Ab initio molecular orbital calculations on furanose sugars: a study with the 6-31G basis set.

Ab initio molecular orbital calculations were performed on 2-deoxy-beta-D-glycero-tetrofuranose (1) using the 6-31G* basis set to evaluate the effect of ring conformation on the molecular parameters (bond lengths, angles, and torsions). Geometric optimizations were conducted on the planar and ten envelope conformers of 1, and these data were compared to those obtained from previous calculations using the STO-3G and 3-21G basis sets. Conformational energy profiles derived from 3-21G and 6-31G* data were found to be qualitatively comparable. The effect of furanose ring conformation on key bond lengths (e.g., C-H, C-O), bond angles (e.g., COC), and bond torsions (e.g., the exoanomeric C-1-O-1 torsion) was examined, and a qualitative agreement was observed between the 3-21G and 6-31G* analyses. The results indicate that, for semi-quantitative ab initio studies of intact carbohydrates, the 3-21G basis set is sufficient, and that the STO-3G basis set should not be employed unless crude structural approximations are desired. The observed concerted behavior of C-O bond lengths in the vicinity of the anomeric carbon of the aldofuranose ring has suggested a possible role of C-1-O-1 bond orientation in affecting the mechanism of glycoside bond hydrolysis.