The Structures of Cellulose - American Chemical Society

Chapter 6

Cross-Polarization-Magic Angle Spinning Carbon-13 NMR Approach to the Structural Analysis of Cellulose F. Horii, A. Hirai, and R. Kitamaru Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan

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CP/MAS C NMR approach has been described to characterize molecular chain conformations and crystal structures of native and regenerated cellulose samples in the dry and hydrated states. First, C isotropic chemical shifts in the solid state are correlated to torsion angles Фand ψ in the β-1,4-glycosidic linkage and χ about the exo-cyclic C5-C6 bond, respectively. On the other hand, the contributions of the crystalline and noncrystalline components to the CP/MAS C NMR results are critically analyzed in terms of the differ ences in C chemical shifts and spin-lattice relaxa tion times Τ and spectra of the respective components are recorded separately. Finally, the crystal struc ture and molecular chain conformations characteristic for native and regenerated celluloses are discussed. 13

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C e l l u l o s e i s a macromolecule o f D-glucoses which a r e j o i n e d by 3 l 4 - g l y c o s i d i c l i n k a g e s . S i n c e each g l u c o s e r e s i d u e adopts preferably the r i g i d C conformation, the molecular chain c o n f o r m a t i o n o f c e l l u l o s e i s s i m p l y d e f i n e d i n terms o f t o r s i o n a n g l e s φ and ψ about C l - 0 and 0-C4 bonds i n t h e β-l,4-glycosidic linkage(see Figure 5 ) . In a d d i t i o n , the o r i e n t a t i o n o f the side CH 0H group depends on t h e t o r s i o n a n g l e χ about t h e e x o - c y c l i c C5-C6 bond. T h e r e f o r e , i f t h e s e t o r s i o n a n g l e s a r e known by an a p p r o p r i a t e a n a l y z i n g t e c h n i q u e , we c a n d i r e c t l y d e s c r i b e t h e m o l e c u l a r c h a i n c o n f o r m a t i o n by u s i n g t h e s e parameters i n s t e a d o f average parameters such a s r a d i u s o f g y r a t i o n . -

f

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^ T h i s work i s a new approach t o d e t e r m i n e t h e t o r s i o n a n g l e s by ^^C i s o t r o p i c c h e m i c a l s h i f t s o b t a i n e d by s o l i d - s t a t e h i g h - r e s o l u t i o n C NMR s p e c t r o s c o p y and t o c h a r a c t e r i z e t h e c o m p l i c a t e d c h a i n c o n f o r m a t i o n o f c e l l u l o s ^ i n t h e 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 regions. However, t h e C c h e m i c a l s h i f t s o b t a i n e d i n t h e s o l i d s t a t e a r e a l s o i n f l u e n c e d by t h e i n t e r a t o m i c c o n t r i b u t i o n , s o - c a l l e d p a c k i n g e f f e c t , a s w e l l a s hydrogen b o n d i n g . T h e r e f o r e , we f i r s t show t h e r e l a t i o n s h i p s between C 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

0097-6156/87/0340-0119$06.00/0 © 1987 American Chemical Society

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i n c a s e s o f monosaccharides and d i s a c c h a r i d e s r e l a t i n g t o c e l l u l o s e and a l s o d i s c u s s t h e e f f e c t s o f t h e p a c k i n g and hydrogen bonding on the c h e m i c a l s h i f t s . ξ^» show t h a t c r o s s - p o l a r i z a t i o n / m a g i c a n g l e spinning(CP/MAS) C NMR s p e c t r a o f d r y and h y d r a t e d c e l l u l o s e s c o n t a i n t h e c o n t r i b u t i o n s o f t h e 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 components and t h a t t h e s p e c t r a o f bo^h components can be s e p a r a t e l y r e c o r d e d by u s i n g t h e d i f f e r e n c e i n C s p i n - l a t t i c e r e l a x a t i o n time Τ . F i n a l l y , we d i s c u s s t h e m o l e c u l a r c h a i n c o n f o r m a t i o n s and 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 on t h e b a s i s o f t h e s e r e s u l t s . Ν

w

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Experimental Samples. C e l l u l o s e s , m o n o s a c c h a r i d e s , and d i s a c c h a r i d e s used i n t h i s work were p r e p a r e d by t h e methods d e s c r i b e d elsewhere(.1-.3). One p a r t of each c e l l u l o s e sample was w e l l d r i e d a t 50 C under vacuum f o r 2-3 days b e f o r e and a f t e r p a c k i n g i n a c o n v e n t i o n a l MAS r o t o r . Another p a r t o f each d r i e d sample was exposed t o atmospheres o f d i f f e r e n t r e l a t i v e h u m i d i t i e s a t 20 C f o r a week, packed i n a MAS r o t o r w i t h an O - r i n g s e a l which i s shown below, and t h e n a l l o w e d t o e q u i l i b r a t e i n the same atmospheres f o r a few more d a y s ( £ ) . Some samples were soaked i n d e i o n i z e d water a t room t e m p e r a t u r e f o r 24 h r and t h e n packed i n t h e r o t o r s . 1 3

1 3

CP/MAS C NMR Measurements. CP/MAS C NMR e x p e r i m e n t s were performed a t room t e m p e r a t u r e by a JEOL JNM-FX200 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 CP/MAS u n i t o p e r a t i n g a t 4.7 T. The HartmançH a h n ^ a t c h e d f i e l d s t r e n g t h γΒ^/2τ\ o f 69 kHz was a p p l i e d t o C a n d ^ H n u c l e i and t h e H r f s t r e n g t h was r e d u c e d t o 54 kHz d u r i n g HC d i p o l a r d e c o u p l i n g . Pulse w a i t i n g time a f t e r the a c q u i s i t i o n o£ an FID was 10-15 s, which was a t l e a s t more t h a n 5 times o f the Η f o r each sample. M a g i c - a n g l e s p i n n i n g was c a r r i e d o u r a t a r a t e o f 3.2-3.5 kHz f o r b o t h d r y and h y d r a t e d samples. In o r d e r t o keep water i n t h e r o t o r d u r i n g sample s p i n n i n g a t such a h i g h r a t e , we have d e v e l o p e d a new r o t o r w i t h an O - r i n g s e a l , which i s shown i n F i g u r e 1, by m o d i f y i n g a conventional b u l l e t - t y p e r o t o r of p o l y ( c h l o r o t r i f l u o r o e t h y l e n e ) (4,_5). T h i s t y p e o f r o t o r can be s t e a d i l y r o t a t e d w i t h o u t p r a c t i c a l l o s s o f water f o r samples w i t h any water c o n t e n t . Such Iji^gh performance i s so enough even f o r one-week measurements t h a t C v a l u e s , which a r e n o r m a l l y o f t h e o r d e r o f 10-1000 s f o j c r y s t a l l i n e components o f polymers, a r e a b l e t o be o b t a i n e d . C c h e m i c a l s h i f t s r e l a t i v e t o t e t r a m e t h y l s i l a n e ( M e ^ S i ) were d e t e r m i n e d u s i n g a narrow c r y s t a l l i n e l i n e a t 33.6 ppm o f a s m a l l c h i p o f l i n e a r p o l y e t h y l e n e i n s e r t e d i n each sample. 3

R e s u l t s and 1.

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C

Discussion

Chemical S h i f t s - T o r s i o n Angles R e l a t i o n s h i p s . 1 3

F i g u r e 2 shows CP/MAS C NMR s p e c t r a o f a ~ D - g l u c o s e , a ~ D - g l u c o s e » H 0 , and β-D-glucose c r y s t a l s t o g e t h e r w i t h t h e spectrum o f D-glucose i n D 0(_2 ). S i n c e a ~ and β-anomers c o e x i s t i n s o l u t i o n , many r e s o n a n c e l i n e s c o r r e s p o n d i n g t o b o t h anomers can be obs*erved(6). On t h e o t h e r hand, each c r y s t a l spectrum c o n t a i n s a group o f r e s o n a n c e l i n e s c o r r e s p o n d i n g t o t h o s e o f 2

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CP-MAS C NMR Approach to Cellulose

rVVi (b)

(a) Figure 1

Schematic diagrams o f MAS r o t o r s . (a) c o n v e n t i o n a l r o t o r ; (b) newly d e v e l o p e d r o t o r w i t h an O - r i n g s e a l . (Reproduced from Ref.4. C o p y r i g h t 1985 Academia R e p u b l i c i i S o c i a l i s t e Romania.)

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Figure 2

60 ppm from TMS

13 25 MHz CP/MAS and s c a l a r - d e c o u p l e d C NMR s p e c t r a o f D-glucoses. A:0l-D-glucose c r y s t a l , B: a-Dglucose^HÔ c r y s t a l , C: D-glucose i n DÔ s o l u t i o n , D: β-D-glucose.

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e i t h e r anomer as shown by broken l i n e s i n F i g u r e 2, where t h e r e s p e c t i v e l i n e s o f each c r y s t a l were a s s i g n e d a c c o r d i n g t o P f e f f e r et al(7#8). I t has t h e r e f o r e been found t h a t each c r y s t a l i s composed o f e i t h e r pure anomer. The C6 l i n e o f t h e a ~ D - g l u c o s e c r y s t a l undergoes a l a r g e d o w n f i e l d s h i f t o f 2.4-2.9 ppm i n comparison t o C6 l i n e s o f t h e α-D-glucose· H 0 as w e l l as t h e β-D-glucose c r y s t a l and t h e i r s o l u t i o n . A c c o r d i n g t o x - r a y analyses(9~.11), t h e c o n f o r m a t i o n s o f t h e CH OH groups about t h e C5-C6 bond a r e g a u c h e - t r a n s f o r t h e Ot-D-glucose and gauche-gauche f o r t h e a~D-glucose»H 0 and β-D-glucose, r e s p e c t i v e l y . Here, f o r example, g a u c h e - t r a n s i n d i c a t e s t h a t t h e C6-06 bond i s gauche t o t h e C5-05 bond and t r a n s t o t h e C4-C5 bond. I t i s , t h e r e f o r e , assumed t h a t t h e l a r g e d o w n f i e l d s h i f t a p p e a r i n g i n t h e C6 l i n e o f t h e a ~ D - g l u c o s e c r y s t a l i s c o r r e l a t e d t o the gauche-trans conformation, while the chemical s h i f t s of the other c r y s t a l s i s c o r r e l a t e d t o t h e gauche-gauche c o n f o r m a t i o n . 2

In o r d e r t o v e r i f y t h i s assumption, t h e c h e m i c a l s h i f t s o f t h e C6 carbons o f g l u c o s e s and g l u c o s e r e s i d u e s o f d i f f e r e n t d i s a c c h a r i d e s a r e p l o t t e d i n F i g u r e 3(2^_3#.12) a g a i n s t t o r s i o n a n g l e s χ about t h e C5-C6 bonds which were o b t a i n e d by x - r a y c r y s t a l a n a l y s e s ( 9 - 1 1 ) . I t i s known from a r e c e n t s u r v e y ( 1 3 ) t h a t t h e CH 0H groups o f l o w - m o l e c u l a r - w e i g h t g l u c o s i d e s p r e f e r a b l y adopt two c o n f o r m a t i o n s o f gauche-gauche and g a u c h e - t r a n s . The o r i e n t a t i o n o f t h e t r a n s - g a u c h e must be s t e r i c a l l y h i n d e r e d due t o t h e i n t r a m o l e c u l a r i n t e r a c t i o n between two OH groups which a r e c o n n e c t e d t o t h e C4 and C6 c a r b o n s . T h e r e f o r e , t h e r e s u l t o f c e l l u l o s e I c r y s t a l which i s d e s c r i b e d l a t e r i s a l s o p l o t t e d , because t h i s i s t h e o n l y one case o f t h e trans-gauche conformation. In n a t i v e c e l l u l o s e t h i s c o n f o r m a t i o n may be p o s s i b l e owing t o hydrogen bonds a s s o c i a t e d w i t h t h e CH OH group. As seen i n F i g u r e 3, t h e c h e m i c a l s h i f t f o r t h i s u n f a v o r a b l e c o n f o r m a t i o n i s v e r y l a r g e compared t o t h e v a l u e s f o r t h e o t h e r conformations. As a r e s u l t , t h e r e seems t o e x i s t a s i m p l e l i n e a r r e l a t i o n s h i p between t h e 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 χ, a l t h o u g h t h e c h e m i c a l s h i f t must be a b r u p t l y d e c r e a s e d f o r t o r s i o n a n g l e s c l o s e t o 360 . According to these r e s u l t s , three p r e f e r r e d c o n f o r m a t i o n s , gauche-gauche, g a u c h e - t r a n s , and t r a n s - g a u c h e , a r e w e l l c o r r e l a t e d t o t h e c h e m i c a l s h i f t s o f 61-62 ppm, 62.7-64.5 ppm, and about 66 ppm, r e s p e c t i v e l y . ^ 2

F i g u r e 4 shows s i m i l a r r e l a t i o n s h i p s between C chemical s h i f t s o f CI and C4 l i n e s and t o r s i o n a n g l e s φ and ψ i n d i f f e r e n t d i s a c c h a r i d e s c o n t a i n i n g β-l,4-glycosidic l i n k a g e s , r e s p e c t i v e l y (3,12^). The c h e m i c a l s h i f t s o f t h e c r y s t a l l i n e components o f c e l l u l o s e samples which a r e shown l a t e r a r e a l s o p l o t t e d a g a i n s t t h e c o r r e s p o n d i n g φ and ψ v a l u e s . In t h e c e l l u l o s e s two o r t h r e e d i f f e r e n t c h e m i c a l s h i f t s o b t a i n e d a r e p l o t t e d a g a i n s t t h e same φ and ψ v a l u e s e x c e p t f o r ramie, because t h e s i n g l e v a l u e s o f φ and ψ have been r e p o r t e d by x - r a y c r y s t a l a n a l y s e s ( 1 4 - 1 7 ) i r r e s p e c t i v e o f t h e o b s e r v a t i o n of the m u l t i p l e chemical s h i f t s . Although the data are somewhat s c a t t e r e d , t h e c h e m i c a l s h i f t s o f CI and C4 carbons can be a l s o w e l l c o r r e l a t e d t o t h e t o r s i o n a n g l e s φ and ψ, r e s p e c t i v e l y . The o r i g i n o f t h e s c a t t e r i n g o f t h e d a t a seen i n F i g u r e s 3 a j ^ 4 i s not c l e a r a t p r e s e n t . However, as d e s c r i b e d i n I n t r o d u c t i o n , C c h e m i c a l s h i f t s a l s o depend on t h e e f f e c t s o f p a c k i n g and hydrogen bonding. F o r example, t h e resonance l i n e o f c e n t r a l CH carbons o f t r i c l i n i c n-C Η c r y s t a l s i s s h i f t e d about 1.3 ppm d o w n f i e l d 2

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120 240 Χ / degree Figure 3

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C c h e m i c a l s h i f t s o f C6 c a r b o n s v s t o r s i o n a n g l e s χ. a: α - D - g l u c o s e , b a - D - g l u c o s e * H 0 , c : β-D-glucose, d:|3-D-cellobiose, e a - D - l a c t o s e H 0 , f : β-D-lactose, α - g l u c o s e and α - g l u c o s e r e s i d u e s , g: c e l l u l o s e I . ο • :i3-glucose and β-glucose r e s i d u e s . (Reproduced from r e f . 3. C o p y r i g h t 1984 American C h e m c i a l S o c i e t y 2

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-AO

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0

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Ψ or Φ /degree Figure 4

13,.

c h e m i c a l s h i f t s o f CI and C4 carbons v s . t o r s i o n a n g l e s φ and ψ . 5 - c e l l o b i o s e , b: β-methyl c e l l o b i o s i d e ^ C H ^ O H , c: a - l a c t o s e H 0, d: β-lactose, e: c e l l u l o s e I ( R e f . l 4 ) , f : c e l l u l o s e I ( R e f . l 6 ) , g: c e l l u l o s e I I ( R e f . l 5 ) , h: c e l l u l o s e I I ( R e f . l 7 ) . (Reproduced from r e f . 12. C o p y r i g h t 1983 I n t e r n a t i o n a l S o c i e t y o f M a g n e t i c Resonance.) e

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compared t o t h e c o r r e s p o n d i n g l i n e s o f η-paraffins w i t h o t h e r c r y s t a l f o r m s ( 1 8 ) , a l t h o u g h the m o l e c l a r c h a i n c o n f o r m a t i o n s a r e almost t h e same i n t h e s e c r y s t a l s . Moreover, s i g n i f i c a n t d o w n f i e l d s h i f t s can be o b s e r v e d f o r carbons c h e m i c a l l y bonded t o h y d r o x y l groups which a r e f o r m i n g hydrogen bonds, when the 0···0 d i s t a n c e i n t h e hydrogen bond i s l e s s t h a n about 2.7 A(19,20). We have e s t i m a t e d t h e s e two e f f e c t s i n the case o f c e l l u l o s e by comparing t h e C c h e m i c a l s h i f t s i n t h e s o l i d s t a t e and t h e c o r r e s p o n d i n g v a l u e s i n s o l u t i o n f o r monosaccharides and d i s a c c h a r i d e s r e l a t i n g t o c e l l u l o s e ( _3) · Here, no comparison was made f o r carbons h a v i n g c o n f o r m a t i o n a l freedom t o n e g l e c t t h e e f f e c t s o f the d i f f e r e n c e s i n c o n f o r m a t i o n between the two s t a t e s . As a r e s u l t , i t has been found t h a t the d i f f e r e n c e Δδ i n c h e m i c a l s h i f t between t h e s o l i d and d i s s o l v e d s t a t e s ranges from -1.51 ppm t o 2.43 ppm, b e i n g independent on the 0···0 d i s t a n c e ( 3 ) . A l t h o u g h such an e x t e n t o f Δδ i s thought as a r e s u l t o f the superposed e f f e c t s o f p a c k i n g and hydrogen bonding, t h e s e e f f e c t s can be e s t i m a t e d t o produce a d o w n f i e l d o r u p f i e l d s h i f t l e s s than a t most 2.5 ppm i n c e l l u l o s e . 2.

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 . I t has been w i d e l y a c c e p t e d t h a t c e l l u l o s e samples c o n t a i n the n o n c r y s t a l l i n e component t o g e t h e r w i t h t h e c r y s t a l l i n e component r e g a r d l e s s o f the s o u r c e o f the samples, a l t h o u g h t h e f r a c t i o n s o f the components d e t e r m i n e d by one method a r e not always i n a c c o r d w i t h t h o s e e s t i m a t e d by a n o t h e r t e c h n i q u e . I t i s t h e r e f o r e very important t o f i r s t examine the c o n t r i b u t i o n s from the two components t o CP/MAS NMR results. F i g u r e 5 shows 50 MHz CP/MAS C NMR s p e c t r a o f c o t t o n c e l l u l o s e w i t h d i f f e r e n t water c o n t e n t s . As p r e v i o u s l y d e s c r i b e d f o r t h e d r y sample shown i n F i g u r e 5a(_l,_3)# the r e s p e c t i v e resonance l i n e s a r e a s s i g n e d t o C l , C4, and C6 carbons from t h e d o w n f i e l d s i d e e x c e p t f o r the c l u s t e r o f resonances a t 80-70 ppm which b e l o n g t o C2, C3, and C5 carbons(21-23). Of t h e s e resonances the C4 and C6 s p l i t i n t o two components, s h a r p d o w n f i e l d and broad u p f i e l d components. These s h a r p and broad components have been a s s i g n e d t o the 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 components, r e s p e c t i v e l y , by comparing the f r a c t i o n s of t h e former component w i t h the degrees o f c r y s t a l l i n i t y d e t e r m i n e d by x - r a y a n a l y s i s f o r samples w i t h w i d e l y d i f f e r e n t c r y s t a l l i n i tiesQ,^). A l t h o u g h such two components cannot be e x p l i c i t l y o b s e r v e d i n the CI l i n e , t h e i r e x i s t e n c e w i l l become c l e a r l a t e r . As seen i n F i g u r e 5 ( 4 ) , each resonance l i n e e v i d e n t l y narrows w i t h i n c r e a s i n g water c o n t e n t , r e s u l t i n g i n c l e a r e r s p l i t t i n g i n t o the 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 components o f the C4 c a r b o n . A s i m i l a r b e t t e r s p l i t t i n g i n t o the two components can be a l s o r e c o g n i z e d i n the C6 l i n e as a r e s u l t o f an u p f i e l d s h i f t o f the noncrystalline line. In a d d i t i o n , a f i n e s p l i t t i n g o f t h e CI resonance i s more c l e a r l y o b s e r v e d w i t h i n c r e a s i n g water c o n t e n t , a l t h o u g h t h i s m u l t i p l i c i t y a r i s e s from b o t h 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 components as d i s c u s s e d l a t e r . S i m i l a r e f f e c t s of water on CP/MAS C s p e c t r a have a l s o been found f o r c e l l u l o s e ( 2 4 ) , (l-3)-3-D-glucans(25) and amylose(5,26). In o r d e r t o know the c o n t r i b u t i o n s o f t h e 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 i n d e t a i l , we have measured C Τ values u s i n g a p u l s e sequence as d e v e l o p e d by T o r c h i a ( 2 2 ) . As a r e s u l t , i t has been found t h a t each resonance l i n e c o n t a i n s two components w i t h r

6.

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ET AL.

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125

6

ppm Figure

5

1 3

50 MHz CP/MAS C NMR s p e c t r a o f c o t t o n c e l l u l o s e w i t h d i f f e r e n t water c o n t e n t s . (a) 0%; (b) 4.2 %; (c) 8.4%; (d) 19.1%; (e) 161%. (Reproduced from R e f . 4 . C o p y r i g h t 1985 Academia R e p u b l i c i i S o c i a l i s t e Romania.)

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Τ s o f 5-35 s and 109-131 s f o r the d r y c o t t o n and 7-18 s and 107-170 s f o r the c o t t o n soaked i n HÔ, which c o r r e s p o n d t o the n o n c r y s t a l l i n e and c r y s t a l l i n e components, r e s p e c t i v e l y (3,4,28). Two Τ v a l u e s have a l s o been o b t a i n e d f o r c e l l u l o s e by Teeaar an Lippmaa(29), a l t h o u g h t h e i r v a l u e s f o r the c r y s t a l l i n e component are r a t h e r longer than ours. On the b a s i s o f t h e s e d i f f e r e n t Τ v a l u e s , we have r e c o r d e d the s p e c t r a o f the 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 components o f c e l l u l o s e s e p a r a t e l y ( 3 , 4 , 2 8 ) . F i g u r e 6b shows t h e spectrum o f the c r y s t a l l i n e component o f c o t t o n soaked i n HÔiwater content=161%), which was o b t a i n e d by T o r c h i a ' s p u l s e sequence(27,28). The d e l a y time between two ΤΓ/2 p u l s e s i n the p u l s e sequence was s e t t o be 100 s. As i s c l e a r l y seen, t h e spectrum shown i n F i g u r e 6b r e f l e c t s the components c o r r e s p o n d i n g t o the d o w n f i e l d s h a r p l i n e s o f C4 and C6 carbons i n the whole spectrum shown i n F i g u r e 6a. A similar crystalline spectrum was o b t a i n e d by o t h e r s ( 2 9 ) u s i n g a l m o s t the same t e c h n i q u e . On the o t h e r hand, F i g u r e 6c i n d i c a t e s the spectrum o f the n o n c r y s t a l l i n e component o f the c o t t o n c e l l u l o s e , which was o b t a i n e d by s u b t r a c t i n g the spectrum o f the c r y s t a l l i n e component shown i n F i g u r e 6b from the whole spectrum shown i n F i g u r e 6a. T h i s spectrum e v i d e n t l y c o r r e s p o n d s t o the components a s s o c i a t e d w i t h the u p f i e l d b r o a d r e s o n a n c e s o f C4 and C6 c a r b o n s . S i m i l a r r e c o r d i n g of the r e s p e c t i v e s p e c t r a were p o s s i b l e f o r o t h e r n a t i v e c e l l u l o s e s such as ramie, b a c t e r i a l , and v a l o n i a c e l l u l o s e s as w e l l as r e g e n e r a t e d c e l l u l o s e s . 3.

C r y s t a l S t r u c t u r e and M o l e c u l a r C h a i n C o n f o r m a t i o n F i g u r e 7 shows the s p e c t r a o f the c r y s t a l l i n e components o f c o t t o n c e l l u l o s e s w i t h the water c o n t e n t s o f 0% and 161%, which were o b t a i n e d by the method d e s c r i b e d i n the p r e v i o u s s e c t i o n ( 4 ) . The m u l t i p l e t o f the CI resonance i s c l e a r l y seen i n t h e s e s p e c t r a ; i n the d r y s t a t e two n o n e q u i v a l e n t l i n e s seem t o c o n s t i t u t e t h i s resonance but they s p l i t i n t o one d o u b l e t and one s m a l l s i n g l e t c e n t e r e d a t the d o u b l e t i n the h y d r a t e d form. Moreover, C4 and C6 resonances t e n d t o s p l i t i n t o a t r i p l e t and a d o u b l e t , r e s p e c t i v e l y . Almost the same s p e c t r a were o b t a i n e d f o r ramie c e l l u l o s e i n b o t h d r y and h y d r a t e d forms. The 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 b a c t e r i a l c e l l u l o s e s a r e shown i n F i g u r e 8 ( 3 0 , 3 J J . In t h i s case the e f f e c t o f water i s a l s o prominent and b o t h CI and C4 resonances s p l i t i n t o d i f f e r e n t t y p e s o f t r i p l e t s from t h o s e o f c o t t o n and ramie c e l l u l o s e s . Almost the same f e a t u r e s o f s p e c t r a i n c l u d i n g CI and C4 f i n e s p l i t t i n g s have a l s o been o b s e r v e d f o r v a l o n i a c e l l u l o s e . A l t h o u g h i t i s v e r y d i f f i c u l t a t p r e s e n t t o d e t e r m i n e whether such f i n e s p l i t t i n g s w i t h i n 1-2 ppm a r e due t o the d i f f e r e n c e i n t o r s i o n a n g l e s , p a c k i n g , o r hydrogen bonding, they a r e c l o s e l y a s s o c i a t e d w i t h c r y s t a l l o g r a p h i c a l i n e q u i v a l e n c e s o f the g l u c o s e r e s i d u e s . I t i s t h e r e f o r e concluded t h a t the c r y s t a l s t r u c t u r e o f c o t t o n and ramie c e l l u l o s e s i s d i f f e r e n t from t h a t o f b a c t e r i a l and v a l o n i a c e l l u l o s e s , c o n t r a r y t o the c l a i m ( JL4,16) t h a t a l l n a t i v e c e l l u l o s e s have the same c r y s t a l structure, c e l l u l o s e I. A t a l l a and VanderHart(_32,_33) have proposed two c r y s t a l l i n e forms f o r c e l l u l o s e I, which they name I and Ιβ, t o e x p l a i n the m u l t i p l i c i t i e s o f CI and C4 l i n e s . Analogous t o t h e i r work, C a e l e t al.(_34) have c o n c l u d e d t h a t the s p e c t r a o f d i f f e r e n t n a t i v e

6. HORII ET AL.

127

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C2,3,5

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120

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13. 50 MHz CP/MAS C NMR s p e c t r a o f c o t t o n c e l l u l o s e w i t h the water c o n t e n t o f 161%. (a) whole spectrum, (b) c r y s t a l l i n e component, ( c ) n o n c r y s t a l l i n e component.

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13 CP/MAS C NMR s p e c t r a o f t h e c r y s t a l l i n e components o f c o t t o n c e l l u l o s e w i t h t h e water c o n t e n t s o f 0% (a) and 161% ( b ) . (Reproduced from R e f . 4 . C o p y r i g h t 1985 A c a demia R e p u b l i c i i S o c i a l i s t e Romania.)

128


c e l l u l o s e s a r e l i n e a r c o m b i n a t i o n s o f two s p e c t r a . In c o n t r a s t t o t h e view o f A t a l l a and VanderHart, however, t h e y have a t t r i b u t e d these s p e c t r a t o the resonances of the glucose r e s i d u e s i n the two-chain and e i g h t - c h a i n u n i t c e l l r e g i o n s o f c e l l u l o s e . According t o t h e i r models, r e l a t i v e i n t e n s i t i e s i n t h e m u l t i p l e t s o f CI and C4 l i n e s can be c a l c u l a t e d ; f o r example, t h e p r e d i c t e d i n t e n s i t y r a t i o s i n t h e CI t r i p l e t a r e ( 1 - f ) : 2 f : ( l - f ) f o r t h e t w o - c r y s t a l model and ( 2 - f • ) : 2 f : ( 2 - f ) f o r t h e t w o - u n i t - c e l l model, r e s p e c t i v e l y . Here, f and f denote t h e f r a c t i o n s o f t h e I c r y s t a l and t h e e i g h t - c h a i n unit c e l l , respectively. In an attempt t o examine t h e v a l i d i t y o f t h e s e models, we e s t i m a t e d t h e i n t e g r a t e d i n t e n s i t y r a t i o s o f t h e CI and C4 l i n e s by a n a l y z i n g t h e m u l t i p l e t s i n terms o f t h r e e L o r e n t z i a n c u r v e s by a computer(30). In t h i s a n a l y s i s such L o r e n t z i a n assumptions were most r e a s o n a b l e . As a r e s u l t , i t has been found t h a t i n many c a s e s t h e e x p e r i m e n t a l r a t i o s a r e not i n a c c o r d w i t h t h o s e p r e d i c t e d by e i t h e r model. Thus, a new model t o f i t t h e e x p e r i m e n t a l r e s u l t s s h o u l d be c o n s t r u c t e d . I t s h o u l d be here emphasized t h a t f o r t h e d e t a i l e d a n a l y s i s o f t h e m u l t i p l e t s t h e c o n t r i b u t i o n from t h e n o n c r y s t a l l i n e component must be removed p r e c i s e l y l i k e our c a s e ; t h i s c o n t r i b u t i o n was not c o n s i d e r e d i n t h e former two c a s e s (32-34). 1

1

1

a

Analogous t o d i f f e r e n t f i n e s t r u c t u r e s i n CI and C4 r e s o n a n c e s , t h e c r y s t a l l i n e l i n e s o f C6 carbons e x h i b i t a d o u b l e t f o r c o t t o n and ramie c e l l u l o s e s and a s i n g l e t f o r b a c t e r i a l and v a l o n i a c e l l u l o s e s , respectively. However, s i n c e t h e s p a c i n g o f t h e d o u b l e t i s as s m a l l as 0.2 ppm, t h e c h e m i c a l s h i f t s o f t h e C6 carbons a r e almost t h e same(about 66.4 ppm) f o r b o t h groups o f c e l l u l o s e . Such h i g h s h i f t v a l u e s seem t o be w e l l c o r r e l a t e d t o t h e t r a n s - g a u c h e c o n f o r m a t i o n , which a g r e e s w i t h t h e r e s u l t s o f x - r a y c r y s t a l a n a l y s e s ( 1 4 , 1 6 ) . F i g u r e 9 shows t h e s p e c t r a o f t h e c r y s t a l l i n e components o b t a i n e d from c u p r a r a y o n w i t h water c o n t e n t s o f 0% and 1 5 8 % ( £ ) . In t h i s c a s e , a l t h o u g h water a l s o s i g n i f i c a n t l y narrows each resonance l i n e , t h e f e a t u r e s o f t h e spectrum remain unchanged; t h e CI and C4 resonances s p l i t i n t o d o u b l e t s with e q u i v a l e n t i n t e n s i t i e s . This r e s u l t i s i n c o n t r a s t t o the case of n a t i v e c e l l u l o s e , r e f l e c t i n g that the c r y s t a l s t r u c t u r e of regenerated c e l l u l o s e i s c e l l u l o s e II(15-17). In a d d i t i o n , t h e c h e m i c a l s h i f t s o f C6 carbons a r e 64.2 ppm i n t h e d r y and h y d r a t e d s t a t e s , whose v a l u e s c o r r e s p o n d t o t h e g a u c h e - t r a n s c o n f o r m a t i o n as i s seen i n F i g u r e 3. This c o n f l i c t s w i t h t h e r e s u l t o f x - r a y c r y s t a l analyses(3^5,17 ) : No l i n e w i t h a higher chemical s h i f t corresponding t o the trans-gauche conformation i s o b s e r v e d , w h i l e both g a u c h e - t r a n s and t r a n s - g a u c h e a r e assumed i n the x-ray a n a l y s e s . Since the v a l u e o f t h e C6 carbon has a l m o s t t h e same o r d e r as l o n g values of the r i n g carbons(3,4,28), t h e r e i s no p o s s i b i l i t y o f t h e r a p i d exchange between g a u c h e - t r a n s and trans-gauche conformations. I t i s d i f f i c u l t at present t o f i n d the reason f o r the c o n f l i c t . F i g u r e 10 shows the s p e c t r a o f t h e n o n c r y s t a l l i n e components o f c o t t o n c e l l u l o s e w i t h water c o n t e n t s o f 0% and 1 6 1 % ( 4 ) . These s p e c t r a were o b t a i n e d by s u b t r a c t i n g t h e s p e c t r a o f t h e c r y s t a l l i n e components from t h e c o r r e s p o n d i n g whole s p e c t r a as shown i n F i g u r e 6. I t i s c l e a r l y seen t h a t t h e l i n e w i d t h s o f t h e CI and C4 r e s o n a n c e s become markedly narrower upon a b s o r b i n g water, w h i l e h o l d i n g t h e c h e m i c a l s h i f t s unchanged. F o r i n s t a n c e , t h e h a l f - v a l u e w i d t h s o f t h e CI resonance l i n e s a r e 60 Hz and 160 Hz, r e s p e c t i v e l y . Such a

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13. CP/MAS C NMR s p e c t r a o f t h e c r y s t a l l i n e components o f b a c t e r i a l c e l l u l o s e w i t h t h e water c o n t e n t s o f 0% (a) and 133% ( b ) .

I

120

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13. CP/MAS NMR s p e c t r a o f t h e c r y s t a l l i n e components o f cupra r a y o n w i t h t h e water c o n t e n t s o f 0% (a) and 155% ( b ) . (Reproduced from R e f . 4 . C o p y r i g h t 1985 Academia R e p u b l i c i i S o c i a l i s t e Romania.)

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marked d i f f e r e n c e i n l i n e w i d t h cannot be p r i m a r i l y a s c r i b e d t o t h e d i f f e r e n c e i n m o l e c u l a r m o b i l i t y o f t h e two samples, because t h e r e i s some e v i d e n c e t h a t water does not s i g n i f i c a n t l y enhance t h e m o b i l i t y of t h e c e j ^ u l o s e c h a i n s under t h e s e e x p e r i m e n t a l c o n d i t i o n s . One i s that the C Τ v a l u e s o f t h e CI and C4 carbons do n o t g r e a t l y d e c r e a s e w i t h i n c r e a s i n g water c o n t e n t . Furthermore, t h e C6 resonance l i n e a l s o becomes narrow o n l y s l i g h t l y , though t h e C6 carbon i n v o l v e d i n t h e m e t h y l o l s i d e group s h o u l d be much more m o b i l e than t h e backbone carbons i f water g r e a t l y enhances t h e m o l e c u l a r mobility. Another s t r o n g e v i d e n c e i n f a v o r o f t h e low m o l e c u l a r m o b i l i t y o f c e l l u l o s e c h a i n s i n the h y d r a t e d s t a t e i s o b t a i n e d from t h e r e s u l t s o f t h e measurements on rayon f i b e r s as d e s c r i b e d below. T h e r e f o r e , t h e o r i g i n o f such narrowing o f t h e CI and C4 resonance l i n e s has t o be sought e l s e w h e r e . The 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 φ and ψ about t h e 3 " 1 , 4 - g l y c o s i d i c l i n k a g e p r o v i d e t h e most l i k e l y ^ x p l a n a t i o n o f t h e s e narrowings. We have a l r e a d y d e s c r i b e d t h a t t h e C chemical s h i f t s of t h e C l , C4 and C6 carbons a r e p r i m a r i l y c o r r e l a t e d on t h e t o r s i o n a n g l e s φ and ψ about t h e 3 ~ 1 / 4 - g l y c o s i d i c l i n k a g e and χ about t h e C5-C6 bonds, r e s p e c t i v e l y . On t h e b a s i s o f t h e s e r e s u l t s , we have concluded(_3) t h a t t h e d i s t r i b u t i o n s i n t h e t o r s i o n a n g l e s φ and ψ a r e r e l a t i v e l y narrow i n t h e d r y n a t i v e c e l l u l o s e , whereas t h e y a r e broad in the dry regenerated c e l l u l o s e . It i s , therefore, plausible that such d i s t r i b u t i o n s a r e f u r t h e r narrowed by water i n t h e h y d r a t e d cotton. Thus i t can be assumed t h a t t h e n o n c r y s t a l l i n e c h a i n s o f c o t t o n a r e i n t h e r e l a t i v e l y o r d e r e d s t a t e i n t h e p r e s e n c e o f water. However, t h e c h a i n s may undergo some d i s t o r t i o n upon d r y i n g and as a r e s u l t t h e t o r s i o n a n g l e s φ and ψ w i l l be d i s t r i b u t e d somewhat b r o a d e r i n t h e d r y sample. F i g u r e 11 shows t h e s p e c t r a o f t h e n o n c r y s t a l l i n e components o f rayon f i b e r s w i t h t h e water c o n t e n t s o f 0% and 1 5 8 % ( 4 ) . In c o n t r a s t to t h e case o f c o t t o n , t h e l i n e w i d t h s o f the r e s p e c t i v e resonances do not remarkably d e c r e a s e by t h e a d d i t i o n o f water; f o r examples, t h e h a l f - v a l u e w i d t h o f the CI resonance l i n e i s 205 Hz f o r t h e h y d r a t e d sample, whereas i t i s 256 Hz f o r the d r y sample. As p o i n t e d out above, t h i s f a c t i m p l i e s t h a t t h e m o l e c u l a r m o b i l i t y o f t h e n o n c r y s t a l l i n e c h a i n s does not g r e a t l y i n c r e a s e w i t h t h e i n c r e a s e o f water c o n t e n t . Moreover, t h e n o n c r y s t a l l i n e component o f r a y o n does not undergo such a s i g n i f i c a n t change o f 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 φ and ψ as o b s e r v e d f o r c o t t o n c e l l u l o s e , p o s s i b l y because t h e m o l e c u l a r c o n f o r m a t i o n o f t h i s component i s r a t h e r random i n t h e d r y s t a t e . In o t h e r words, such a d i s o r d e r e d c o n f o r m a t i o n may h a r d l y a l l o w marked d i s t o r t i o n o f t h e n o n c r y s t a l l i n e c h a i n s t o be produced upon d r y i n g c u p r a r a y o n . F i g u r e 12 shows s c h e m a t i c s t r u c t u r a l models o f n a t i v e and r e g e n e r a t e d c e l l u l o s e samples, which have been proposed i n an attempt to e x p l a i n our o b s e r v a t i o n ( 3 0 , 31 ). H a i g l e r e t a l . (3J5) have r e c e n t l y found i n t h e i n c u b a t i o n o f A c e t o b a c t e r x y l i n u m i n t h e p r e s e n c e o f a f l u o r e s c e n t b r i g h t n e r t h a t the nascent f i b r i l of b a c t e r i a l c e l l u l o s e i s composed o f bundles o f c h a i n s i n t h e t a c t o i d a l and n o n c r y s t a l l i n e phase. Furthermore, Kai(2§) has r e p o r t e d u s i n g s i m i l a r t e c h n i q u e s t h a t t h e c e l l u l o s e c h a i n s a r e not o r i e n t e d a t random i n t h e c r o s s - s e c t i o n o f t h e n a s c e n t f i b r i l but a r e i n t h e form o f monomolecular l a y e r s c o r r e s p o n d i n g t o t h e (110) p l a n e o f c e l l u l o s e I . On t h e b a s i s o f t h e s e r e s u l t s , i t seems p l a u s i b l e t o assume t h a t

HORII ET AL.

i3


C2,3,5

13 F i g u r e 10 CP/MAS C NMR s p e c t r a o f t h e n o n c r y s t a l l i n e components of c o t t o n c e l l u l o s e w i t h t h e water c o n t e n t s o f 0% (a) and 161% ( b ) . (Reproduced from Ref.4. C o p y r i g h t 1985 Academia R e p u b l i c i i S o c i a l i s t e Romania.)

C2,3 5 f

13 F i g u r e 11 CP/MAS C NMR s p e c t r a o f t h e n o n c r y s t a l l i n e components o f c u p r a rayon w i t h t h e water c o n t e n t s o f 0% (a) and 155% ( b ) . (Reproduced from Ref.4. C o p y r i g h t 1985 A c a demia R e p u b l i c i i S o c i a l i s t e Romania.)

132


F i g u r e 12 Schematic s t r u c t u r a l models o f n a t i v e and c e l l u l o s e samples.

regenerated

6.

HORII

ET AL.

13


133

n a t i v e c e l l u l o s e s i n c l u d i n g c o t t o n and ramie a r e c r y s t a l l i z e d from t h e o r d e r e d s t a t e such a s l i q u i d c r y s t a l . In t h a t c a s e t h e c h a i n s which remain as n o n c r y s t a l l i n e p o r t i o n s a f t e r c r y s t a l l i z a t i o n a r e a l s o r e l a t i v e l y o r d e r e d a s shown i n F i g u r e 12. Such o r d e r e d n o n c r y s t a l l i n e c h a i n s w i l l undergo some d i s t o r t i o n upon d r y i n g b u t t h i s k i n d o f d i s t o r t i o n must be r e l a x e d by a b s o r b i n g water a g a i n . T h i s p r o c e s s i s thought t o i n d u c e such n a r r o w i n g o f resonance l i n e s as shown i n F i g u r e 10. On t h e o t h e r hand, t h e n o n c r y s t a l l i n e c h a i n s i n r a y o n f i b e r s must be c o n s i d e r a b l y d i s o r i e n t e d because t h e f i b e r s a r e spun and c r y s t a l l i z e d from t h e s o l u t i o n where c h a i n s adopt t h e a l m o s t random c o n f o r m a t i o n . T h e r e f o r e , t h e n o n c r y s t a l l i n e sequences h a r d l y undergo any change i n c o n f o r m a t i o n when d r i e d o r h y d r a t e d as shown i n F i g u r e 12. Such a n o n c r y s t a l l i n e s t a t e i s thought t o be r e f l e c t e d on t h e s m a l l change i n l i n e w i d t h as seen F i g u r e 11.

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RECEIVED

April 1,1987

The Structures of Cellulose - American Chemical Society

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