Cellulose conformational rigidity

Figure 7. A "snapshot" of a typical cellulosic chain trajectory taken from a Monte Carlo sample of cellulosic chains, all based on die conformational energy map of Fig. 6. Filled circles representing glycosidic oxygens, linked by virtud bonds spanning the sugar residues (not shown), allow one to trace the instantaneous chain trajectory in a coordinate system that is rigidly fixed to the residue at one end of the chain. Projections of the chain into three mutually orthogonal planes assist in visualization of the trajectory in three dimensions.

The bridged biphenyl (472) (67ZN(B)1296> has a rigid twist-boat-chair conformation in the crystal (72CC985) and is an eight-membered heterocycle with two torsional constraints in a 1,3 relationship. It has been resolved by chromatography on cellulose acetate, and AG for racemization is 120 kJ mol-1 (67M1323). 

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]

The unusual properties of xanthan undoubtedly result from its structural rigidity, which in turn is a consequence of its linear, cellulosic backbone that is stiffened and shielded by the trisaccharide side chains. The conformation of xanthan in solution is a matter of debate. It does appear that the conformation changes with conditions. 

Structural features of cellulosic materials that determine their susceptibility to enzymatic degradation include (1) the moisture content of the fiber (2) the size and diffusibility of the enzyme molecules involved in relation to the size and surface properties of the gross capillaries, and the spaces between microfibrils and the cellulose molecules in the amorphous regions (3) the degree of crystallinity of the cellulose (4) its unit-cell dimensions (5) the conformation and steric rigidity of the anhydroglucose units (6) the degree of polymerization of the cellulose ... 

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