Structures of Native and Regenerated Celluloses - American

4 Structures of Native and Regenerated Celluloses

Downloaded by MICHIGAN STATE UNIV on June 7, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0048.ch004

JOHN BLACKWELL, FRANCIS J. KOLPAK, and KENNCORWIN H. GARDNER Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106

We have i n v e s t i g a t e d the c r y s t a l s t r u c t u r e s of c e l l u l o s e I and I I by x-ray methods, based on i n t e n s i t y data from f i b e r diagrams o f V a l o n i a and rayon c e l l u l o s e s . T h i s has been a long standing problem d a t i n g from the 1920s. From the work o f Meyer and M i s c h ( l ) the n a t i v e s t r u c t u r e was shown t o have a monoclinic u n i t cell with dimensions a = 8.35Å, b = 7.9Å, c = 10.3Å ( f i b e r a x i s ) and γ = 96°. S i m i l a r l y , from the data c o l l e c t e d by W e l l a r d ( 2 ) , the u n i t cell f o r c e l l u l o s e I I is a l s o monoclinic with dimensions a = 7.93Å, b = 9;18Å, c = 10.34Å and γ = 117.3° (average values over a v a r i e t y o f p r e p a r a t i o n s ) . The u n i t c e l l s for both forms contain c e l l o b i o s e residues of two chains, and the space groups are g e n e r a l l y thought to be P21. The c e l l u l o s e chains a r e thought to possess two-fold screw symmetry, and t o be with t h e i r axes through the o r i g i n (0,0) and center (1/2,1/2) o f the a. b_ p r o j e c t i o n s . The important consequence o f t h i s i s that the space group symmetries a r e s a t i s f i e d whether the two mole c u l e s passing through the u n i t c e l l have the same ( p a r a l l e l ) o f opposite ( a n t i p a r a l l e l ) sense. Considerable e f f o r t has been d i r e c t e d t o determine the chain p o l a r i t i e s f o r the two forms and thence t o e l u c i d a t e t h e i r hydrogen bonding networks. T h i s i n f o r m a t i o n i s c l e a r l y neces sary i n order to understand the process of c e l l u l o s e b i o s y n t h e s i s . However, previous workers (notably Jones)(3,4) have g e n e r a l l y reported that models c o n t a i n i n g chains with both the same or a l t e r n a t i n g p o l a r i t i e s could be b u i l t f o r both forms and could not be d i s t i n g u i s h e d i n terms of agreement with the x-ray data. In the absense o f d e f i n i t e evidence, most researchers favored a n t i p a r a l l e l chain s t r u c t u r e s . Probably the most d e t a i l e d pro posed model i s that f o r n a t i v e c e l l u l o s e due to Liang and Marchessault. (5_). T h i s model was based on p o l a r i z e d i n f r a r e d s p e c t r a and contained the hydrogen bonding proposed by Frey Wyssling(6) and Hermans e t a l . ( 7 ) . Our r e i n v e s t i g a t i o n o f the s t r u c t u r e s of the two polymorphs used r i g i d body l e a s t squares refinement methods. From model b u i l d i n g and conformational a n a l y s i s ( 8 ) i t was c l e a r that the

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In Cellulose Chemistry and Technology; Arthur, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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chain backbone conformation must be a t l e a s t c l o s e t o that pro posed by Hermann et a l . ( 7 ) , i . e . a 2- h e l i x with i n t r a m o l e c u l a r (0(3)-Η···05') hydrogen bonding. Models could t h e r e f o r e be constructed c o n t a i n i n g two such chains per u n i t c e l l , and the p o s i t i o n s of these chains could be r e f i n e d by l e a s t squares methods, u s i n g the methods developed f o r biopolymers by Arnott and Wonacott(9). T h i s work i s summarized below, a f t e r which the s t r u c t u r e s determined a r e d e s c r i b e d and d i s c u s s e d . More extensive d e s c r i p t i o n s o f the refinement process f o r the two s t r u c t u r e s are given elsewhere(10,11).


Experimental C e l l u l o s e I. Samples o f the c e l l w a l l s of the a l g a V a l o n i a v e n t r i c o s a were p u r i f i e d by b o i l i n g i n a l a r g e excess o f 1% aqueous NaOH f o r 6 h r s , w i t h a change o f a l k a l s o l u t i o n a f t e r 3 h r s . They were then allowed t o stand overnight i n 0.05N HC1 at room temperature, r i n s e d and s t o r e d i n d i s t i l l e d water. Specimens f o r x-ray work were prepared as bundles o f p a r a l l e l f i b e r s drawn from the p u r i f i e d c e l l w a l l s . C e l l u l o s e I I . Samples of regenerated c e l l u l o s e (rayon f i b e r s ) were obtained from Celanese F i b e r s Company. Small i n d i v i d u a l f i b r i l s were drawn from the l a r g e r rayon f i b e r s and prepared as a p a r a l l e l bundle f o r x-ray work. X-ray f i b e r diagrams were recorded on Kodak No-screen f i l m u s i n g CuKa r a d i a t i o n and a f l a t p l a t e vacuum camera, the c o l l i m a t o r c o n s i s t e d o f two 400μ p i n h o l e s separated by 12cm. The d-spacings were c a l i b r a t e d using i n o r g a n i c s a l t s . The i n t e n s i t i e s f o r c e l l u l o s e I were measured using a Joyce L o e b l densitometer. The areas under r a d i a l scans through each r e f l e c t i o n were c o r r e c t e d i n the manner d e s c r i b e d by C e l l a et a l . ( 1 2 ) . For c e l l u l o s e I I a new method was devised using a Photometries densitometer which produces an o p t i c a l d e n s i t y map o f the e n t i r e x-ray photograph. Contours o f equal o p t i c a l d e n s i t y are drawn from which the i n t e g r a t e d i n t e n s i t y of the r e f l e c t i o n i s determined f o l l o w i n g s u b t r a c t i o n o f the back ground. A few very weak r e f l e c t i o n s were estimated by eye i n each case. Unobserved r e f l e c t i o n s p r e d i c t e d t o occur w i t h i n the s c a t t e r i n g angle covered by the x-ray data were assigned a value such that the s t r u c t u r e amplitude was 2/3 o f a judged t h r e s h o l d value f o r the weakest r e f l e c t i o n which could be d e t e c t ed i n that r e g i o n o f the x-ray photograph. Unit C e l l

Parameters

The x-ray f i b e r diagrams f o r c e l l u l o s e I and I I a r e shown i n F i g u r e s 1 and 2 r e s p e c t i v e l y . Least squares refinement o f the u n i t c e l l f o r V a l o n i a c e l l u l o s e I r e s u l t e d i n dimensions a = 16.34Â, b - 15.72Â, ç. = 10.38Â and γ = 97.0°. From



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X-Ray diffraction pattern of a bundle of oriented fibers of Valonia cellulose

Figure 2.

X-Ray diffraction pattern of a bundle of rayon fibers



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d e n s i t y measurements, t h i s u n i t c e l l contains s e c t i o n s e i g h t chains. Of the 39 observed non m e r i d i o n a l r e f l e c t i o n s , 36 can be indexed by planes with even h and k i n d i c e s , i . e . they a r e indexed s a t i s f a c t o r i l y by a two chain (Meyer and Misch) u n i t c e l l with dimensions a = 8.17Â, b = 7.86Â, ç = 10.38Â and γ = 97.0°. The remaining three r e f l e c t i o n s a r e those n e c e s s i t a t i n g doubling of the a and b axes t o give the e i g h t chain c e l l , f i r s t reported by Honjo and Watanabe (13) f o r V a l o n i a . These three r e f l e c t i o n s are very weak; furthermore, a l a r g e number of r e f l e c t i o n s with odd h o r k are p r e d i c t e d w i t h i n the range of the x-ray photograph but are too weak t o be detected. Hence the d i f f e r e n c e s between the four two-chain u n i t s making up the e i g h t - c h a i n c e l l must be very s m a l l , and the two chain Meyer and Misch u n i t c e l l i s an adequate approximation t o the c e l l u l o s e I s t r u c t u r e . For c e l l u l o s e I I the r e f i n e d u n i t c e l l i s monoclinic with dimensions a = 8.01Â, b = 9.04Â, ç = 10.36Â and γ = 117.1°. The experimental e r r o r f o r the dimensions f o r both c e l l u l o s e I and I I i s ±0.05Â, and the d i f f e r e n c e i n f i b e r repeats between the two forms i s not s i g n i f i c a n t . The only systematic absenses i n each case a r e f o r odd order 001 r e f l e c t i o n s and the space groups a r e both Ρ 2 · 1

Molecular Model Consistant with the P2- symmetries, models were constructed f o r the c e l l u l o s e chain as two f o l d h e l i c e s with the a p p r o p r i a t e repeats. Standard bond lengths and bond angles f o r pyranose r i n g s t r u c t u r e s (14) were used, and the model contained an 0(3)-Η···0(5 ) i n t r a m o l e c u l a r hydrogen bond. Standard numbering of the atoms on the c e l l o b i o s e r e s i d u e i s shown i n F i g u r e 3. The C-O-C g l y c o s i d i c bond angle was 114.8°. The i n t r a m o l e c u l a r hydrogen bond length was 2.75Â and 2.69Â i n c e l l u l o s e I and I I , due t o the s l i g h t l y d i f f e r e n t f i b e r repeats and g l y c o s i d i c t o r s i o n angles. The c e l l u l o s e chain i s completely r i g i d except f o r p o s s i b l e r o t a t i o n o f the CHÔH s i d e chain. This r o t a t i o n i s d e f i n e d by the d i h e d r a l angle χ, which i s zero when C(6)-0(6) i s c i s t o C(4)-C(5). T h i s o r i e n t a t i o n i s a l s o defined r e l a t i v e t o the C(4)-C(5) and C(5)-0(5) bonds: gg guache t o both C(5)-0(5) and C(4)-C(5) (χ = -60°); _gt, gauche t o C(5)-0(5) and trans to C(4)-C(5) (χ = 180°), and t£, trans t o C(5)-0(5) and gauche t o C(4)-C(5) (χ = +60°). (15). f

Chain

Packing

The symmetry requirements o f the P2^ space group a r e s a t i s f i e d by p l a c i n g two independent chains so that t h e i r screw axes are p a r a l l e l t o £ and pass through (0,0) and (1/2,1/2) i n the a. ID plane. Two chain u n i t c e l l s were constructed f o r both forms, c o n t a i n i n g p a r a l l e l and a n t i p a r a l l e l chains; each model was then



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Molecular model for the cellobiose residue showing the numbering of the atoms


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r e f i n e d s e p a r a t e l y against the x-ray data. The p o s i t i o n s of the r i g i d c e l l u l o s e chains a r e completely d e f i n e d by three packing parameters: the s h i f t of one chain along c_with respect t o the other chain, and two parameters d e f i n i n g the independent r o t a t i o n s of the two chains about t h e i r h e l i x axes. A survey of p o s s i b l e packing followed by comparison of the observed and c a l c u l a t e d s t r u c t u r e amplitudes i n d i c a t e d that four b a s i c two chain models need t o be considered. In each case, the chain through (0,0) has the g l y c o s i d i c oxygen 0 ( l ) a t ζ = 0 and " s h i f t d e s c r i b e s the c_ a x i s displacement of the second chain through (1/2, 1/2): The chain sense i s d e f i n e d as "up" when z , _ v > ζ \ · The four models a r e : P ^ p a r a l l e l chains o r i e n t e d "up" with a s h i f t of ^c/4; P ~ p a r a l l e l chains o r i e n t e d "down" with a s h i f t of ^c/4; a - - a n t i p a r a l l e l chains with an "up" chain a t (0,0) and a down chain a t (1/2,1/2), with a s h i f t of ^-c/4; and a - a n t i p a r a l l e l chains as i n a^ but with a s h i f t of M-c/4. f

1 1

n /

U


47

ρ / ς

W

C

W

2

9

2

Refinement Each of the above s t r u c t u r e s i s d e f i n e d by parameters determining the p o s i t i o n and o r i e n t a t i o n of the r i g i d chains and t h e i r pendant -CH 0H groups. The l e a s t squares procedure r e f i n e s these parameters to give the best agreement between the observed and c a l c u l a t e d s t r u c t u r e amplitudes. The seven r e f i n a b l e para meters f o r each model a r e : 1) SHIFT, the stagger of the center chain along c with respect to the chain a t the o r i g i n ; 2) φ , the r o t a t i o n of the o r i g i n chain about i t s h e l i x a x i s ; 3) φ , the r o t a t i o n of the center chain about i t s h e l i x a x i s ; 4) χ, the o r i e n t a t i o n of the -CHÔH groups of the o r i g i n c h a i n ; 5) χ , the o r i e n t a t i o n of the -CflÔH groups of the center chain; 6) K, a s c a l e f a c t o r f o r comparison of the observed and c a l c u l a t ed s t r u c t u r e amplitudes, and 7) B, the i s o t r o p i c temperature f a c t o r . The refinement w i l l be discussed i n terms of the r e s i d u a l s , which give a measure of the agreement between the observed and c a l c u l a t e d s t r u c t u r e amplitudes. These are d e f i n e d : ?

1

τ

κ = z||F l lF ]| Σ |F 1 f t

7

0

r

,

_

~

ZW(|F 1-|F„1)

2 =

?

ν™ IΊ7 I

Λ

EWQFJ-IFJ)

2

τ™ν EwF 02

where F and F a r e the observed and c a l c u l a t e d s t r u c t u r e amplitudes and w i s a weigh assigned t o each observed s t r u c t u r e amplitude. C

Cellulose I The i n i t i a l refinement was done f o r models where both chains had the same o r i e n t a t i o n f o r the CHÔH groups i . e . χ = χ . In l a t e r work i t was found tjiat models with a l l but very small !

American Chemical Society Library

1155 16th St., N.W. Washington, D.C. 20036 In Cellulose Chemistry and Technology; Arthur, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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d i f f e r e n c e s between χ and χ are not compatible with the x-ray data f o r stereochemical requirements. These small d i f f e r e n c e s do not give s i g n i f i c a n t improvement i n the f i t between the observed and c a l c u l a t e d s t r u c t u r e amplitudes, and hence i n the s t r u c t u r e s described below, χ = χ , and the refinement i s f o r 6 v a r i a b l e s . When the models were r e f i n e d against the 36 observed r e f l e c t i o n s only, the r e s u l t i n g R values were R =0.207, R =0.249, R =0.179, and R =0.202. In a l l four r e f i n e d models, the 'planes of the pyranose r i n g s are approximately i n the a c_ plane and the value of SHIFT staggers the g l y c o s i d i c oxygens by ^c/4. The χ value f o r models a^, a^ and P- places the -CH 0H groups near the t g p o s i t i o n , such that 0(2 ;-Η··-0(6) i n t r a m o l e c u l a r and reasonable i n t r a m o l e c u l a r hydrogen bonds can be formed. For model p and χ value places the CH 0H group intermediate between the t£ and _gt p o s i t i o n s , which does not allow f o r hydrogen bond i n g of the 0(2)-H groups. S t a t i s t i c a l t e s t s (15) i n d i c a t e the model p^ gives s i g n i f i c a n t l y b e t t e r agreement with the x-ray data than model p « Model a can be r e j e c t e d i n favor of model a^ on the same b a s i s . Thus models a- and p^ were the most l i k e l y a n t i p a r a l l e l and p a r a l l e l chain models and were considered f o r f u r t h e r refinement. At t h i s stage the unobserved r e f l e c t i o n s were i n c l u d e d i n the refinement where the c a l c u l a t e d s t r u c t u r e amplitude was l a r g e r than the t h r e s h o l d value, under these circumstances. A weighting scheme of w=l f o r observed and w=l/2 f o r unobserved r e f l e c t i o n s was used at t h i s p o i n t . The f i n a l r e s i d u a l s were R =0.233, R =0.299, R" =0.215 and R" =0.270. A p p l i c a t i o n o f the Hamilton s t a t u l t i c a l t e s t t l 6 ) t o these data i n d i c a t e a preference f o r the p a r a l l e l chain model (p-) at a s i g n i f i c a n c e l e v e l of 0.5%, i . e . , the p a r a l l e l model i s p r e f e r e d by a f a c t o r o f more than 200 to 1. The ab and ac p r o j e c t i o n s of the s t r u c t u r e are shown i n F i g u r e 4. The s t r u c t u r e has no bad contacts on the b a s i s of accepted stereochemical c r i t e r i a . The r e f i n e d value of φ i s 0.4° from that of φ , and the Hamilton t e s t i n d i c a t e s that the c o n s t r a i n e d model with φ = φ i s i n as good agreement with the data as the model w i t h φ as a separate v a r i a b l e . The f i n a l value of φ=19.4° ( a r b i t r a r y o r i g i n ) p l a c e s the chains so that the "planes" of the r i n g s are approximately i n the ac plane (see F i g u r e 4). The r e f i n e d value of SHIFT=0.266c:. T h i s d e v i a t i o n from a p e r f e c t quarter stagger i s not unexpected: the weak 002 m e r i d i o n a l would be absent f o r a stagger of 0.25c. The r e f i n e d value of χ=80.3° p l a c e s the CH 0H groups w i t h i n ^20° of the t£ p o s i t i o n (χ=60°). T h i s o r i e n t a t i o n d i d not s h i f t s i g n i f i c a n t l y from that r e f i n e d f o r the observed r e f l e c t i o n s only. The i s o t r o p i c temperature f a c t o r i s B=2.50. 1

2

2

1

9

2

f


2 >

2

2

2

f

f

P

1

1

1

2

Hydrogen Bonding i n C e l l u l o s e I The hydrogen bonding network i n c e l l u l o s e I i s shown i n Figure 4c. A l l the hydroxyl groups form hydrogen bonds with acceptable bond lengths and angles. In a d d i t i o n t o the



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co

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Ο

es

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0(3)-Η···0(5 ) hydrogen bond o f l e n g t h 2.75Â d e f i n e d i n the model, there i s a second i n t r a m o l e c u l a r bond: 0(2 )-Η···06 of l e n g t h 2.87Â. These i n t r a m o l e c u l a r bonds run on both s i d e s of the c e l l u l o s e chain. In a d d i t i o n , there i s an i n t e r c h a i n hydrogen bond between 0(6)-H and 0(3) of the next chain along the a a x i s ; t h i s bond i s 2.79Â i n length. No hydrogen bonding occurs along the u n i t c e l l d i a g o n a l s , r a t h e r the hydrogen bonding i s a l l i n the 020 planes, and the s t r u c t u r e i s seen as a s e r i e s of hydrogen bonded sheets o f chains, where s u c c e s s i v e sheets are staggered and a l l the chains have the same sense. !


Cellulose I I For c e l l u l o s e I I , study of molecular models i n d i c a t e d that the two chains probably have d i f f e r e n t conformations f o r the -CHÔH groups, and hence a l l seven v a r i a b l e s were considered i n the refinement. Refinement against the 44 observed r e f l e c t i o n s gave r e s i d u a l s of R ^0.254, R =0.188 and R =0.195, R =0.171. Of these four modell, only a^ ?s s t e r e o c h e m i c a l ^ acceptable, and t h i s gives the lowest r e s i d u a l . Model P^ contains four bad contacts and models p and a^ c o n t a i n two each (The worst o f these contact d i s t a n c e s are nonbonded oxygen-oxygen d i s t a n c e s of 2.17Â, 2.05Â and 2.11Â i n p^, p and a r e s p e c t i v e l y , which a r e t o t a l l y unacceptable). E f f o r t s t o remove these contacts by i n c o r p o r a t i o n o f non-bonded c o n s t r a i n t s were not s u c e s s f u l : the R values increased t o R =0.272, R =0.219 and R=0.230, but although the contact d i s t a n c e s werl lengthened, the stereochemical c r i t e r i a were s t i l l not s a t i s f i e d . A l l four models were then r e f i n e d against the t o t a l observed and unobserved data, as was done f o r c e l l u l o s e I . The bad contacts f o r models p^, p and a^ were not removed and these s t r u c t u r e s remain unacceptable. For model a , a short oxygenoxygen contact o f 2.49Â was introduced, but t h i s was e r r a d i c a t e d with an a p p r o p r i a t e nonbonded c o n s t r a i n t . The f i n a l R values were R =0.219 and R"=0.167. Thus an a n t i p a r a l l e l chain model i s proposed f o r c e l l u l o s e I I . The ab and ac p r o j e c t i o n s o f the s t r u c t u r e are shown i n F i g u r e 5a and 5b. The r e f i n e d values o f φ and φ o r i e n t the chains so that the r i n g s a r e almost p a r a l l e l t o the ac planes, although not q u i t e so c l o s e as f o r c e l l u l o s e I. The r e l a t i v e stagger o f the chains i s 0.216c. The s i d e chains have d i f f e r e n t conformations f o r the corner "up" chains (through 0,0), χ=186.3°, p l a c i n g the -CHÔH group c l o s e t o the £t p o s i t i o n (χ=180°), f o r the center "down" chains (through 1/2,1/2), = 7 0 . 2 ° , p l a c i n g these -CH 0H groups c l o s e t o the t £ p o s i t i o n (χ=60°). The r e f i n e d i s o t r o p i c temperature f a c t o r i s B=19.96. 2

al

2

2

2

2

2

2

?

f

1

f

x

2

Hydrogen Bonding i n C e l l u l o s e I I The hydrogen bonding network i n c e l l u l o s e I I i s more complex



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than i n c e l l u l o s e I , and i s shown i n F i g u r e 5b-e. A l l o f the hydroxyl groups form hydrogen bonds with acceptable bond lengths and angles. Each chain has the 0(3)-Η···0(5') i n t r a m o l e c u l a r bond o f l e n g t h 2.69Â, as d e f i n e d i n the model. The -CIÔH groups of the center "down" chains a r e c l o s e t o the t& p o s i t i o n and these chains have a second i n t r a m o l e c u l a r 0(2)-Η···0(6) bond o f l e n g t h 2.73Â. The 0(6)-H group of t h i s chain a l s o forms an i n t e r m o l e c u l a r 0(6)-Η···0(3) bond of l e n g t h 2.67Â t o the next ("down") chain along the a a x i s , with a r e s u l t that the "down" chains form hydrogen bonded sheets i n the 020 planes s i m i l a r to those i n c e l l u l o s e I. The sheet of down chains i s shown i n Figure 5c. For the "up" corner chains the -CHÔH groups a r e c l o s e t o the £t p o s i t i o n , and form 0(6)-Η···0(2) i n t e r m o l e c u l a r bond of length 2.73Â to the next chain along the a a x i s , again i n the 020 plane. The sheet o f "up" chains i s shown i n Figure 5d. For the _gt_ conformation the 0(2)-H group cannot form an i n t r a molecular bond, but i s i n v o l v e d i n an i n t e r m o l e c u l a r 0(2)-Η···0(2 ) bond o f l e n g t h 2.77Â t o the next "down" chain on the d i a g o n a l along the 110 plane, as shown i n F i g u r e 5e. Hence the c e l l u l o s e I I s t r u c t u r e i s an a r r a y o f staggered hydrogen bonded sheets. The chain sense i s the same w i t h i n the sheets, but the sheets have a l t e r n a t i n g p o l a r i t i e s and are hydrogen bonded together along the long diagonal o f the u n i t c e l l . ?

Discussion A p a r a l l e l chain s t r u c t u r e f o r c e l l u l o s e I e f f e c t i v e l y r u l e s out chain f o l d i n g during s y n t h e s i s o f c e l l u l o s e m i c r o f i b r i l s . Native m i c r o f i b r i l s are t h e r e f o r e shown t o be extended-chain polymer s i n g l e c r y s t a l s , which are h i g h l y d e s i r a b l e s t r u c t u r e s i n terms o f mechanical p r o p e r t i e s . For c e l l u l o s e I I , the chains are a n t i p a r a l l e l , which i s c e r t a i n l y compatible with f o l d e d chains, although there i s no d e f i n i t e evidence f o r such a c r y s t a l l i z a t i o n process. C e l l u l o s e I I i s the s t a b l e polymorphic form, i n that i t i s p o s s i b l e to convert form I t o form I I , but not v i c a v e r s a . The c e l l u l o s e I I s t r u c t u r e contains the a t t r a c t i v e f e a t u r e o f a hydrogen bond between adjacent sheets of chains, which may account f o r t h i s s t a b i l i t y . In a d d i t i o n the hydrogen bonds have an average length o f 2.72Â i n c e l l u l o s e I I , as compared t o 2.80Â i n c e l l u l o s e I. The r e s o l u t i o n a t t a i n a b l e i n the x-ray r e f i n e ments i s not s u f f i c i e n t t o determine i n d i v i d u a l bond l e n g t h s , but t h i s d i f f e r e n c e i n the average bond lengths i s probably s i g n i f i cant, and r e f l e c t s a t i g h t e r chain packing i n c e l l u l o s e I I , c o n s i s t a n t with the higher s t a b i l i t y o f t h i s form. Of the v a r i o u s p o s s i b i l i t i e s f o r the chain p o l a r i t i e s i n the two forms, the p a r a l l e l I - a n t i p a r a l l e l I I s o l u t i o n seems t o be the most reasonable. R e s u l t s from x-ray and packing analyses by Sarko and Muggli (17) a l s o favor these p o l a r i t i e s . I f


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Figure 5. Projections of the antiparallel chain model for cellulose II. (a) Projection perpendicular to the ac plane. The center "down" chains (dark) are staggered with respect to the corner "up" chains, (b) Projection perpendicular to the a b plane along the fiber axis. The 0(2)-Η· · -0(2') hydrogen bond along the 110 planes is indicated.


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4.

Figure 5. (c) Hydrogen bonding network in the 020 plane for the center "down" chains. These sheets are very simihr to those for cellulose I. (d) Hydrogen bonding network in the 020 plane for the corner "up" chains, (e) Hydrogen bonding between antiparallel chains in the 110 plane.



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c e l l u l o s e I was an array o f a n t i p a r a l l e l chains, i t i s d i f f i c u l t to see why these would not adopt the c e l l u l o s e I I l a t t i c e . A consequence o f the p a r a l l e l chain s t r u c t u r e , however, i s that i t r e q u i r e s a r e l a t i v e l y complex b i o s y n t h e s i s mechanism w i t h p o l y m e r i z a t i o n followed very c l o s e l y by c r y s t a l l i z a t i o n . I f the two steps were to be w e l l separated then a r a y o n - l i k e s t r u c t u r e would be produced. The r e s u l t s f o r c e l l u l o s e I I were obtained f o r rayon. There i s no reason to b e l i e v e they do not apply to mercerized c e l l u l o s e , although we are c u r r e n t l y r e i n v e s t i g a t i n g the l a t t e r s t r u c t u r e . The m e r c e r i z a t i o n process i n v o l v e s s w e l l i n g i n c a u s t i c soda s o l u t i o n and i s accompanied by only a s m a l l change i n l e n g t h . Chanzy e t al.(18) have r e c e n t l y shown that shish-kabob s t r u c t u r e s of low molecular weight c e l l u l o s e with the form I I l a t t i c e w i l l e p i t a x i a l l y c r y s t a l l i z e on c e l l u l o s e I f i b e r s . Such e p i t a x i a l c r y s t a l l i z a t i o n i s to be expected s i n c e h a l f of the sheets i n c e l l u l o s e I I a r e the same as those i n c e l l u l o s e I. M e r c e r i z a t i o n proceeds showly and never goes t o completion. The unconverted c e l l u l o s e I could maintain the f i b e r dimensions and serve as a template f o r c r y s t a l l i z a t i o n of c e l l u l o s e I I . Acknowledgements This work was supported by N.S.F. Grant No. DMS 75-01028 and N.I.H. Career Development Award No. AM 70642 (to J.B.).

Abstract The crystal structures of native and regenerated celluloses have been determined using x-ray diffraction and least squares refinement techniques. Both structures have monoclinic unit cells containing sections of two chains with 2 screw axes. Models containing both parallel and antiparallel chains were refined in each case by comparison with the x-ray intensities for Valonia cellulose I and rayon cellulose II. For native cellulose, the results show a preference for a system of parallel chains (i.e. all the chains have the same sense). The refinement orients the -CHOH groups close to the tg conformation such that an 0(6)···Η-0(2') intramolecular hydrogen bond is formed. The structure also contains an 0(3)-Η···0(3) intermolecular bond along the a axis. All these bonds lie in the 020 plane, and the native structure is an array of staggered hydrogen bonded sheets. In contrast, for regenerated cellulose the only acceptable structure contains antiparallel chains (i.e. the chains have alternating sense). The CHOH groups of the corner chain are oriented close to the gt position; those of the center chain are close to the tg position. Both center and corner chains have the 0(3)-Η···0(5') intramolecular bond and the center chain also has an 0(2')-Η···0(6) intramolecular bond. Intermolecular hydrogen bonding occurs along the 020 planes: 0-(6)-Η···0(2) 1

2

2


4.

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bonds for the corner chains and 0(6)-Η···0(3) bonds for the center chains, and also along the 110 planes, with a hydrogen bond between 0(2)-H of the corner chain and 0(2') of the center chain. The major consequence of these structures is that native cellulose is seen as extended chain polymer single crytals. The cellulose II structure is compatible with regular chain folding, although there is no direct evidence for such folding.


Literature Cited 1. Meyer, K.H., and Misch, L. Helv. Chim. Acta. (1937) 20, 232-244. 2. Wellard, N.J., J. Polymer Sci. (1954) 13, 471-476. 3. Jones, D.W., J. Polymer Sci. (1958) 32, 371-394. 4. Jones, D.W., J. Polymer Sci. (1960) 42, 173-188. 5. Liang, C.Y. and Marchessault, R.H., J. Polymer Sci. (1959) 37, 385-395. 6. Frey-Wyssling, Α., Biochim, Biophys. Acta. (1955) 18, 166-168. 7. Hermann, P.H., DeBooys, J., and Maan, C., Kolloid-Z. (1943) 102, 169-180. 8. Rao, V.S.R., Sundararajan, P.R., Ramakrisnan, C., and Ramachandan, G.N., in Conformation of Biopolymers, (G.N. Ramachandran, ed.), (1967), Vol. 2, p. 271. Academic Press, New York. 9. Arnott, S., and Wonacott, A.J., Polymer (1966) 7, 157-166. 10. Gardner, K.H., and Blackwell, J., Biopolymers (1974) 14, 1975-2001. 11. Kolpak, F.J., and Blackwell, J. (1976) 9, 273-278. 12. Cella, R.J., Lee, Β., and Hughes, R.E. Acta Cryst. (1970) A26, 118-124. 13. Honjo, G., and Watanabe, Μ., Nature (1958) 181, 326-328. 14. Arnott, S., and Scott, W.E., J. Chem. Soc. Perkin. Trans. II (1972) 324-335. 15. Sundaratingham, Μ., Biopolymers (1966) 6, 189-213. 16. Hamilton, W.C., Acta Cryst. (1965) 18, 502-510. 17. Sarko, Α., and Muggli, R. Macromolecules (1974) 7, 486-494. 18. Chanzy, Η., Roche, Ε., and Vuong, R. Appl. Polymer Sci. Symposia (in press).


Structures of Native and Regenerated Celluloses - American

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