Cellulose and Amylose

The isomeric polysaccharides amylose (a component of starch) and cellulose also show the significance of stereoisomerism on polymer properties. Cellulose and amylose have the structures shown in Fig. 8-8. (Amylopectin, the other component of starch, has the same structure as amylose except that it is branched at carbon 6.) Both are polymers of glucose in which the 

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Cellulose and amylose are comprised of the same glucose subunits as the cyclodexttins. In the case of cellulose, the glucose units are attached through 1,4-P-linkages resulting ia a linear polymer. In the case of amylose, the 1,4-a-linkages, as are found ia the cyclodexttins, are thought to confer heUcity to the polymeric chain. 

As mentioned previously, cellulosic phases as well as amylosic phases have also been used extensively for enantiomeric separations more recently (89,90). Most of the work ia this area has been with various derivatives of the native carbohydrate. The enantioresolving abiUties of the derivatized cellulosic and amylosic phases are reported to be very dependent on the types of substituents on the aromatic moieties that are appended onto the native carbohydrate (91). Table 3 fists some of the cellulosic and amylosic derivatives that have been used. These columns are available through Chiral Technologies, Inc. and J. T. Baker, Inc. 

The small change in stereochemistry between cellulose and amylose creates a large difference in their overall shape and in their properties. Some of this difference can be seen in the strorcture of a short portion of fflnylose in Figure 25.9. The presence of the a-glycosidic linkages imparts a twist to the fflnylose chain. Where the main chain is roughly linear- in cellulose, it is helical in anylose. Attractive forces between chains are weaker in fflnylose, and fflnylose does not form the same kind of strong fibers that cellulose does. 

Examples with other Pirkle-type CSPs have also been described [139, 140]. In relation to polysaccharides coated onto silica gel, they have shown long-term stability in this operation mode [141, 142], and thus are also potentially good chiral selectors for preparative SFC [21]. In that context, the separation of racemic gliben-clamide analogues (7, Fig. 1-3) on cellulose- and amylose-derived CSPs was described [143]. 

Polysaccharide-based CSPs incorporate derivatives of cellulose and amylose adsorbed on silica gel. The selectivity of these CSPs depends upon the nature of the substituents introduced during the derivatization process. The secondary structure of the modified polysaccharide is believed to play a role in selectivity, but the chiral recognition mechanisms have not been fully elucidated [55]. 

Okamoto et al [85] performed the optical resolution of primaquine and other racemic drugs by high performance liquid chromatography using cellulose and amylose tris-(phenylcarbamate) derivatives as chiral stationary phases. Primaquine and other compounds were effectively resolved by cellulose and/or amylose derivatives having substituents such as methyl, tertiary butyl, or halogen, on the phenyl groups. 

Among optically active polymers, polysaccharide derivatives are particularly valuable. Polysaccharides such as cellulose and amylose are the most readily available optically active polymers and have stereoregular sequences. Although the chiral recognition abilities of native polysaccharides are not remarkable, they can be readily converted to the esters and carbamates with high chiral recognition abilities. The chiral recognition mechanism of these derivatives has been clarified to some extent. 

Cellulose and amylose derivative CSPs are mostly used, in the normal-phase mode, with n-hexane-based mobile phases containing some alcohol as modifier. Chromatographic performances, retention and selectivity, are reported to be affected by the composition of the mobile phase... 

Table I. Measured and Computed Room Temperature Characteristic Ratio and Temperature Coefficients for Cellulosic and Amylosic Chains...

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