A DFT AND AIM TOPOLOGICAL STUDIES OF α-D-GLUCOPYRANOSIDE AND α-D GALACTOPYRANOSID

Density functional theory calculations on three different conformers: gt (gauchetrans), gg (gauche-gauche) and tg (trans-gauche) of two glycosides, namely, n-octyl-α-D-glucopyranoside (C8O-α-Glc) and n-octyl-α-D-galactopyranoside (C8O-α-Gal) were performed for geometry optimization at the B3LYP/6-311G* level of theory. Both molecules are stereoisomers (epimers) differing only in the orientation of the hydroxyl group at the C4 position. Thus it is interesting to investigate electronically the effect of the direction (axial/equatorial) of the hydroxyl group at the C4 position and also consider the effect of the orientation of hydroxyl rotamer on the C6 position. The structure parameters of X-H∙∙∙Y intramolecular hydrogen bonds were analyzed, while the nature of these bonds and the intramolecular interactions were considered using the atoms in molecules (AIM) approach. The results of the gas and solvated systems show that the numbers of the intramolecular hydrogen bonds and also the exocyclic rotamer, are two important factors in determining the stability of the sugar molecule. Therefore, C8O-α-Glc with two intramolecular hydrogen bonds in gg and tg conformers is more stable than in gt conformer with only one intramolecular hydrogen bond.