Effect of acetate substituents on the conformations of di- and polysaccharides

Abstract

In an effort to analyse the conformational differences which arise due to acetyl substituents in polysaccharides, crystal structure data on free and acetylated disaccharides are compared. Marked differences are found in the torsional angles around the skeletal bonds as well as the glycosidic bonds in cellobiose compared with acetyl cellobiose. The cause and the significance of such variations are examined in terms of calculated conformational energy maps. Calculations suggest the possibility of extrapolating the results on disaccharides to predict the structural differences in the respective free and acetylated polysaccharide counterparts. It is concluded that any theoretical investigation on free or acetylated polysaccharide should be conducted only with data derived from the identically related oligomeric crystal structures. When this is done for acetyl xylan and acetyl mannan, one can readily explain why the former changes from threefold to twofold chain symmetry and vice versa for the latter when one goes from the native polysaccharide to the acetate derivative. Comparison of cellobiose and acetyl cellobiose conformational energy maps shows that in the former intramolecular hydrogen bonds are much more important conformational influences than intermolecular ones, while in the latter non-bonded steric effects are the dominant factor. © 1975, Walter de Gruyter. All rights reserved.