Effect of acetate substituents on the conformations of di- and polysaccharides
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. INTRODUCTION Many of the naturally occurring polysaccharides are often found to be partially acetylated" 2, Analysis of crystalline conformations by x-ray diffraction methods has shown that the chain symmetry of free and acetyl derivatives of polysaccharides differs significantly3'°. Examples of this are listed in Table 1. It seems likely that the observed changes in the fibre repeat and the screw symmetry of the helical polymer chain are the result of steric imperatives. Such spatial adjustments could be due either to intramolecular or intermolecular interactions, or both. The importance of understanding the role played by the substituents in their influence on the preferred conformation of the polymer cannot be overemphasized, since the local conformation of a polysaccharide greatly influences its chemical and physical properties, and appears to be a determining factor in enzymatic recognition11. Evidence has accumulated in recent years that the short-range preferred conformation of a polymer is close to that of the related dimer or oligomer. Differences in the conformational features of free and acetyl derivatives of