The Structures of Cellulose - American Chemical Society

Chapter 17

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

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

Downloaded by UNIV OF PITTSBURGH on February 10, 2015 | http://pubs.acs.org Publication Date: June 22, 1987 | doi: 10.1021/bk-1987-0340.ch017

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 pattern of dissolution of cellulose and related polysaccharides in the SO-amine-DMSO system 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. 2

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 . M e r c e r i z e d samples were p r e pared from t h e s e c e l l u l o s e s by s t i r r i n g i n 24% NaOH s o l u t i o n cont 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


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 d c e l l u l o s e samples p r e p a r e d from ramie, c o t t o n l i n t e r s and A v i c e l do n o t d i s s o l v e , even a f t e r 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 .


17.

ISOGAI ET AL.


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). In t h e l a t t e r c a s e , p a r t i a l l y s u b s t i t u t e d c e l l u l o s e e t h e r s such as m e t h y l - and c a r b o x y m e t h y l - c e l l u l o s e s were 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


296


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 o f a d i s p e r s i o n o f m i c e l l e s . I t seems t o us more l i k e l y t h a t some s t r o n g o r s t e r i c a l l y p r o t e c t e d i n t e r 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



ISOGAI ET AL.

1.

Figure


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


298


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-glucan ( 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

4.2

1.0 2.3

4.5

4.1 10.0

2.3 3.0

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


17.

ISOGAI ET AL.

Intra- and Intermolecular Hydrogen Bonds in Cellulose


Cell-(OH)

3

+

DMSO

3S0

+

2

R=H

3Et NR 2

or Et

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 .


299



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 e f f e c t , i t s i n a b i l i t y t o s o l u b i l i z e r e g e n e r a t e d c e l l u l o s e , o r any o f the o t h e r p o l y s a c c h a r i d e s c a n be a c c o u n t e d f o r . 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.


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, R.; Hirano, S. Proceedings of the Second International Conference on Chitin and Chitosan in 1982 (Japan), p. 72.

RECEIVED March 5, 1987


The Structures of Cellulose - American Chemical Society

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