Cellulose chain geometry

Fig. 82.—Geometry of the cellulose chain unit. (After Benoit. q...

Cellulose I(S and cellulose II have monoclinic, two-chain unit cells. Cellulose IIIj has a monoclinic one-chain cell [227], and the one-chain unit cell of la is triclinic with no 2i symmetry. Still, all of the chain shapes are very similar to each other. It had been speculated that cellulose chain linkage geometries would alternate between the quite different linkages found in crystalline p-cellobiose and in methyl 3-cellobioside [196]. That idea is now obsolete. Such a departure from symmetry would be far greater than indicated by the above high-resolution studies. When molecules from the high-resolution structures for all of the polymorphs are superimposed, differences in their backbone structures are barely visible. 

On the other hand, if the thousands of glucose units are linked by beta 1-4 bonds, that molecule is called cellulose. Plants use this polymer as part of their structural material for building stems and leaves. The geometry of these beta linkages allows the long cellulose chains to align themselves side-by-side and thus create strong fibrous strands. 

Figure 15-7. Drawing of a cellulose chain segment in a folding conformation. The cp and v / values are indicated. This bend was energy minimized with MM3. The lower portions retain the linkage geometries of crystalline cellotetraose (Gessler et al. 1995) See Color Plate of this figure beginning on page 555)...

This extremely narrow distribution of structures is expanded considerably when the cellulose chain is considered to be a colleetion of P-l,4-linked glucose residues that have one of the geometries found in our survey of related small molecule crystals, such as cellobiose or cellobiose acetate. These extrapolated structures compare well with the experimentally determined cellulose derivatives and complexes if the latter are taken to be left-handed. Previously, we calculated... 

According to a recent report, the unit cell of cellotetraose hemihydrate in single crystals contains two antiparallel chains, which are conformationally distinct—especially in the sugar geometries.74 However, all hydroxymethyl groups adopt similar gt orientations. Whether this oligosaccharide morphology can be implemented for cellulose II in fibers remains to be seen. 

Using the two-chain unit-cell,3 with a = 0.817 nm, b = 0.785 nm, c = 1.034 nm, andy = 96.38°, the modified intensity-data of Mann and coworkers,37 and several residue-geometries, the structure of native ramie cellulose was refined. The resulting R factors were 15.8%, 18.5%, and 17.5% for, the antiparallel, parallel-up, and parallel-down models, respectively. A temperature factor of 0.23 nm2 was necessary in order to obtain a good fit with the observed data. It was suggested that the antiparallel packing of the chains cannot be discounted for cotton and ramie celluloses. 

Models of polymer chain extension were first used to compare the effect of the glycosidic linkage geometry of simple polysaccharide chains, eg cellulose and amylose (43). Both polymers are 1,4-linked glucans the only difference is in the anomeric configuration on the C-1 atom of the monomeric unit, a for amylose and for cellulose. The calculated data show a remarkable pseudohelical chain... 

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