Computational insight into networking H-bonds in open and cyclic forms of galactose

Abstract

In this work, the intramolecular H-bonds in galactose were studied using DFT at the B3LYP/6–311++G (d,p) levels of theory, atoms in molecules (AIM), and natural bond orbital (NBO). AIM and NBO analysis revealed a cooperative network of trifurcated, bifurcated, and normal H-bonds for the conjugate bases of open galactse (O-Gal). While for the conjugate base of the cyclic form of galactose, we identified bifurcated and normal H-bonds, which may highlight a crucial feature of the biological activity of a whole class of natural sugars. The O-H…O bonds are categorized as mostly electrostatic, strong H-bonds and more favorable, whereas for multiple interactions involving C=O...H, C-H…O and C-H…H-C (dihydrogen bonding) interactions are categorized as weak H-bonds bonds in the conjugate base of considered sugar. The theoretical results indicated that based on acidity values (ΔHacid), galactose in open form (O-Gal) is more acidic than cyclic form (Gal).The acidity value for galactose in open and cyclic forms exhibit significantly lower values (i.e., stronger acid) compared to the reported acidity value for α-/ß- anomers of D-glucopyranose and simple alcohols because their conjugate bases are more stabilized through intramolecular H-bonds like trifurcated and bifurcated, which are studied in details using AIM and NBO analysis.