.09 ml o f 0.291 g S02/ml DMSO s o l u t i o n ) and 1.92 mo DEA, i n t h i s o r d e r a t room temperature. S u c c e s s f u l d i s s o l u t i o n was judged by v i s u a l e x a m i n a t i o n a f t e r s t i r r i n g f o r 1 day. R e s u l t s and D i s c u s s i o n 1. S o l u b i l i t i e s o f c e l l u l o s e s and o t h e r i n t h e SO2-DEA-DMSO system

polysaccharides

In o u r p r e v i o u s work on t h e dérivâtizations o f c e l l u l o s e U_-J>), we have found t h e SO2-DEA-DMSO system t o be the most e f f e c t i v e nonaqueous c e l l u l o s e s o l v e n t medium f o r t h e p r e p a r a t i o n o f h i g h l y substituted c e l l u l o s e ethers. The s o l u b i l i t i e s o f v a r i o u s c e l l u l o s i c samples and o t h e r p o l y s a c c h a r i d e s i n t h i s nonaqueous s o l v e n t 3ystem were t e s t e d i n the c o u r s e o f t h e i n v e s t i g a t i o n s . As shown i n

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

294

THE STRUCTURES OF CELLULOSE

T a b l e I , n a t i v e c e l l u l o s e s such as ramie, c o t t o n l i n t e r s and A v i c e l , b o t h b e f o r e and a f t e r m e r c e r i z a t i o n d i s s o l v e d c o m p l e t e l y i n t h i s s o l v e n t system. More r e c e n t l y we have e s t a b l i s h e d t h a t h i g h l y c r y s t a l l i n e a l g a l c e l l u l o s e s are also readily dissolved i n t h i s system.

Table

I.

S o l u b i l i t i e s of Various Polysaccharides i n S02-diethylamine-DMS0 System

Soluble

N a t i v e c e l l u l o s e s (ramie, l i n t e r , A v i c e l ) M e r c e r i z e d c e l l u l o s e s (ramie, l i n t e r , A v i c e l )

Insoluble

Amylose, s t a r c h , glucomannan, x y l a n , p e c t i n R e g e n e r a t e d c e l l u l o s e (ramie, l i n t e r , A v i c e l )

In s h a r p c o n t r a s t , r e g e n e r a t e ramie, c o t t o n l i n t e r s an d e c r y s t a l l i z a t i o n by b a l l - m i l l i n g . Furthermore, amylose, s t a r c h , glucomannan, x y l a n and p e c t i n were a l s o found t o be i n s o l u b l e i n t h i s system. Our f i n d i n g c o n c e r n i n g r e g e n e r a t e d samples c o n s i s t e n t w i t h t h e r e p o r t by Yamazaki and Nakao (13) t h a t c o m m e r c i a l l y a v a i l a b l e r a y o n s , even w i t h DPv as low as 300, a r e i n s o l u b l e i n a l l S 0 2 ~ a m i n e - o r g a n i c s o l v e n t systems. Hie patterns o f s o l u b i l i t y , o r l a c k t h e r e o f , a r e c u r i o u s because r e g e n e r a t e d c e l l u l o s e s g e n e r a l l y have lower c r y s t a l l i n i t i e s than n a t i v e ones. Moreover, t h e o t h e r p o l y s a c c h a r i d e s so f a r examined have lower m o l e c u l a r w e i g h t s and c r y s t a l l i n i t i e s than n a t i v e c e l l u l o s e s . In a d d i t i o n , amylose and s t a r c h a r e s o l u b l e i n DMSO a l o n e above 60°C. A l l t h e c e l l u l o s e samples and the p o l y s a c c h a r i d e s used i n t h i s work a r e s o l u b l e i n o t h e r nonaqueous c e l l u l o s e s o l v e n t systems such as p a r a f o r m a l d e h y d e DMSO ( 1 4 ) , L i C l - d i m e t h y l a c e t a m i d e ( 1 5 ) , N-methylmorpholine N-oxide (16) ( c o n t a i n i n g s m a l l amounts o f water) and o t h e r s . Thus, t h e i n s o l u b i l i t y o f r e g e n e r a t e d c e l l u l o s e s was o b s e r v e d o n l y f o r t h e S02~amine systems. We b e l i e v e t h a t t h e d i f f e r e n c e s i n s o l u b i l i t y r e p o r t e d i n T a b l e I may r e f l e c t d i f f e r e n t p a t t e r n s o f i n t r a - and i n t e r m o l e c u l a r hydrogen b o n d i n g i n d i f f e r e n t c e l l u l o s e s . w

e

r

e

In t h e p r e v i o u s work i n which ^H- and l^C-NMR used ( 1 7 ) , the d i s s o l u t i o n o f c e l l u l o s e i n t h e SO2-DEA-DMSO system has been e x p l a i n e d i n terms o f complex f o r m a t i o n between t h e -OH o f c e l l u l o s e , and SO2 and DEA, as shown i n F i g u r e 1. The p a t t e r n o f s o l u b i l i t i e s noted e a r l i e r s u g g e s t s t h a t t h e complex f o r m a t i o n r e a c t i o n i n F i g u r e 1 i s s p e c i f i c t o p a r t i c u l a r i n t r a - and/or i n t e r m o l e c u l a r hydrogen bonding p a t t e r n s p e c u l i a r t o n a t i v e and m e r c e r i z e d c e l l u loses. The d i s s o l u t i o n c h a r a c t e r i s t i c s o f n a t i v e , m e r c e r i z e d and r e g e n e r a t e d c e l l u l o s e s , and t h e i r i n t e r p r e t a t i o n i n terms o f hydrogen bonding d i f f e r e n c e s , p o i n t t o some i n t e r e s t i n g r e l a t i o n s h i p s . N a t i v e and m e r c e r i z e d c e l l u l o s e s would seem t o have some common hydrogen bonding p a t t e r n s , a l t h o u g h they have d i f f e r e n t x - r a y d i f f r a c t i o n p a t t e r n s . On t h e o t h e r hand, m e r c e r i z e d and r e g e n e r a t e d c e l l u l o s e s would d i f f e r from each o t h e r w i t h r e s p e c t t o i n t e r m o l e c u l a r hydrogen bonds, a l t h o u g h they have t h e same x - r a y p a t t e r n .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

17.

ISOGAI ET AL.

Intra- and Intermolecular Hydrogen Bonds in Cellulose 295

R e c e n t l y , VanderHart and A t a l l a (18) proposed,on t h e b a s i s o f CP-MAS C NMR o f d i f f e r e n t c e l l u l o s e samples, t h a t a l l n a t i v e c e l l u l o s e a r e composites o f two c r y s t a l l i n e m o d i f i c a t i o n s , c e l l ΐ and I g , even though they have s i m i l a r x - r a y p a t t e r n s . These have been i n t e r p r e t e d i n terms o f s i m i l a r heavy atom l a t t i c e s w i t h d i f ­ f e r e n t hydrogen b o n d i n g p a t t e r n s (19,20). Thus, i t may w e l l be t h a t m e r c e r i z e d and r e g e n e r a t e d c e l l u l o s e s d i f f e r i n t h e same way and c a n have d i f f e r e n t i n t e r m o l e c u l a r hydrogen bonding p a t t e r n s , which r e s u l t i n d i f f e r e n c e s i n t h e i r s o l u b i l i t y i n t h e SO2-DEA-DMSO system. l 3

α

2.

R e l a t i v e r e a c t i v i t i e s o f h y d r o x y l groups i n g l u c o s e r e s i d u e s o f c e l l u l o s e toward e t h e r i f i c a t i o n s under b a s i c c o n d i t i o n s

We have p r e v i o u s l y r e p o r t e d s t u d i e s on t h e d i s t r i b u t i o n o f s u b s t i ­ t u e n t s i n p a r t i a l l y e t h e r i f i e d c e l l u l o s e s which were p r e p a r e d from h e t e r o g e n e o u s a l k a l i c e l l u l o s e and from homogeneous nonaqueous c e l l u l o s e s o l u t i o n s (21). I c e l l u l o s e e t h e r s such a p r e p a r e d from SO2-DEA-DMSO s o l u t i o n s o f c e l l u l o s e by a d d i t i o n s o f powdered NaOH as a base. When the nonaqueous c e l l u l o s e s o l v e n t was used as a medium, t h e o r d e r o f r e a c t i v i t i e s was 6-OH>2-OH~3-OH. T h i s o r d e r i s s i m i l a r t o o b s e r v a t i o n s i n the c a s e o f s i m p l e a l c o h o l s . Thus, a l t h o u g h t h e p r i m a r y h y d r o x y l group 6-OH has t h e h i g h e s t r e a c t i v i t y , and t h e s e c o n d a r y h y d r o x y l groups 2-OH and 3-OH have almost e q u a l r e a c ­ t i v i t i e s , t h e d i f f e r e n c e o f r e a c t i v i t i e s between 6-OH, 2-OH and 3-OH i s small. On t h e o t h e r hand, when h e t e r o g e n e o u s a l k a l i c e l l u l o s e systems were used, where c e l l u l o s e was s w o l l e n i n aqueous a l k a l i r a t h e r than b e i n g i n s o l u t i o n , t h e o r d e r o f r e a c t i v i t y was 2-OH>6-OH>>3-OH a t a l l c o n c e n t r a t i o n s o f aqueous a l k a l i ( 2 2 ) . This o r d e r i s not c o n s i s t e n t w i t h t h e p a t t e r n f o r lower m o l e c u l a r weight compounds and s u g g e s t s c o n s i d e r a t i o n o f e f f e c t s o f i n t r a - and i n t e r ­ m o l e c u l a r hydrogen bonds which may remain even i n s w o l l e n a l k a l i cellulose. The secondary h y d r o x y l group 2-OH has a h i g h e r r e a c ­ t i v i t y than t h e primary a l c o h o l 6-OH, and t h e r e i s a remarkable d i f ­ f e r e n c e i n r e a c t i v i t i e s between t h e secondary a l c o h o l s 2-OH and 3- OH. These r e s u l t s suggest t h a t some o f t h e i n t r a m o l e c u l a r h y d r o ­ gen bonds between the 3-OH groups and 0-5 i n the a d j a c e n t anhydro­ g l u c o s e r e s i d u e s , known t o o c c u r i n n a t i v e c e l l u l o s e s , a r e r e t a i n e d even i n s w o l l e n a l k a l i c e l l u l o s e , and thus have an e f f e c t on t h e e t h e r i f i c a t i o n process. The r e s i s t a n c e t o d i s r u p t i o n by a l k a l i a l s o s u g g e s t s some s t r o n g i n t e r m o l e c u l a r hydrogen bonds i n c e l l u l o s e . Such s t r o n g hydrogen bonds may w e l l be the key t o r e t e n t i o n o f the f i b e r form o f a l k a l i c e l l u l o s e at a l l c o n c e n t r a t i o n s o f a l k a l i . In t h e c a s e o f most polymers, s w e l l i n g and d i s s o l u t i o n i n s o l v e n t s p r o c e e d i n sequence. That i s , f i r s t t h e a c c e s s i b l e p a r t s a r e s w o l l e n by a s o l v e n t and t h i s i s f o l l o w e d by d i s s o l u t i o n . However, i n t h e case o f c e l l u l o s e i n aqueous a l k a l i , a l t h o u g h i t i s s w o l l e n and undergoes a l a t t i c e t r a n s f o r m a t i o n , i t does not d i s s o l v e . I f t h e h y d r o x y l groups a r e s o l v a t e d w i t h aqueous a l k a l i and i f a l l i n t r a - and i n t e r m o l e c u l a r h y d r o g e n bonds a r e c l e a v e d , c e l l u l o s e f i b e r s cannot be e x p e c t e d t o keep t h e i r form nor t o form a new l a t t i c e . This suggests that

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

296

THE STRUCTURES OF CELLULOSE

some o f t h e s t r o n g i n t e r m o l e c u l a r hydrogen bonds i n n a t i v e c e l l u l o s e may s u r v i v e i n s w o l l e n a l k a l i c e l l u l o s e s and c o n t r i b u t e t o t h e d e f i n i t i o n o f the new l a t t i c e . Such a p r o p o s a l seems a p l a u s i b l e a l t e r n a t i v e t o that o f N a s t a b i l i z a t i o n o f the l a t t i c e . It has been proposed t h a t i n a l k a l i c e l l u l o s e , a l k a l i (NaOH) and water b r i d g e t h e c e l l u l o s e m o l e c u l e s ( 2 3 ) . However, i t does not seem l i k e l y t h a t a l l d i r e c t i n t e r m o l e c u l a r hydrogen bonds i n a l k a l i c e l l u l o s e a r e d i s r u p t e d , inasmuch as t h e f i b r o u s morphology i s retained. A d d i t i o n a l l y , t h e r e have n o t been any r e p o r t s o f any p r e c i p i t a t e s o f complexes between o l i g o m e r s and a l k a l i i n aqueous solutions. There has been a n o t h e r p r o p o s a l t h a t p l a n e - s t r u c t u r e s c o n s i s t i n g o f c e l l u l o s e m o l e c u l e s i n t h e 101 p l a n e o f n a t i v e c e l l u l o s e a r e h e l d t o g e t h e r by h y d r o p h o b i c i n t e r a c t i o n s even i n t h e p r e s e n c e o f a l k a l i , and t h a t h y d r o p h i l i c s u r f a c e s o f the 101 p l a n e - s t r u c t u r e s a r e s o l v a t e d w i t h a l k a l i and water ( 2 4 ) . However, i f such p l a n a r s t r u c ­ t u r e s were s o l v a t e d w i t h aqueous a l k a l i they would be e x p e c t e d t o r e s u l t i n the formation us more l i k e l y t h a t som m o l e c u l a r hydrogen bonds o f n a t i v e c e l l u l o s e s u r v i v e even i n a l k a l i cellulose. On t h e o t h e r hand, s i n c e some hydrogen bonds a r e c l e a v e d by NaOH and water which p e n e t r a t e i n t o t h e c r y s t a l l i n e l a t t i c e o f c e l l u l o s e , new l a t t i c e p l a n e s c a n be formed a s , f o r example, i n NaC e l l u l o s e I o r o t h e r soda c e l l u l o s e s . In a s s e s s i n g t h e s t a b i l i t y o f i n t e r m o l e c u l a r hydrogen bonds, t h e p a t t e r n o f c h e m i c a l r e a c t i v i t y o f the h y d r o x y l groups may be an indicator. On t h e b a s i s o f t h e r e s u l t s o f r e l a t i v e r e a c t i v i t i e s o f the h y d r o x y l groups i n t h e a n h y d r o g l u c o s e r e s i d u e s , t h e more s t a b l e i n t e r m o l e c u l a r hydrogen bonds o f c e l l u l o s e appear l i k e l y t o i n v o l v e 6-OH, because i t o f t e n shows lower r e a c t i v i t y f o r e t h e r i f i c a t i o n s t h a n the 2-OH. +

3.

S o l i d - s t a t e 13C-NMR d a t a

of various

c e l l u l o s e samples

l^C-NMR s p e c t r a l s h i f t s f o r c e l l u l o s e and some o l i g o m e r s a r e sum­ m a r i z e d i n F i g u r e 2 (6_-_9). H o r i i et_ a_l. (9^) d i s c u s s e d t h e c h e m i c a l s h i f t s o f C-6 c a r b o n s , and proposed t h a t t h e d i f f e r e n c e between C e l l u l o s e I and C e l l u l o s e I I i s caused by c o n f o r m a t i o n a l d i f f e r e n c e s a t t h e 6-OH groups. The s h i f t s i n t h e c r y s t a l l i n e p a r t s o f C e l l u ­ l o s e I were a s s i g n e d t o t - g c o n f o r m a t i o n s ( c a . 66 ppm f o r C-6), and those f o r C e l l u l o s e I I and t h e amorphous p a r t s o f a l l c e l l u l o s e s t o g-t c o n f o r m a t i o n s ( c a . 63 ppm f o r C - 6 ) . In c o n t r a s t , c h e m i c a l s h i f t s o f t h e C-4 carbons change l a r g e l y depending on whether c e l l u l o s e i s i n a s o l i d s t a t e o r i n s o l u t i o n (Figure 2). H o r i i et^ al_. ( 2 5 ) s u g g e s t e d t h a t t h i s change i n t h e c h e m i c a l s h i f t o f C-4 i s a l s o caused by c o n f o r m a t i o n a l d i f f e r e n c e . However, such a h i g h d o w n f i e l d s h i f t o f C-4 carbons (Δ = c a . 10 ppm) seems u n l i k e l y t o a r i s e from a c o m f o r m a t i o n a l d i f f e r e n c e a l o n e . Changes i n c h e m i c a l s h i f t s o f c a r b o n s i n low m o l e c u l a r s u g a r s , which have been r e p o r t e d by H o r i i e t a l . (9^), were e x p l a i n e d m a i n l y i n terms o f changes i n s h i e l d i n g due t o d i f f e r e n t hydrogen b o n d i n g p a t ­ terns associated with d i f f e r e n t conformations. The s o l i d - s t a t e l^c-NMR s p e c t r a o f c u r d l a n , amylose and c h i t i n have a l s o been r e p o r t e d ( 2 6 ) , and g e n e r a l l y C - l and C-4, o r C-3 i n

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

ISOGAI ET AL.

1.

Figure

Intra- and Intermolecular Hydrogen Bonds in Cellulose 297

D i s s o l u t i o n mechanism o f c e l l u l o s e i n SO2-amine-DMSO systems.

CI

C4

C6

Cellulose soin. Cellobiose soin. Cellobiose cryst Amorphous c e l l . Cellopentaose Mercerized c e l l . Regenerated c e l l Linter c e l l . 110

100

90

80

70

60

ppm Figure

2.

l^C-chemical samples.

s h i f t s of carbon i n various

cellulose

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

298

THE STRUCTURES OF CELLULOSE

the case o f c u r d l a n , have h i g h c h e m i c a l s h i f t s i n t h e i r s o l i d s t a t e s i n comparison w i t h s h i f t s i n s o l u t i o n . T a b l e I I shows the d i f f e r ­ ence (Δ) between the c h e m i c a l s h i f t s i n the s o l i d s t a t e and i n s o l u ­ t i o n f o r the above g l u c a n s as w e l l as c e l l u l o s e . The t a b l e shows t h a t Δ f o r C - l carbons ranges from 1.0 t o 3.0 ppm. In the case o f amylose and c h i t i n A's f o r C-4, and i n the case o f c u r d l a n Δ f o r C-3, a r e o n l y s l i g h t l y h i g h e r (4.1-4.5 ppm). On the o t h e r hand, c e l l u l o s e has a much l a r g e r Δ o f 10 ppm f o r C-4. T h i s excep­ t i o n a l l y h i g h c h e m i c a l s h i f t f o r C-4 s u g g e s t s a s p e c i a l d i f f e r e n c e for solid-state structures of celluloses.

Table

II.

Sample Curdlan Amylose Chitin

Cellulose

1 3

D i f f e r e n c e (Δ, ppm) o f C - C h e m i c a l Between S o l i d and S o l u t i o n S t a t e s

Linkages (l-3)-3-D-gluca ( 1 -4) -ot-D-g l u c a n (l-4)-3-D-2acetamido-2-deoxyglucan (l-4)-$-D-glucan

Shifts

A, ppm C-3

C-l

C-4

2.3

4.5

2.3 3.0

4.1 10.0

We propose here a p o s s i b l e e x p l a n a t i o n f o r the h i g h v a l u e o f the c h e m i c a l s h i f t s o f C-4 i n c r y s t a l l i n e c e l l u l o s e , namely, as r e p r e ­ s e n t e d i n F i g u r e 3, an e x c e p t i o n a l l y s t r o n g i n t e r m o l e c u l a r h y d r o g e n bond between a 6-OH and a g l y c o s i d i c l i n k a g e oxygen atom. We s p e c u ­ l a t e t h a t i t i s the 6-OH o f an a d j a c e n t c h a i n because o f the anoma­ l o u s l y low r e a c t i v i t y o f the p r i m a r y h y d r o x y l s i n soda c e l l u l o s e s . S t a b i l i z a t i o n o f such an i n t e r m o l e c u l a r h y d r o g e n bond can be f a c i l i ­ t a t e d by a t r a n s f e r of e l e c t r o n d e n s i t y to the g l y c o s i d i c oxygen atom from the C-4 carbon. T h i s , i n t u r n , can reduce the s h i e l d i n g a t C-4 and r e s u l t i n the d o w n f i e l d s h i f t o f i t s resonance. Electron d e n s i t y at C - l i s c l e a r l y too low f o r i t to make a s i g n i f i c a n t c o n t r i b u t i o n because o f i t s anomeric c h a r a c t e r ; t h i s i s r e f l e c t e d i n i t s d o w n f i e l d s h i f t beyond 100 ppm. Strong i n t e r m o l e c u l a r hydrogen bonds s t a b i l i z e d by the s h i f t o f e l e c t r o n d e n s i t y c o u l d be r e s i s t a n t t o c l e a v a g e even i n c o n c e n t r a t e d aqueous a l k a l i , and thus s t a b i l i z e the morphology o f the f i b e r . Discussion The r e s u l t s d i s c u s s e d i n the p r e v i o u s s e c t i o n s suggest one p r o p e r t y common to n a t i v e and m e r c e r i z e d c e l l u l o s e s which i s not shared by regenerated c e l l u l o s e s . That i s t h e i r s o l u b i l i t y i n the S02-amineDMSO system. On the o t h e r hand, m e r c e r i z e d c e l l u l o s e and r e g e n e r ­ ated c e l l u l o s e possess very s i m i l a r x-ray d i f f r a c t i o n p a t t e r n s , though t h e i r response to the s o l v e n t system i s not the same. These p a t t e r n s suggest d i f f e r e n c e s between m e r c e r i z e d and r e g e n e r a t e d c e l l u l o s e s analogous to the d i f f e r e n c e s between I and I g , among the components o f n a t i v e c e l l u l o s e s . Thus i t i s proposed t h a t , a l t h o u g h a

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

17.

ISOGAI ET AL.

Intra- and Intermolecular Hydrogen Bonds in Cellulose

Cell-(OH)

3

+

DMSO

3S0

+

2

3Et NR 2

Cell-

JN

Et

F i g u r e 3.

P o s s i b l e i n t e r m o l e c u l a r h y d r o g e n bond i n c e l l u l o s e .

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

299

THE STRUCTURES OF CELLULOSE

300

m e r c e r i z e d and r e g e n e r a t e d c e l l u l o s e s p o s s e s s the same o r v e r y s i m i ­ l a r heavy atom l a t t i c e s , the p a t t e r n s o f h y d r o g e n bonding d e v e l o p e d d u r i n g r e g e n e r a t i o n a r e u n l i k e t h o s e which r e s u l t from m e r c e r i z a ­ tion. Furthermore, i t appears t h a t t h e system o f hydrogen bonds t h a t i s the s p e c i f i c s i t e o f a t t a c k o f the S02-amine-DMS0 system i s not formed d u r i n g the r e g e n e r a t i o n o f c e l l u l o s e from s o l u t i o n . Whether t h e hydrogen bond o f the primary h y d r o x y l t o the g l y c o ­ s i d i c l i n k a g e oxygen i s a p a r t o f t h i s system remains an open q u e s t i o n , s i n c e the u n u s u a l l y h i g h d o w n f i e l d s h i f t o f t h e l^C-NMR resonance o f C-4 a l s o o c c u r s i n r e g e n e r a t e d c e l l u l o s e . It seems more l i k e l y t h a t t h i s hydrogen bond i s p a r t o f a system o f h y d r o g e n bonds t h a t a c t i n c o n c e r t and r e s u l t i n a c o o p e r a t i v e s t a b i l i z a t i o n o f the system f o r n a t i v e and m e r c e r i z e d c e l l u l o s e s . Thus, w h i l e t h e C-6 h y d r o x y l hydrogen bond t o t h e g l y c o s i d i c l i n k a g e may be formed i n r e g e n e r a t i o n , t h e o t h e r components o f the c o o p e r a t i v e hydrogen bond system do not o c c u r . I f t h e a c t i o n o f the SO2-amine-DMSO s o l v e n t i s p a r t i c u l a r l y a t t u n e d t o the c o o p e r a t i v e hydrogen b o n d i n g effect, i t s inability t the o t h e r p o l y s a c c h a r i d e The p r o p o s a l we make h e r e i s a s p e c u l a t i v e one. We b e l i e v e i t v a l u a b l e t o p r e s e n t i t , however, p a r t i c u l a r l y i n view o f the d i f f e r ­ ences i n hydrogen bonding p a t t e r n s o f the I and Ig c e l l u l o s e s a l l u d e d t o above. The i m p l i c a t i o n s o f o u r p r o p o s a l a r e q u i t e important i n the c o n t e x t o f s t r u c t u r a l s t u d i e s on c e l l u l o s e i n g e n e r a l , f o r i t f o l l o w s from t h e p r o p o s a l t h a t the p a c k i n g o f t h e heavy atom l a t t i c e i s d e t e r m i n e d p r i m a r i l y by the shape o f t h e mole­ c u l e s i n t h e i r d i f f e r e n t c o n f o r m a t i o n s and t h a t more than one s t a b l e p a t t e r n o f hydrogen bonding i s p o s s i b l e f o r each o f the heavy atom lattices. a

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Isogai, Α.; Ishizu, Α.; Nakano, J. J. Appl. Polym. Sci. 1984, 2 9 , 2097. Isogai, Α.; Ishizu, A.; Nakano, J. ibid. 1984, 29, 3873. Isogai, A.; Ishizu, Α.; Nakano, J. ibid. 1985, 30, 345. Isogai, Α.; Ishizu, Α.; Nakano, J. ibid. in press. Isogai, Α.; Ishizu, A.; Nakano, J.; Eda, S.; Kato, K. Carbohyd. Res. 1985, 138, 99. Atalla, R. H.; Gast, J. C.; Sindorf, D. W.; Bartuska, V. J.; Maciel, G. E. J. Am. Chem. Soc. 1980, 102, 3249. Earl, W. L.; VanderHart, D. L. ibid. 1980, 102, 3251. Kunze, J.; Scheler, G.; Schroter, B.; Phillip, B. Polym. Bull. 1983, 10, 56. Horii, F.; Hirai, Α.; Kitamaru, R. ibid. 1983, 10, 357. TAPPI Standard, Τ 230 om-82 (1985). Jayme, G.; Lang, F. Methods in Carbohyd. Chem., Vol III, Academic Press, New York, p. 80 (1963). Timell, T. E. Methods in Carbohyd. Chem., Vol V, Academic Press, New York, p. 134, 137 (1965). Yamazaki, S.; Nakao, O. Sen-i Gakkaishi 1974, 30, T234. Gruber, E.; Gruber, R. Cellulose Chem. Technol. 1978, 12, 345. Gagnaire, D.; Germain, J. S.-; Vincendon, M. J. Appl. Polym. Sci., Appl. Polym. Symp. 1983, 87, 261.

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

17.

16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

ISOGAI ET AL.

Intra- and intermolecular Hydrogen Bonds in Cellulose

301

Chanzy, H.; Noe, P.; Paillet, M.; Smith, P. ibid. 1983, 87, 239. Isogai, Α.; Ishizu, Α.; Nakano, J. J. Appl. Polym. Sci. under contribution. VanderHart, D. L.; Atalla, R. H. Macromolecules 1984, 17, 1465. Atalla, R. H. Proceedings of Paper Making Raw Materials, Vol. III, Oxford p. 59 (1985). Wiley, J. H.; Atalla, R. H. Paper in this ACS Symposium Series. Isogai, Α.; Ishizu, Α.; Nakano, J. Sen-i Gakkaishi 1984, 40, T504. Rowland, S. P. Cellulose Chem. Technol. 1980, 14, 423. For example, T. Okano and A. Sarko J. Appl. Polym. Sci. 1985, 30, 325. For example, F. J. Kolpak and J. Blackwell Macromolecules 1976, 9, 275. Horii, F.; Hirai, Α.; Kitamaru R ACS Symposium Series No 1984, 260, p. 27. Saito, H.; Tabeta, International Conference on Chitin and Chitosan in 1982 (Japan), p. 72.

RECEIVED March 5, 1987

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Author Index A t a l l a , R a j a i Η., 1,88,151,272, B e r t o n i e r e , N o e l i e R., 255 B l a c k w e l l , John, 199 Bresee, R a n d a l l R., 214 Chanzy, Η., 189 E c k h a r d t , Β., 68 F a t e l e y , W i l l i a m G., 214 F e l l e r s , John F., 233 F i n k , H.-P., 178 F r e n c h , A. D., 15,68 G e n t i l e , V i c t o r Μ., 272 Hatano, Masahiro, 135 H a y a s h i , J i s u k e , 135 H e n r i s s a t , B e r n a r d , 38 H i r a i , Α., 119 H o r i i , F., 119 I s h i z u , Α., 292 I s o g a i , Α., 292 K i t a m a r u , R., 119 Kon, H i r o s h i , 135 Kunze, J . , 178 Kurz, D a v i d , 199 Lee, David Μ., 199

L i n , J . S., 233 M i l l e r , D. P., 15 Nakano, J . , 292 N i s h i m u r a , H i s a o , 169 Nozawa, T s u n e n o r i , 135 Okano, T a k e s h i , 169 P e r e n i c h , Theresa Α., 214 P e r e z , Serge, 38 P h i l i p p , B., 178 Quenin, I . , 189 Roughead, W. Α., 15

Su, Mao-Yao, 199 T a k a i , M i t s u o , 135 Tang, Ming-Ya, 233 Tvaroska, I g o r , 38 VanderHart, D. L., 88 W i l e y , James Η., 151 W i n t e r , W i l l i a m Τ., 38 Yang, C h a r l e s Q., 214 Young, R. Α., 68 Z e r o n i a n , S. H a i g , 255

Affiliation Index Academy o f S c i e n c e s o f the German Democratic R e p u b l i c , 178 Case Western Reserve U n i v e r s i t y , 199 Centre N a t i o n a l de l a Recherche S c i e n t i f i q u e , 38,189 Clemson U n i v e r s i t y , 15 Georgia I n s t i t u t e o f Technology, 68 Hokkaido U n i v e r s i t y , 135 I n s t i t u t e o f Paper C h e m i s t r y , 1,88,151,272,292 Kansas S t a t e U n i v e r s i t y , 214

Kyoto U n i v e r s i t y , 119 N a t i o n a l Bureau o f S t a n d a r d s , 88 Oak Ridge N a t i o n a l L a b o r a t o r y , 233 Southern R e g i o n a l Research C e n t e r , 255 S t a t e U n i v e r s i t y o f New York, 169 Tohoku U n i v e r s i t y , 135 U.S. Department o f A g r i c u l t u r e , 15,68 U n i v e r s i t y o f C a l i f o r n i a — D a v i s , 255 U n i v e r s i t y o f G e o r g i a , 214 U n i v e r s i t y o f Tennessee, 233 U n i v e r s i t y o f Tokyo, 292

304

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Subject Index

A Accessibility c a l c u l a t i o n for moisture-regain method, 264 c o t t o n , 264t endgroups i n a l k a l i n e d e g r a d a t i o n , 283 f i b r o u s h y d r o c e l l u l o s e , 274-275 m e r c e r i z e d c o t t o n , 264t See a l s o H y d r o x y l a c c e s s i b i l i t Acetobacter xylinum b a c t e r i a c e l l u l o s e , 89 carbon-13 CP-MAS s p e c t r a , 9 6 f c r y s t a l l i n e n a t i v e c e l l u l o s e , 11 n a s c e n t f i b r i l o f c e l l u l o s e , 130 NMR s p e c t r a , 113 Acid hydrolysis c e l l u l o s e , 257 e f f e c t on c o t t o n l i n t e r s , 9 8 f e f f e c t on s p e c t r a l changes o f c o t t o n l i n t e r s , 97 See a l s o H y d r o l y s i s Algal cellulose c r y s t a l l i n e polymorphy, 115 o r i e n t a t i o n , 156f u n i t c e l l , 153 See a l s o R h i z o c l o n i u m h i e r o g l y p h i c u m A l k a l i t r e a t m e n t , c e l l u l o s e , 178-187 A l k a l i n e degradation d e s c r i p t i o n , 272 h y d r o c e l l u l o s e , 280f s t r u c t u r e changes, 274-281 Alkaline peeling—See Peeling Allomorphs carbon-13 NMR s p e c t r a , 146-149 cellulose, 3 c e l l u l o s e I I f a m i l y , 142 CH^ s t r e t c h i n g bands, 141t Cladophora c e l l u l o s e , 105-106 g l u c o s e r i n g s t r e t c h i n g , I44t i n f r a r e d s p e c t r a , 138-146 irreversibility of cellulose I , 135-136 m a g n e t i c a l l y i n e q u i v a l e n t s i t e s , 110 n a t i v e c e l l u l o s e s , 88-116 OH s t r e t c h i n g bands, 1 3 8 , 1 4 1 t s p e c t r a changes from m e c h a n i c a l b e a t i n g , 97 Amorphous f r a c t i o n , c e l l u l o s e sample, 263 Amylose e f f e c t o f water on CP-MAS s p e c t r a , 124

Amylo se — C o n t i n u e d polymorphs, 189 Angular modulation frequency, FT-IR-PAS, 217 Anhydroglucose assignment o f c a r b o n - 1 3 r e s o n a n c e s , 95 n o n e q u i v a l e n c e a l o n g c h a i n s , 113 u n i t c e l l , 111 A n i s o t r o p i c b u l k magnetic

Anselme Payen, 2 A n t i p a r a l l e l model c e l l o t e t r a o s e , 82-83t,86 c e l l u l o s e I , 200 c e l l u l o s e I-ethylenediamine complexes, 207 c e l l u l o s e I I , 200-203 X-ray d i f f r a c t i o n p a t t e r n s f o r c e l l o t e t r a o s e , 76f,85f Attenuated t o t a l r e f l e c t a n c e , c e l l u l o s e t e x t i l e m a t e r i a l s , 214 A v a i l a b i l i t i e s , hydroxyl g r o u p s , 261-262

Β

Bacterial cellulose CP-MAS NMR s p e c t r a , 129f c r y s t a l l i n e s p e c t r a , 126 n a s c e n t m i c r o f i b r i l s , 149 Raman s p e c t r a , 1 5 8 , l 6 2 f s i z e o f the c r y s t a l l i t e s , 161-164 Band a s s i g n m e n t s , Raman s p e c t r o s c o p y , 161 Biosynthesis, cellulose, 4 Bromine s o r p t i o n , measurement o f a c c e s s i b i l i t y , 265 Browning's method, 94

C

Carbon-13 chemical s h i f t s cellulose, 297f See a l s o Chemical s h i f t s Carbon-13 l o n g i t u d i n a l r e l a x a t i o n time, observations, 102-106

305 In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

306

THE STRUCTURES OF CELLULOSE

Carbon-13 NMR s p e c t r a a l l o m o r p h s , 146-149 c e l l o t e t r a o s e , 63-64 c e l l u l o s e , 296 c e l l u l o s e d i s o r d e r i n g by a l k a l i t r e a t m e n t , 178 c e l l u l o s e resonance m u l t i p l i c i t i e s , 90-91 cotton linters, l80f,181f,182-184,I86f guanidonium h y d r o x i d e - t r e a t e d c e l l u l o s e , 184-185 h y d r o c e l l u l o s e , 275 hydrocellulose during a l k a l i n e degradation, 279f,280f i s o l a t i o n o f c r y s t a l l i n e c o r e , 100 methyl 0 - D - c e l l o t r i o s i d e , 63-64 n a t i v e c e l l u l o s e , 88-116,153 procedure i n a l k a l i treatmen s t u d y , 179 structural irreversibilit s t u d y , 138 viscose f i b e r , l80f Carbon-13 s p i n exchange Dante sequence, 94 h y d r o c e l l u l o s e from c o t t o n l i n t e r s , 110-111 C e l l symmetry ramie c e l l u l o s e , 18-19 See a l s o Symmetry Cellobiose charge d e n s i t i e s , 47-48 c o n f o r m a t i o n a l energy w e l l , 4 4 f c o n f o r m a t i o n s , 45 c r y s t a l l i n e state, 50f energy map, 45 numbering o f atoms and t o r s i o n a n g l e s , 39 p a c k i n g i n c r y s t a l s , 48 p r e p a r a t i o n i n s t r u c t u r a l s t u d y , 41 r e l a t i v e energies o f stable conformers, 4 5 t schematic r e p r e s e n t a t i o n , 3 9 f Cellodextrins conformations, 62f X-ray d i f f r a c t i o n , 39 Cellotetraose a n t i p a r a l l e l o r i e n t a t i o n , 63 computer model b u i l d i n g , 73 c r y s t a l data, 55t c r y s t a l l i z a t i o n , 42 f r a c t i o n a l c o o r d i n a t e s o f atoms, 6 0 t micrograph o f m i c r o c r y s t a l s , 5 7 f m o l e c u l a r f o r m u l a , 73 p r o j e c t i o n onto base p l a n e , 6 1 f Rietveld c r y s t a l structure application, 7 s i m i l a r i t y t o c e l l u l o s e I I , 68 s m a l l - a n g l e powder d i f f r a c t i o n , 7 1 f space group, 68-69,74 s t r u c t u r e , 39,59t,63 u n i t c e l l content, 6 l f

Cellotetraose—Continued wide-angle n e u t r o n powder diffractogram, 56f wide-angle X-ray powder d i f f r a c t i o n , 71f X-ray d i f f r a c t i o n d a t a , 55-58 Cellulose a c i d h y d r o l y s i s , 257 a l g a e , c r y s t a l l i n e composite h y p o t h e s i s , 93 a l k a l i t r e a t m e n t , 178-187 a l k a l i n e degradation p r o c e d u r e , 289-290 allomorphs, 3 amorphous, a l k a l i n e d e g r a d a t i o n , 281 a n t i p a r a l l e l models, 75 a v a i l a b i l i t y o f hydroxyl g r o u p s , 261-262 band assignment i v i b r a t i o n a l

c a r b o n - 1 3 c h e m i c a l s h i f t s , 297f c h a i n a x i s , o r i e n t a t i o n , 154 chains glycosidic l i n k a g e s , 8-9,185,286-287 m o b i l i t y i n hydrated s t a t e , 128-130 c h e m i c a l c h a r a c t e r i z a t i o n , 255-269 comparison o f s t r u c t u r a l t y p e s , 12 complexes f o r m a t i o n i n s t r u c t u r a l s t u d y , 204 X-ray s t u d i e s , 199-212 c o n f o r m a t i o n , 167 conformational s t a b i l i t y i n d e r i v a t i z a t i o n r e a c t i o n s , 10 c o n t r i b u t i o n s from c r y s t a l l i n e and n o n c r y s t a l l i n e r e g i o n s , 124-126 c o r r e l a t i o n s between c h e m i c a l s h i f t s and d i h e d r a l a n g l e s , 10 CP-MAS c a r b o n - 1 3 NMR s t u d y , 119-133 c r o s s - s e c t i o n s p e c t r a , 166 c r y s t a l s t r u c t u r e , 18,126-133,35 c r y s t a l l i n e a l l o m o r p h , 3,110-111,234 crystallinity d e t e r m i n a t i o n s , 255-257 c r y s t a l l i t e l e n g t h , 265 c r y s t a l l i z a t i o n i n m o l e c u l a r weight s t u d y , 190 crystals e l e c t r o n micrographs, 195f two-step growth p a t t e r n , 197 d e s c r i p t i o n , 119,234 d e u t e r a t e d , 142 d i f f r a c t o m e t r i c s t u d i e s , 5-7 d i s s o l u t i o n , 294,297f e f f e c t o f a c i d h y d r o l y s i s , 113-114 e f f e c t o f complexing agent, 211 esters, irreversible c o n v e r s i o n s , 136 e t h e r i f i c a t i o n s , 295-296 f a m i l i e s , c h a i n c o n f o r m a t i o n , 148

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

307

INDEX

Cellulose—Continued fiber c o m p o s i t i o n , 174 photoacoustic infrared s p e c t r a , 220f f i r s t recognized, 2 f o r m y l a t i o n , 257-258 frame r e l a x a t i o n t i m e s , 100 g l y c o s i d i c l i n k a g e t r e a t m e n t , 185 hydrogen bonds, 197,299f hydroxyl a c c e s s i b i l i t y measurement, 290 l a t e r a l o r d e r d i s t r i b u t i o n , 266 line positions after a l k a l i treatment, l 8 0 t low and h i g h temperature polymorphs, 190 m e r c e r i z e d , hydrogen bonds, 292-300 m i c r o c r y s t a l l i t e s , 251 microfibrils, biosynthesis mobility o f c r y s t a l chains morphology o f r e c r y s t a l l i z e d , 196-198 NMR s p e c t r a o f a l k a l i - t r e a t e d , 179-185 oligomers CP-MAS, 5 4 f s t r u c t u r e s , 58-65 o r g a n i c s o l v e n t s , 199 o r i e n t a t i o n , 164-167 p a r a l l e l c h a i n s , 200 p a r a l l e l - u p models, 75 polymorphy, 161-164,196 pore volume f r a c t i o n , 249t p r e p a r a t i o n i n CP-MAS carbon-13 NMR s t u d y , 130 recrystallized polymorphic and m o r p h o l o g i c a l a s p e c t s , 189-198 X-ray diagram, 193f r e g e n e r a t e d , hydrogen bonds, 292-300 resemblance t o mannan c r y s t a l s , 196-197 r e s i s t a n c e t o d i s r u p t i o n by a l k a l i , 295 SAXS s o u r c e , 245 s o l i d - s t a t e carbon-13 NMR s p e c t r a , 296-298 s o l i d - s t a t e s t r u c t u r a l changes, 187 s o l u b i l i t i e s i n SO -DEA-DMS0 system, 293-295 s p e c t r o s c o p y , 7-10 spectrum o f f u l l y d e u t e r a t e d , 158 s t r u c t u r a l models, 2,132f structure d e s c r i p t i o n , 1-12,234-235 determination d i f f i c u l t i e s , 2 f u t u r e d i r e c t i o n s , 11-12 m u l t i d i s c i p l i n a r y s t u d i e s , 10 q u e s t i o n s o f g e n e r a l i n t e r e s t , 19 Valonia ventricosa, 4 t e t r a m e r p a c k i n g , 75

Cellulose—Continued textile materials c h e m i c a l a d d i t i v e e x a m i n a t i o n , 230 FT-IR-PAS, 214-230 t h r e e f o l d h e l i c e s , 172 t r a n s f o r m a t i o n t o c e l l u l o s e I I , 135 t r a n s f o r m a t i o n t o N a - c e l l u l o s e , 171f treatment w i t h aqueous guanidonium h y d r o x i d e , 185 u n i f o r m i t y o f chemical a d d i t i v e s , 221-226 u n i t c e l l symmetry, 5 v i b r a t i o n a l spectrum, 155-161 X-ray d i f f r a c t i o n s t u d i e s , 15 Cellulose I base plane o f t h e u n i t c e l l , 16 c h a i n geometry, 136 chemical stopping i n a l k a l i n e

d i f f e r e n c e from c e l l u l o s e I I , 296 d i f f e r e n c e from N a - c e l l u l o s e I , 170 i n t r a c h a i n hydrogen bonds, 142 nonequivalent g l y c o s i d i c l i n k a g e s , 8 p a r a l l e l p a c k i n g , 137 p e e l i n g i n h i b i t i o n , 289 p o l a r i t y o f a d j a c e n t c h a i n s , 200 Raman s p e c t r o s c o p y , 8 refinements o f s t r u c t u r e s , 6 spectrum, 111 s t r u c t u r a l i r r e v e r s i b i l i t y , 135-149 s t r u c t u r e , 200,201f u n i t c e l l , I6,17f See a l s o N a t i v e c e l l u l o s e C e l l u l o s e 1-1,3-diaminopropane complex, s t r u c t u r e , 2 1 0 f C e l l u l o s e I - e t h y l e n e d i a m i n e complex, s t r u c t u r e , 207,208f Cellulose I I c e l l o t e t r a o s e s i m i l a r i t y , 68 c h a i n c o n f o r m a t i o n , 148 c h a i n geometry, 136 chemical stopping i n a l k a l i n e d e g r a d a t i o n , 284-285 conversion product o f N a - c e l l u l o s e I I B , 176 c r y s t a l s t r u c t u r e , 64 c r y s t a l s , e l e c t r o n micrographs, 1 9 3 f d i f f e r e n c e s from n a t i v e c e l l u l o s e , 164 d i f f r a c t i o n , 191 e f f e c t o f degree o f p o l y m e r i z a t i o n on morphology, 197 f a m i l y , p a r a l l e l d i c h r o i s m , 142-144 models, 63 p e e l i n g i n h i b i t i o n , 289 preparation i n i r r e v e r s i b i l i t y s t u d y , 137 Raman s p e c t r a , 154 spectra o f high c r y s t a l l i n i t y samples, 9

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

308

THE STRUCTURES OF CELLULOSE

Cellulose II—Continued s t a b i l i t y compared w i t h c e l l u l o s e I , 203 structural difference to cellulose I, 8 s t r u c t u r e , 38-65,202f u n i t c e l l , 200-203 C e l l u l o s e I I - h y d r a z i n e complex A, 2 1 2 f Cellulose I I I preparation i n i r r e v e r s i b i l i t y s t u d y , 137 u n i t c e l l , 148 C e l l u l o s e IV preparation i n i r r e v e r s i b i l i t y s t u d y , 137 X-ray d i f f r a c t i o n , 191,192 C e 1 l u l o s e - d iaminopropane complexes, u n i t c e l l s , 206t C e 1 l u l o s e - e t h y l e n e d iamine complex u n i t c e l l s , 206t C e 1 l u l o s e - h y d r a z i n e complexes c e l l s , 206t Cellulose triacetate, fibers, spectral c h a r a c t e r i s t i c s , 219 Cellulose t r i n i t r a t e I , saponified i n t o c e l l u l o s e I , 136 Chain a x e s , c e l l u l o s e I I , 192 Chain c l e a v a g e pseudo-first-order rate e x p r e s s i o n , 288 r a t e c o e f f i c i e n t s , 288t Chain c o n f o r m a t i o n c e l l u l o s e , I40f,l46 c e l l u l o s e I , 4,138-141,200 c e l l u l o s e I-ethylenediamine complex, 205 c e l l u l o s e I I , 4,138-141,148,200-203 c e l l u l o s e I I - h y d r a z i n e complex, 211 N a - c e l l u l o s e I , 170 ramie c e l l u l o s e , 18 u n i t c e l l , 142 use o f c a r b o n - 1 3 NMR s p e c t r a , 178 Chain packing N a - c e l l u l o s e I V , 172 p o l a r i t y o f c e l l u l o s e I and N a - c e l l u l o s e I , 170-172 Chain p o l a r i t y cellulose, 4 t r a n s f o r m a t i o n from c e l l u l o s e I t o N a - c e l l u l o s e , 170 Chain s t a c k i n g , c e l l u l o s e , 176 Chemical m i c r o s t r u c t u r a l a n a l y s i s , d e s c r i p t i o n , 258-262 Chemical s h i f t s a l l o m o r p h s , 146-148 c e l l u l o s e , dependence on p a c k i n g and hydrogen b o n d i n g , 122-124 c r y s t a l l i n e c e l l u l o s e , 298 See a l s o Carbon-13 c h e m i c a l s h i f t s Chemical s t o p p i n g a l k a l i n e degradation o f h y d r o c e l l u l o s e , 284-286

Chemical s t o p p i n g — C o n t i n u e d r a t e c o e f f i c i e n t s , 284t Chitin c e l l u l o s e I I complex s i m i l a r i t y , 211 s t r u c t u r e , 207 Cladophora c e l l u l o s e a l l o m o r p h s , 105-106,116 carbon-13 NMR s p e c t r a , 97-99,104f,111 c a n d i d a t e s f o r CP-MAS s p e c t r a , 112f changes i n l i n e s h a p e , 105 h y d r o l y s i s , 115 m u l t i p l e c r y s t a l l i n e forms, 100 See a l s o Cladophora g l o m e r a t a Cladophora g l o m e r a t a b e a t i n g o f a l g a l c e l l u l o s e , 94 s p e c t r a , 95-97 See a l s o Cladophora c e l l u l o s e Coefficient of variation,

d a t a , 4-7,21-25 Conformational a n a l y s i s , c e l l u l o s e I I s t r u c t u r a l s t u d y , 41-42,45-47 C o n f o r m a t i o n s , c e l l u l o s e s I and I I , 137 C o n t r a s t v a r i a t i o n , t e c h n i q u e , 235 Cotton a c c e s s i b i l i t y , 264t aggregate f r a c t a l s , 250-252 p e r m e a b i l i t y l i m i t s , 267t SAXS, 234 s t r u c t u r a l s i m i l a r i t y t o r a m i e , 16 t h e r m a l d i f f u s i o n l e n g t h , 227t volume a c c e s s i b l e t o w a t e r , 267t See a l s o G r e i g e c o t t o n Cotton c e l l u l o s e aggregate s t r u c t u r e , 251-252 CP-MAS carbon-13 NMR s p e c t r a , 124,125f,127f,131f growth p r o c e s s , 235 SAXS, 236-238 spectra c r y s t a l l i n e components, 126-133 n o n c r y s t a l l i n e components, 128-130 use, 214 WAXD, 238-245 Cotton f a b r i c s c a r b o n y l peak i n t e n s i t i e s , 226t d i s t r i b u t i o n o f the f i n i s h i n g a g e n t s , 224 f i n i s h i n g a g e n t s , 224 w r i n k l e r e c o v e r y p r o p e r t i e s , 224-226 Cotton f i b e r chemical m i c r o s t r u c t u r a l a n a l y s i s , 262 me t h i n e o r i e n t a t i o n , 166 Raman s p e c t r a , 154 Cotton h y d r o c e l l u l o s e CP-MAS s p e c t r a , 103f spectra as f u n c t i o n o f delay t i m e , 102

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

309

INDEX

Cotton l i n t e r s carbon-13 NMR s p e c t r a , 92f,179-185 c e l l u l o s e I WAXS p a t t e r n , 182 s p e c t r a l changes accompanying a c i d h y d r o l y s i s , 97 Cotton y a r n c a r b o n y l peak a f t e r s i z i n g , 223t,224t depth o f s i z i n g agent examined, 226-230 photoacoustic infrared s p e c t r a , 220f,222f,225f,228f,229f s i z i n g agent d e t e c t i o n , 219-221 C r o s s p o l a r i z a t i o n (CP), use i n NMR studies, 9 Cross-polarization-magic angle s p i n n i n g (CP-MAS) c e l l u l o s e oligomers, 54f c e l l u l o s e I , I47f c e l l u l o s e I I , 47-48,I48 c r y s t a l l i n e allomorphs, 92 measurements, 120 n a t i v e c e l l u l o s e s , 95-97 structural analysis o f c e l l u l o s e , 119-133 s o l i d c a r b o h y d r a t e s , 47 See a l s o Magic a n g l e s p i n n i n g (MAS) Crystal structure c e l l u l o s e , 18,126-133 n a t i v e c e l l u l o s e , 126 ramie c e l l u l o s e , 126 r e g e n e r a t e d c e l l u l o s e , 128 R i e t v e i d method, 68-86 Crystalline alkali-cellulose complexes, m e r c e r i z a t i o n i n t e r m e d i a t e s , 169-171 C r y s t a l l i n e c e l l u l o s e , chemical s h i f t s , 298 C r y s t a l l i n e r e g i o n s , d e f i n i t i o n , 263 Crystallization e f f e c t o f t e m p e r a t u r e , 189 measurement methods, 257 Cupra r a y o n , CP-MAS NMR spectra, 128,129f,131f C u r d l a n , c a r b o n - 1 3 NMR s p e c t r a , 296-298

D

Dante i r r a d i a t i o n , d e s c r i p t i o n , 93-94 Degree o f p o l y m e r i z a t i o n e f f e c t on c e l l u l o s e morphology, 197 h y d r o c e l l u l o s e , 273-274 n a t i v e c e l l u l o s e s , 293,294t r e l a t i o n s h i p to hydrocellulose weight a f t e r h y d r o l y s i s , 268f Depth p r o f i l i n g t e c h n i q u e , FT-IR-PAS, 226-230

Desizing e f f e c t i v e n e s s , 223 FT-IR-PAS, 221 Deuteration, c e l l u l o s e samples, 137-138 D i c h r o i s m , c o m p l i c a t i o n s , 155 Diffractometrie studies, c e l l u l o s e , 5-7 Dime t h y l o I d i h y d roxye t h y l e n e u r e a , s t r u c t u r e , 218 D i p o l a r c o u p l i n g , p r o t o n s , 100 Disaccharides carbon-13 c h e m i c a l s h i f t s , 124 c h e m i c a l s h i f t s and t o r s i o n a n g l e s , 122 p r e p a r a t i o n i n CP-MAS carbon-13 NMR s t u d y , 130 structures, 7

Ε

Electron density methyl 0 - D - c e l l o b i o s i d e i n c r y s t a l conformation, 49t r e l a t i o n s h i p t o c h e m i c a l s h i f t , 48 E l e c t r o n micrographs, c e l l u l o s e c r y s t a l s , 193f,195f Endgroups accessibility i n alkaline t r e a t m e n t , 283 a l k a l i n e degradation o f h y d r o c e l l u l o s e s , 287 a n a l y t i c a l d e t e r m i n a t i o n , 290 E q u a t o r i a l l a y e r l i n e , X-ray d i f f r a c t i o n data, 28t E t h e r i f i c a t i o n s , c e l l u l o s e , 295-296

F

F i b r i l s , a l g a l s p e c i e s , 95-97 Foam f i n i s h i n g t e c h n i q u e s , d e s c r i p t i o n , 224 F o r m y l a t i o n , c e l l u l o s e , 257-258 Fortisan cellulose II-hydrazine complex A, s t r u c t u r e , 209 Fourier transform infrared spectrometer, 216f Fourier transform infrared photoacoustic spectroscopy (FT-IR-PAS) advantages, 215 depth p r o f i l i n g , 226-230 d i s a d v a n t a g e s , 230 procedure i n t e x t i l e m a t e r i a l s t u d y , 218-219

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

310

THE STRUCTURES OF CELLULOSE

Fourier transform infrared photoacoustic spectroscopy ( FT-IR-PAS) — C o n t i n u e d scheme o f o p e r a t i o n , 215 See a l s o P h o t o a c o u s t i c s p e c t r o s c o p y Fractal analysis, cotton c e l l u l o s e , 233-252 Fractals c e l l u l o s e a g g r e g a t e s , 252 c h a r a c t e r i z a t i o n , 235

G

G l a u c o c y s t i s , m i c r o f i b r i l s , 203 Glucans, c a r b o n - 1 3 c h e m i c a l s h i f t s , 298t Glucomannan polymers, m o l e c u l a i n f l u e n c e on polymorphism /3-D-Glucose c a r b o n - 1 3 NMR s p e c t r a , 121f charge d e n s i t i e s , 47-48 Glucose r i n g s t r e t c h i n g , a l l o m o r p h s o f c e l l u l o s e , I44t 0-1,4-Glycosidic l i n k a g e c e l l u l o s e c h a i n s , 8-9 c l e a v a g e i n a l k a l i n e media, 286-289 d i s t r i b u t i o n s i n t o r s i o n a n g l e s , 130 stable conformations, 8 symmetry, 148 Gold c o l l o i d s , f r a c t a l s t r u c t u r e , 236 Greige c o t t o n Guinier p l o t s , 241f i s o i n t e n s i t y contour p l o t s , 239f,242f s c a t t e r i n g i n t e n s i t y d a t a , 240f See a l s o Cotton Guanidonium h y d r o x i d e t r e a t m e n t , c e l l u l o s e , 184-185

H

H a u s d o r f f dimension, 235 Higher p l a n t c e l l u l o s e s a l l o m o r p h s , 116 CP-MAS s p e c t r a , 9 6 f s i m i l a r i t y , 113 Hydrocellulose a l k a l i n e d e g r a d a t i o n , 280f amorphous and f i b r o u s s t r u c t u r e s , 274-281 carbon-13 NMR s p e c t r a d u r i n g d e g r a d a t i o n , 279f c e l l u l o s e I I c h a r a c t e r , 281 CP-MAS s p e c t r a , 104f,112f degree o f p o l y m e r i z a t i o n , 273-274 e f f e c t o f p h y s i c a l s t r u c t u r e on a l k a l i n e d e g r a d a t i o n , 272-290

Hydrocellulose—Continued f i b r o u s and amorphous p e e l i n g , 285 f i b r o u s changes d u r i n g t r e a t m e n t , 281 hydroxyl a c c e s s i b i l i t y during d e g r a d a t i o n , 276f l o n g i t u d i n a l r e l a x a t i o n t i m e , 102 preparation i n a l k a l i n e degradation s t u d y , 273 Raman s p e c t r a , 275,278f random c h a i n c l e a v a g e , 288 s p e c t r a , 101 X-ray d i f f r a c t o g r a m s , 275,277f Hydrocellulose I I Guinier p l o t s , 241f i s o i n t e n s i t y c o n t o u r p l o t s , 239f s c a t t e r i n g i n t e n s i t y i n SAXS, 245 s p e c i f i c i n n e r s u r f a c e , 250

c e l l u l o s e , 138-141,292-300 d i f f e r e n c e s among l a t t i c e p a c k i n g s , 300 u n i t c e l l , 11-12 Hydrolysis Cladophora g l o m e r a t a , 94-95 r e l a t i o n s h i p between r e s i d u e weight and t i m e , 259t See a l s o A c i d h y d r o l y s i s Hydroxyl a c c e s s i b i l i t y a l k a l i n e d e g r a d a t i o n , 274,287 hydrocellulose during a l k a l i n e d e g r a d a t i o n , 276f See a l s o A c c e s s i b i l i t y H y d r o x y l groups a v a i l a b i l i t i e s i n c e l l u l o s e , 262t hydrogen bonding i n f o r m a t i o n , 258-262 r e a c t i v i t y i n glucose residues o f c e l l u l o s e , 295-296

I I n c i d e n t l i g h t , plane o f p o l a r i z a t i o n i n Raman s p e c t r a , 155 Infrared spectroscopy, i n f r a r e d a b s o r p t i o n spectrum, 214 I n o s i t o l s , v i b r a t i o n a l s p e c t r a , 12 Intensity ratios, two-unit-cell model, 128 I n v a r i a n t v a l u e s , c e l l u l o s e , 247-249 I o d i n e s o r p t i o n , measurement o f a c c e s s i b i l i t y , 265 IR s p e c t r a , 1 3 8 , l 4 0 f , l 4 3 f L L a t e r a l order d i s t r i b u t i o n , c e l l u l o s e , 266

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

311

INDEX

L e v e l i n g - o f f degree o f p o l y m e r i z a t i o n , 265-266 Linked-Atom L e a s t - S q u a r e s (LALS) program cellotetraose, 55-58 m o l e c u l a r model g e n e r a t i o n , 43 use, 20,26t X-ray i n t e n s i t i e s , 200

M

Magic a n g l e s p i n n i n g (MAS) diagrams o f r o t o r s , 121f speeds i n n a t i v e c e l l u l o s e s t u d y , 93 use, 39 See a l s o C r o s s - p o l a r i z a t i o n - m a g i a n g l e s p i n n i n g (CP-MAS Mannan, polymorphism, 196 Mercerization crystalline alkali-cellulose complexes as i n t e r m e d i a t e s , 169-171 e f f e c t on pore volume f r a c t i o n , 249 mechanism, 202-203 t r a n s f o r m a t i o n s , 174 Mercerized c e l l u l o s e hydrogen bonding p a t t e r n s , 294 random c h a i n c l e a v a g e , 288 s i m i l a r i t y to regenerated c e l l u l o s e , 298 Mercerized cotton a c c e s s i b i l i t y , 264t c r y s t a l l i n e f r a c t i o n , 264 p e r m e a b i l i t y l i m i t s , 267t volume a c c e s s i b l e t o water, 267t M e r c e r i z e d ramie c o n f o r m a t i o n , 167 Raman s p e c t r a , I 6 2 f , l 6 4 Methyl /3-D-cellobioside charge d e n s i t i e s , 47-48 c r y s t a l l i n e state, 50f p a c k i n g i n c r y s t a l s , 48 r e l a t i v e energies o f stable conformers, 4 5 t Methyl /3-D-cellotrioside c r y s t a l data, 51t c r y s t a l s t r u c t u r e , 53f,63 d i f f r a c t i o n methods, 42-43 d i f f r a c t i o n p a t t e r n , 51 hexagonal c r y s t a l l i n e p l a t e l e t , 5 2 f model f o r c e l l u l o s e I I , 51 s t r u c t u r e , 63 X-ray d i f f r a c t i o n , 5 2 f Meyer-Misch c e l l , 33 Microfibrils c e l l u l o s e I , 174 d e f i n i t i o n , 234 M i l l e r i n d i c e s , 38 MM2 CARB program, 4 l , 4 6 t

M o i s t u r e r e g a i n , d e f i n i t i o n , 263 M o i s t u r e - r e g a i n t e c h n i q u e , measurement o f c e l l u l o s e d i s o r d e r , 256 Molecular conformations, c e l l u l o s e s I and I I , 12 M o l e c u l a r weight e f f e c t on i n t e r n a l water a c c e s s i b l e to s o l u t e , 268f e f f e c t on polymorphism, 189-198 Monomeric g e o m e t r i e s , e f f e c t i n r i g i d models, 2 7 t Monosaccharides carbon-13 c h e m i c a l s h i f t s , 124 p r e p a r a t i o n i n CP-MAS carbon-13 NMR s t u d y , 130 MULTAN program, u s e , 42

Na-cellulose, transition to celluloses I and I I , 185 Na-cellulose I a n t i p a r a l l e l s t r u c t u r e , 174 f i b r o u s o r i e n t a t i o n , 170 X-ray f i b e r d i f f r a c t i o n , 171f Na-cellulose IIB c r y s t a l s t r u c t u r e , 172 t h r e e f o l d h e l i c a l s t r u c t u r e , 174 u n i t c e l l , 173f X-ray f i b e r d i f f r a c t i o n , 171f N a - c e l l u l o s e IV c r y s t a l s t r u c t u r e , 172 u n i t c e l l , 173f X-ray f i b e r d i f f r a c t i o n , 171Γ Native c e l l u l o s e a l l o m o r p h s , 89,111,295 a n t i p a r a l l e l c h a i n arrangement, 4 carbon-13 NMR s p e c t r a , 9,95-97,153 c a t e g o r i e s o f Raman s p e c t r a , 158 crystal structure, 88-89,126,130-133,169,234 c r y s t a l l i n e polymorphy, 11,102 d i f f e r e n c e s o f c e l l u l o s e , 164 d i s t r i b u t i o n s i n t o r s i o n a n g l e s , 130 hydrogen bonds, 292-300 OH s t r e t c h i n g bands, 142 p e e l i n g , 272-273 preparation i n i r r e v e r s i b i l i t y study, 137 Raman s p e c t r a , 9 s t r u c t u r a l c o m p l e x i t y , 11 s u r f a c e l a y e r s o f the c r y s t a l l i t e s , 91 u n i t c e l l parameters, 90 See a l s o C e l l u l o s e I Near-neighbor s p e c t r a a l l o m o r p h c o n t r i b u t i o n s , 110 i s o l a t i o n , 106-113 s o u r c e , 106

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

312

THE STRUCTURES OF CELLULOSE

Near-surface a n a l y s i s , c e l l u l o s e fibers, 219-221 NMR—See a l s o C a r b o n - 1 3 NMR s p e c t r a 0

Oligomers p a c k i n g f e a t u r e s , 48 X-ray c r y s t a l l o g r a p h y , 63 Organic s o l v e n t s , c e l l u l o s e , 199

Ρ

Packing c e l l u l o s e I-ethylenediamin complexes, 207 c e l l u l o s e s I and I I , 154 ramie c e l l u l o s e , 1 8 , 2 9 , 3 U , 3 5 use o f c a r b o n - 1 3 NMR s p e c t r a , 178 See a l s o P a r a l l e l p a c k i n g P a r a l l e l dichroism, c e l l u l o s e I I f a m i l y , 142-144 P a r a l l e l - d o w n model d e s c r i p t i o n , 32t,34t See a l s o P a r a l l e l model Parallel-down structure, d e s c r i p t i o n , 18 P a r a l l e l model c e l l o t e t r a o s e , 80-81t,86 c e l l u l o s e I , 200 c e l l u l o s e I-ethylenediamine complexes, 207 d i f f r a c t i o n patterns f o r cellotetraose, 85f X-ray d i f f r a c t i o n p a t t e r n s f o r cellotetraose, 76f See a l s o P a r a l l e l - d o w n model P a r a l l e l packing c e l l u l o s e I , 137 See a l s o P a c k i n g P a r a l l e l - u p s t r u c t u r e , d e s c r i p t i o n , 18 Peeling pseudo-first-order rate expression, 281-283 rate c o e f f i c i e n t s , 283t r a t e s f o r h y d r o c e l l u l o s e , 285t reactions, a l k a l i n e degradation o f h y d r o c e l l u l o s e , 281-283 Periodate o x i d a t i o n , d e s c r i p t i o n , 258-262 Permeability l i m i t s c o t t o n , 2671 m e r c e r i z e d c o t t o n , 267t Perturbative Configuration Interaction w i t h L o c a l i z e d O r b i t a l s (PCILO) charge d e n s i t i e s , 47-48 d e s c r i p t i o n , 41 P h o t o a c o u s t i c sample c e l l u n i t , 2 1 6 f

Photoacoustic spectroscopy e f f e c t i v e s a m p l i n g d e p t h , 217 See a l s o F o u r i e r t r a n s f o r m i n f r a r e d photoacoustic spectroscopy (FT-IR-PAS) P h y s i c a l stopping d e s c r i p t i o n , 285 maximum number o f s i t e s , 286 p e e l i n g r e a c t i o n , 274 pseudo-first-order rate e q u a t i o n , 285-286 rate c o e f f i c i e n t s f o r h y d r o c e l l u l o s e , 286t r a t e f a c t o r s , 289 PITMOS, m o l e c u l a r d r a w i n g s , 43 Plant c e l l w a l l s , molecular o r g a n i z a t i o n , 151-152 P o l y e t h y l e n e , carbon-13 NMR c h e m i c a l

difficulty, 5 Polymers computer programs, 20 s p e c t r a o f nonrandomly o r i e n t e d , 154 structure analyses p r e c i s i o n , 2 Polymorphs c e l l u l o s e I , 153 c o m p o s i t i o n , 192f n e a r - n e i g h b o r spectrum r e s u l t s , 107 p a r a m e t e r s , 189 r e c r y s t a l l i z e d c e l l u l o s e , 189-198 Polysaccharides c r y s t a l l i z a t i o n b e h a v i o r , 189 s o l u b i l i t i e s , 292-300 Pore s i z e , methods f o r c e l l u l o s e samples, 250 Pore s t r u c t u r e , c h a r a c t e r i z a t i o n , 266-269 Pore volume f r a c t i o n , c e l l u l o s e , 249t Porod s law d e v i a t i o n , 246-247 SAXS, 246 P o t a s s i u m bromide p e l l e t method, c e l l u l o s e a n a l y s i s , 214 Power law b e h a v i o r , SAXS, 245 P r i n c i p l e o f economy, 6 Proton-spin d i f f u s i o n , o b s e r v a t i o n s , 100-102 P r o t o n s , d i p o l a r c o u p l i n g , 100 PS-79 program, 20,26t 1

R

R factors parallel-down chains, 23f SRRC, 2 2 t , 3 4 s y m m e t r i c a l models, 3 1 t X-ray d i f f r a c t i o n s t u d i e s , 2 0 - 2 1 See a l s o R v a l u e s

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

313

INDEX

R values c e l l o t e t r a o s e , 75 ramie c e l l u l o s e , 35 R i e t v e i d method, 72-73 w e i g h t i n g schemes, 21 See a l s o R f a c t o r s Raman microprobe e x p e r i m e n t s , 156f purpose, 151 Raman s p e c t r a band a s s i g n m e n t s , 153 c a t e g o r i e s f o r c e l l u l o s e , 166 c e l l u l o s e s , 151-167 h y d r o c e l l u l o s e , 275,278f,279f n a t i v e c e l l u l o s e s , 9,158 Valonia, I 6 l t Raman s p e c t r o s c o p y , 7 - 9 Ramie c e l l u l o s e c e l l symmetry, 18-19 c h a i n c o n f o r m a t i o n , 18 c r o s s - s e c t i o n a l Raman s p e c t r a c r y s t a l s t r u c t u r e , 126 d i f f e r e n c e s from V a l o n i a c e l l u l o s e , 161-164 monomeric geometry, 25 p a c k i n g , 18 Raman s p e c t r a , 155,161t,I62f s e l e c t i o n i n X-ray d i f f r a c t i o n s t u d y , 15-16 u n i t c e l l , 16 Ramie c e l l u l o s e I , X-ray d i f f r a c t i o n s t u d i e s , 15-36 Ramie c e l l u l o s e 1-1,3-diaminopropane complex, s t r u c t u r e , 209 Ramie c e l l u l o s e I - e t h y l e n e d i a m i n e , X-ray d i f f r a c t i o n s t u d i e s , 205-208 Ramie f i b e r , Raman s p e c t r a , 159f Rayon r e c r y s t a l l i z e d , 191 s o l u b i l i t y i n S0 -amine-organic s o l v e n t systems, 294 REFIN program, 72 Regenerated c e l l u l o s e , c r y s t a l s t r u c t u r e , 128 Resonance i n t e n s i t y emphasis i n c r y s t a l l i n e i n t e r i o r , 100 h y d r o c e l l u l o s e , 101 Rhizoclonium c e l l u l o s e CP-MAS s p e c t r a , 108f c r y s t a l l i n e l a t e r a l d i m e n s i o n s , 110 c r y s t a l l i n e polymorphism, 107-109 i s o l a t i o n o f near-neighbor s p e c t r a , 106-108 See a l s o R h i z o c l o n i u m h i e r o g l y p h i c u m Rhizoclonium hieroglyphicum s p e c t r a , 95-97,103f See a l s o A l g a l c e l l u l o s e , Rhizoclonium c e l l u l o s e R i e t v e i d method a n t i p a r a l l e l model f o r cellotetraose, 79f 2

Rietveid method—Continued cellotetraose, 68-86 d e s c r i p t i o n , 70-72 i m p o r t a n t f e a t u r e s , 72 p a r a l l e l model f o r cellotetraose, 77f t e r m i n a t i o n o f r e f i n e m e n t , 72 use, 69

S

Scan v e l o c i t y , F o u r i e r t r a n s f o r m i n f r a r e d spectrometer, 2 l 8 t Scanning t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s , f i b r i l l a r network o f n a t i v e c e l l u l o s e s , 99

progra d e s c r i p t i o n and use, 19-20 R v a l u e s , 34 R-value c a l c u l a t i o n , 25 Silica particles, fractal s t r u c t u r e , 236 S i n g l e - c r y s t a l d i f f r a c t i o n , u s e , 39 S i z i n g agent amount removed by d e s i z i n g , 223 degree o f p e n e t r a t i o n , 223 d e t e c t i o n by FT-IR-PAS, 219-221 warp y a r n s , 219 S m a l l - a n g l e X-ray s c a t t e r i n g (SAXS) c o t t o n c e l l u l o s e , 233-252 procedure i n c e l l u l o s e s t u d y , 237 s c a t t e r i n g source f o r c e l l u l o s e , 245 SO -d i e thy lamine -d ime thy 1 su 1 fox i d e (SO -DEA-DMSO), s o l u b i l i t i e s o f c e l l u l o s e , 292-300 Soda c e l l u l o s e , c o n v e r s i o n t o c e l l u l o s e I , 136 S o l i d - s t a t e NMR, use i n c e l l u l o s e I I s t r u c t u r a l s t u d y , 41 S o l u b i l i t i e s , p o l y s a c c h a r i d e s , 292-300 Solvent c r y s t a l l i z a t i o n , role i n polymorphism, 189 Sorption d e u t e r a t i o n method, 262-263 m o i s t u r e - r e g a i n method, 263-265 Southern R e g i o n a l Research Center (SRRC) program R f a c t o r s , 22t,34 R-value c a l c u l a t i o n s , 21 temperature f a c t o r s , 32t use, 20 X-ray d i f f r a c t i o n d a t a f o r ramie c e l l u l o s e , 26t,29 Space group c e l l o t e t r a o s e , 55,68-69 parameters, c e l l o t e t r a o s e s t r u c t u r a l s t u d y , 74-75 V a l o n i a c e l l u l o s e , 200

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

314

THE STRUCTURES OF CELLULOSE

S p e c i f i c inner surface, hydrocellulose I I and V a l o n i a , 250 S p e c t r a l s u b t r a c t i o n technique, a p p l i c a t i o n , 221 S p e c t r o s c o p y , c e l l u l o s e , 4-5,7-10 S t r u c t u r a l a n a l y s e s , problems, 2 Structural i r r e v e r s i b i l i t y , celluloses I and I I , 135-149 Structural levels, cellulose, 3 S t r u c t u r a l models, c e l l u l o s e , 2 Symmetry unit c e l l , 5 See a l s o C e l l symmetry, Twofold screw a x i s symmetry Τ

Temperature f a c t o r importance i n X-ray d i f f r a c t i o s t u d i e s , 31 SRRC program, 3 2 t Temperature o f c r y s t a l l i z a t i o n , e f f e c t on c e l l u l o s e I I - c e l l u l o s e IV r a t i o s , 191 Thermal d i f f u s i o n l e n g t h c o t t o n , 227t p h o t o a c o u s t i c s p e c t r o s c o p y , 217 Torsion angles d e t e r m i n a t i o n , 119 d i s t r i b u t i o n s about β-1,4-glycoside l i n k a g e s , 130 r e l a t i o n s h i p to chemical s h i f t s , 120-124 T w o - c r y s t a l model, i n t e n s i t y r a t i o s , 128 T w o - u n i t - c e l l model, i n t e n s i t y r a t i o s , 128 Twofold screw a x i s symmetry c e l l u l o s e , 18 coincidence with molecular chain axis, 6 v a l i d i t y o f assumption, 5 See a l s o Symmetry

U

Unit c e l l a l g a l c e l l u l o s e , 153 anhydroglucose s i t e s , 113 anhydroglucose u n i t s , 105,111 c e l l o t e t r a o s e , 69,86 c e l l u l o s e I , 17f,172,175f,200 c e l l u l o s e 1-1,3-diaminopropane complex, 209 c e l l u l o s e I - h y d r a z i n e complexes, 205 c e l l u l o s e I I , 200-203

Unit c e l l — C o n t i n u e d c e l l u l o s e II-hydrazine complex, 209-211 c e l l u l o s e I I I , 148 cellulose-diaminopropane complexes, 206t c e 1 l u l o s e - e t h y l e n e d iamine complexes, 206t e e l l u l o s e - h y d r a z i n e complexes, 206t c e l l u l o s e s I and I I , I 4 l , 1 7 5 f c h a i n c o n f o r m a t i o n s , 142 8 - c h a i n v e r s u s 2 - c h a i n , 36 conformation to c r y s t a l l o g r a p h i c c o n v e n t i o n , 35 c r y s t a l s t r u c t u r e , 115 hydrogen bonding p a t t e r n , 11-12 i n e q u i v a l e n c e s , 115 N a - c e l l u l o s e I I B , 173f 173f

s p e c t r a l l i n e s , 109 t e t r a m e r s o f c e l l o t e t r a o s e , 75-78 V a l o n i a , 91,154 X-ray d i f f r a c t i o n s t u d i e s , 16-18

V

Valonia cellulose c r y s t a l l i n e s p e c t r a o f d r y and h y d r a t e d , 126 Guinier p l o t s , 241f i s o i n t e n s i t y c o n t o u r p l o t s , 240f p r e p a r a t i o n i n SAXS s t u d y , 236-237 Raman s p e c t r a , 1 5 7 f , l 6 l - l 6 4 s c a t t e r i n g i n t e n s i t y i n SAXS, 245 s p e c i f i c i n n e r s u r f a c e , 250 s p e c t r a , 159f s t r u c t u r a l parameters, 246t transformation o f c e l l u l o s e I to I I , 138 X-ray d i f f r a c t i o n data f o r ramie c e l l u l o s e , 25 See a l s o V a l o n i a macrophysa, Valonia ventricosa V a l o n i a macrophysa a g g r e g a t e s , 155 carbon-13 NMR s p e c t r a , 95-97 See a l s o V a l o n i a c e l l u l o s e Valonia ventricosa carbon-13 NMR s p e c t r a , 95-97 cellulose structure, 4 d e s c r i p t i o n , 237 resonance m u l t i p l e t s , 91 X-ray i n t e n s i t y d a t a , 200 See a l s o V a l o n i a c e l l u l o s e V i b r a t i o n a l s p e c t r a , i n o s i t o l s , 12

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

315

INDEX

W

Warp y a r n s , s i z i n g a g e n t s , 219 Water e f f e c t on c o t t o n c e l l u l o s e , 124-126,128-130 s o r p t i o n e q u a t i o n , 264 W e i g h t i n g schemes, R v a l u e s , 21 Wide-angle X-ray d i f f r a c t i o n (WAXD) data, cotton c e l l u l o s e , 244f Wide-angle X-ray s c a t t e r i n g (WAXS) a l k a l i - t r e a t e d c e l l u l o s e , I82,l84t cotton l i n t e r s treated with a l k a l i , 184 procedure i n a l k a l i treatment s t u d y , 179 procedure i n c e l l u l o s e s t u d y , 238

X-ray d i f f r a c t i o n a b i l i t y t o d i s t i n g u i s h models, 32 c e l l o d e x t r i n s , 39 c e l l o t e t r a o s e , 56f,76f,78-86 c e l l u l o s e c r y s t a l l i t e s , 154,191 change i n d i s t a n c e due t o c h a i n r o t a t i o n , 33t c r y s t a l l i n e o l i g o m e r s , 48-50 dependence on models, 39 methyl 0 - D - c e l l o t r i o s i d e , 5 2 f N a - c e l l u l o s e , 170,171f procedure i n c e l l u l o s e complexes s t u d y , 204-205 structural irreversibility s t u d y , 138 use i n R i e t v e i d method, 69 X-ray d i f f r a c t o g r a m c e l l u l o s e I , 138,139f 138

X X-ray c r y s t a l l o g r a p h y ,

o l i g o m e r s , 63

complexes, 199-212

Production by Barbara J. Libengood Indexing by Keith B. Belton Jacket design by Caria L. Clemens Elements typeset by Hot Type Ltd., Washington, DC Printed and bound by Maple Press Co., York, PA

In The Structures of Cellulose; Atalla, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.