Conformational studies of α-glucans

Abstract

A study of the effect of linkage on the possible conformations of di-and polysaccharides of α -D-glucose and also the probable intramolecular hydrogen bonds has been made. The differences in the nature of linkage is shown to effect the energetically preferred conformations; (1 → 2) linkages lead only to righthanded helical conformations, (1 → 3) linkages lead to extended as well as both left and righthanded helical conformations; (1 → 4) linkages lead to both right-and lefthanded wide helical conformations. The possible hydrogen bonds between adjacent residues are also dependent on the nature of the linkage. A comparison of the conformational data of α-D-glucans with those of β -D-glucans has indicated that the favored conformations and hydrogen bonds between contiguous residues in the chain are influenced by the configuration at the anomeric carbon atom in all the three types of polysaccharides. From the energy calculations a probable conformation (φM = -10°, ΨM = -30°, φN = -23°, ΨN = -19°) has also been proposed for crystalline mycodextran in conformity with x-ray data. This conformation contains two types of hydrogen bonds between contiguous residues one between 0-2 and 0-3 atoms at (1 → 4) linkage and the other between 0-2 and 0-4 atoms at (1 → 3) linkage in the chain. The conformation of maltose unit (-10°,-30°) that is likely to occur in the crystalline mycodextran coincides with the minimum energy conformation of maltose.