Conformational analysis of polysaccharides. Part V. The characterization of linkage conformations (chain conformations) by optical rotation at a single wavelength. Evidence for distortion of cyclohexa-amylose in aqueous solution. Optical rotation and the amylose conformation

Abstract

A relationship is suggested between the optical rotation measured at a single wavelength for a di-, oligo-, or polysaccharide, and the conformation at the glycosidic linkage as expressed in torsion angles about the C–O and O–C bonds; it is based on Kauzmann's additivity principles for optical rotation, Brewster's empirical but universal treatment in terms of screw patterns of polarizability, and Whiffen's earlier suggestions for carbohydrates. By use of conformation details from crystal structures with the optical rotations of component monosaccharide derivatives, it is then possible to predict the molecular rotations of β-cellobiose and α-lactose derivatives in water, and of cyclohexa-amylose in dimethyl sulphoxide, to within a few degrees. The molecular rotation of cyclohexaamylose in water shows an anomaly of 30° per residue but this vanishes when the inclusion complex is formed with saturated hydrocarbon derivatives; thus it appears that the macro-ring suffers some distortion in water, which is relieved when a complex is formed with cosolute, and which does not occur in dimethyl sulphoxide solution.

The agreement between observed and calculated rotations is not so good for maltose derivatives, but the discrepancy is resolved by conformational analysis, which shows why the crystal conformation is not retained in solution, and is supported by ¹H n.m.r. spectroscopy. The optical rotations of α-cellobiose derivatives, larger cycloamyloses, and amylose, are used to deduce the solution conformations; these are highly compatible with principles of conformational analysis. In amylose, there is evidence for weakly co-operative hydrogen bonding along the chain, even in aqueous solution.

Optical rotation appears to be highly sensitive to conformation at the glycosidic linkage, and a very promising method for the exploration of polysaccharide stereochemistry, although interpretation is not straightforward in the presence of uronic acid residues, and possibly of acetamido-groups.