Pcilo calculations on 2-acetamido-2-deoxy-α- d-galactose and O-(2-acetamido-2-deoxy-α- d-galactosyl)-(1→3)-2-acetamido-2-deoxy- d-galactose

Abstract

PCILO semi-empirical quantum-chemical calculations have been conducted on 2-acetamido-2-deoxy-α- d-galactose (GaINAc) and the α- d-(1→3)-linked di-GaINAc disaccharide component of the Forssman antigen, in order to determine the favored orientations of their side groups, and the mutual orientations of the two pyranoid rings in the latter. For the N-acetyl group, three stable positions have been located, in one of which the plane of the amide unit is normal to the mean plane of the pyranoid ring. The orientation of the hydroxyl group attached to the anomeric carbon atom is found to be in accord with the exo-anomeric predictions. Furthermore, all of the side-group orientations of GaINAc found in the present study are in good agreement with the results of X-ray investigations. A grid-search method was used to locate all low-energy conformational regions for the glycosidic dihedral angles of O-(2-acetamido-2-deoxy-α- d-galactosyl)-(1→3)-2-acetamido-2-deoxy- d-galactose. Two allowed regions were found, and energy minimization was then performed in each of them. For this disaccharide, three sets of favored orientations of the (reducing) pyranose ring relative to the nonreducing ring were found. One set of glycosidic torsion angles is very close to that found from our X-ray diffraction results, n.m.r.-spectral observations, and hard-sphere exo-anomeric calculations.