Rotamers arising from restricted motions and electromc interactions of acetoxymethyl groups in carbohydrates

Abstract

The two dimensional conformational spaces available for acetoxymethyl groups in carbohydrates are reported. The studies were performed on two molecules differing in their C-4 configurations, i.e. gluco and galacto. The acetoxymethyl group was assigned a mean geometry derived from the computation of 19 equivalent groups as found in pertinent acetylated carbohydrates. The internal energy calculations were performed by including the partitioned contributions arising from the non-bonded energies, electrostatic interactions, and torsion energy. An instability rating of 3 kcal/mol was assigned to conformations involving peri interactions, i.e., trans–gauche and gauche–gauche forms in gluco and galacto carbohydrates, respectively. The results which are best described in term of topology show that several conformations corresponding to the trans and gauche states of the C—O single bond are preferred. However, significant shifts from perfect staggering are observed. The predicted regions of low energy are found to be in good agreement with experimental data from crystal structures of acetylated carbohydrates.