Torsion angles, cellulosics

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. 

While most CA s of disaccharides have depended only on intrinsic characteristics of the molecule, experimental results depend strongly on the environment. By experiment, Kamide and Saito ( ) have shown that the degree of flexibility of cellulose and its derivatives is strongly dependent on the dielectric constant of the solvent as well as the exact type and degree of substitution. Since a substantial portion of the polymer flexibility depends on the extent of variability of the torsion angles at the intermonomer linkage, the dependence of polymer flexibility on type of solvent and substitution means that the disaccharide flexibility also should depend on those factors. Non-polar solvents allowed the molecules to have greater flexibility than did polar solvents (35). 

FIGURE 7-19 Conformation at the glycosidic bonds of cellulose, amylose, and dextran. The polymers are depicted as rigid pyranose rings joined by glycosidic bonds, with free rotation about these bonds. Note that in dextran there is also free rotation about the bond between C-5 and C-6 (torsion angle [Pg.251]

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 ).]...

Our recent CP/MAS 3c NMR work(l, 2) on native and regenerated celluloses has shown that the resonance lines can be analyzed in terms of the contribution from the crystalline and noncrystalline components. The chemical shifts of Cl, C4, and C6 carbons thus ascribed to the respective components could be correlated to the torsion angles i(> and Tp, which define the conformation about the 3-1,4-glycosidic linkage, and x about the exo-cyclic C5-C6 bond,... 

Figure 6 C chemical shifts of the Cl carbons vs. torsion angles . a g-D-celloblose, b 3-D-methyl cellobloslde CH OH, c a-D-lactose monohydrate, d 3-D-lactose, e cellulose I (29), f cellulose I ( ), g cellulose II (30), h cellulose II (32). (Data a-d, g-h Ref. 4.)...

Table II shows that the noncrystalline components of native cellulose have no significant difference in chemical shift and line width from each other. The chemical shifts of the regenerated cellulose fibers, except for the amorphous cellulose, are also almost the same as those of native cellulose. This may suggest that the most probable conformation of the regenerated fibers is very similar to that of native cellulose. However, there is a marked difference in line width Av of the Cl carbon between native and regenerated celluloses the line width of the Cl carbon of native cellulose is about half that of the regenerated cellulose. Although the cause of the line broadening in CP/MAS spectra is not clear as yet, it is most likely that the line width of the Cl carbon is primarily dependent on the distribution of the torsion angle <(). Thus, it is suggested that the distribution in the angle <() is relatively narrow for the non-...

Figure 3 C chemical shifts of C6 carbons vs. torsion angles X a flt-D-glucose, b d-D-glucose H O, c 3 D-glucose, d B-D-cellobiose, e a-D-lactose H O, f 0-D-lactose, g cellulose I. o d-glucose and O-glucose residues,...

Figure 11 shows the spectra of the noncrystalline components of rayon fibers with the water contents of 0% and 158%( ). In contrast to the case of cotton, the linewidths of the respective resonances do not remarkably decrease by the addition of water for examples, the half-value width of the Cl resonance line is 205 Hz for the hydrated sample, whereas it is 256 Hz for the dry sample. As pointed out above, this fact implies that the molecular mobility of the noncrystalline chains does not greatly increase with the increase of water content. Moreover, the noncrystalline component of rayon does not undergo such a significant change of distributions in torsion angles 4> vjj as observed for cotton cellulose, possibly because the molecular conformation of this component is rather random in the dry state. In other words, such a disordered conformation may hardly allow marked distortion of the noncrystalline chains to be produced upon drying cupra rayon. 

Figure 1. Schematic representation of the configuration of (a) cellulose and (b) amylose with conventional atom labeling and the introduction of the torsion angles 0 and y/-...

The second topic reviewed is a quantitative analysis of DD MAS NMR spectra of native celluloses. The resonant lines of Cl, C4 and C6 carbon species for these samples are deconvolved into the crystalline and non-crystalline components using the difference in the spin-lattice relaxation time between both components. The chemical shifts of these lines are determined separately for the both components of samples, and correlated to the torsion angles (f and ip about -1,4-glycosidic linkage and x> about the exocyclic C5-C6 bond, respectively. On the basis of these results, the characteristic chain conformations of the crystalline and non-crystalline components of native and regenerative cellulose are discussed. 

Next, we have examined the relationship between the chemical shifts and the molecular chain conformation. The versatility of chain conformation of cellulose molecules is expressed in three torsion angles as shown in Fig. 14 (j> and xp, rotations around the /3-l,4-glycosidic linkages and the rotation of methylol side groups around the C5-C6 bonds. In order to find out the relationship between the chemical shifts of cellulose samples and the torsion angles, the... 

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