Conformational analysis, cellulose

The torsion angles predicted by conformational analysis agree closely with those of crystalline cellobiose as measured by X-ray diffraction, the conformation of which is restricted by two chain-stabilising intramolecular hydrogen bonds between 0(3 )-H and 0(5) and also between 0(2 )-H and 0(6) (Figure 4.3). These are also found in cellulose and they assist in maintaining the highly extended conformation which allows it to function as a structural polymer. 

Influence of Physical Structure. The hydrolytic behavior of cellulose is much influenced by its physical structure and lateral order [121-132]. Wood cellulose was hydrolyzed twice as fast as cotton [125]. Hydrolysis rate was significantly increased by physical or chemical pretreatment, with the effect depending on the source of cellulose. Hill and coworkers [127,128] reported that mercerization increased the hydrolysis rate of cotton (by 40%) and of ramie (7%), whereas the opposite effect was observed for linen and a-cellulose samples showing an approximately 30% reduction. Based on kinetic analysis, they concluded that the end-attach model proposed by Sharpies [121] can only be applied to the cellulose II structure and not to the cellulose I crystallite. Thus, the conformation of cellulose is also a significant factor affecting its reactivity and possibly the hydrolytic mechanism as well. 

Fig. 2.—Conformational Analysis of Cellulose.31 [ and if> are the torsion angles shown in 2 and are assumed to be the only degrees of freedom possessed by the chain. Combinations of and if that require no steric compression are enclosed by the continuous line ( fully allowed conformations ). The broken lines enclose conformations in which there is slight steric compression ( marginally allowed conformations ), All other conformations involve bad steric clashes ( disallowed ).]...

X-ray diffraction techniques are the only way of determining the crystal structure of natural and synthetic polymers, although the x-ray data itself obtained from a crystalline polymeric fiber or film is not sufficient to allow complete refinement of the structure. Conformational analysis and electron diffraction represent complementary methods which will facilitate the determination of the structure. The necessary requirements for the x-ray approach are crystallinity and orientation. X-ray data cannot be Obtained from an amorphous sample which means that a noncrystalline polymeric material must be treated in order to induce or improve crystallinity. Some polymers, such as cellulose andchitin, are crystalline and oriented in the native state.(1 )... 

X-ray diagrams of lichenan were recorded and conformational analysis( l) allows the proposal of a model for the lichenan chain. The analogy between lichenan and regenerated cellulose was found to be valid, although it would have been more precise to refer to "water" cellulose. The fiber diagram reveals that the equatorial 101 and 002 reflections of "water" cellulose are on the first layer line in lichenan, an observation which led to the proposal of a pleated sheet" structure. (61,62)... 

Fig, Chain conformations of cellulose proposed by X-ray analysis 28). A bent form chain for cellulose I, B bent and twisted form chain deviated from 2 for cellulose II. There are two kinds of the distance of 03 - 05 Internal rotation angle around glucosidic linkage is according to Jone s expression 32). 01 - 01 distance is 5.16A, C - C,... 

The structure and arrangement of cellulosic chains play an important role in the formation of liquid crystals. At present, neither the conformation of cellulosics nor the solvent bound to the chain in the case of a lyotropic mesophase are known for these liquid-crystalline systems. Nevertheless, these structural features form the basis for a discussion of structural and thermodynamic aspects. Information on cellulosics is available for the two borderline cases next to the LC state, i.e., for the solvent built-in solid state as well as for the pure solid state, obtained by X-ray, NMR, and potential energy analysis on one side, and for the semi-dilute state from light-scattering experiments on the other side. These data have to be evaluated for a discussion of possible structures and models in liquid-crystalline phases. 

The size of the cellulose molecule is normally expressed in terms of their degree polymerization (DP), i.e, the number of anhydroglucose units present in a chain. However, the conformational analysis of cellulose indicates that cellobiose (4-0-a-D-glucopyranosyl-a-D-glucopyranose, Fig. 10.2a) is its basic structural unit [10]. The conformation of the repeating unit of cellulose can be explained if we consider the model proposed for the biosynthesis of glucose [11]. 

Examination of (1 4)-linked xylans have indicated a three-fold, left-handed helical structure [65]. hi the case of the AX from rice endosperm flour [66], this structure was confirmed by both X-ray diffraction and conformational analysis using the PS79 computer program. Although it does not seems to be a desirable conformation for xylans to make a complex firmly associated with the two-fold ribbon-like structure of cellulose, the existence of such interactions was documented [67]. 

Rees D.A. and Skerrett R.J. 1968. Conformational analysis of cellobiose, cellulose and xylan. Carbohydr Res 7 334-348. 

The proposal of multiple crystalline forms in native celluloses implies that all native celluloses are compositions of two distinct forms, which has been earlier indicated for Acetobacter and Vallonia celluloses, 8). From the resolution of the NMR spectra an estimate of about 60-70 % of the la form in Acetobacter cellulose and of 60-70 % of the lb form in cotton was obtained. A further detailed analysis of conformational features in celluloses seemed to need X-ray diffractometric and Raman spectroscopic confirmation 19-56). 

N 153 Analysis of Cooperative Conformational Transitions hi Cellulose and Amylose Triearbanilates"... 

NMR measurements to deduce conformations of sugar rings, three-dimensional structures, and the degree of conformational flexibility in various parts of N-linked oligosaccharides (Fig. 4-21). Measurement of C-O-C -C spin-coulping constants is also of value.270 Use of multidimensional NMR has permitted analysis of mixtures of cellulose oligosaccharides.269... 

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