Polysaccharide conformation. Part 10. Solvent and temperature effects on the optical rotation and conformation of model carbohydrates

Abstract

The optical rotations of simple carbohydrates vary with temperature and solvent even after allowance for mutarotaton and local field effects. The variation can be as large as 70° in molecular rotation. For monosaccharides we show that the sign and magnitude can be estimated by an extension of Brewster's treatment of the optical activity of sugars. The mechanism of solvent action is probably through direct spectroscopic perturbation of the optically active transitions. The conformation of the glycosidic linkage in crystalline di- and oligo-saccharides is often but not always preferred in solution. At least for the examples we have studied, change of solvent does not have a major influence on the conformation at equatorial–equatorial linkages. In the series of di- and oligosaccharides that have an axial glycosidic bond, however, it seems that change of solvent can induce a redistribution between conformational states at the glycosidic linkage. For methyl β-maltoside, α,α-trehalose, and cyclohexa-amylose, transfer from non-aqueous to aqueous solvent appears to favour ‘folded’ conformations in which adjacent sugar rings shield C–H bonds from water and expose hydroxy-groups in an array that is more compatible with the structured component of liquid water. For all the solvent influences that we have observed, the largest effects are seen between the least polar solvent (dioxan) and water. Water differs from other solvents in its influence more than other solvents differ between themselves and shows its most extreme behaviour when it contains a maximum of hydrogen bonded structure. Breakdown of this structure by heating or addition of alkali, moves the properties towards those of, for example, dimethyl sulphoxide.