Influence of H‐bonds on acidity of deoxy‐hexose sugars

Abstract

The unusual monosaccharaides such as deoxy‐hexose sugars, including methyl‐pentose and aldo‐pentose, are promising and important sugars in life science. However, little research on H‐bond interactions in these systems has been reported. The aldo‐pentose has a proton instead of the CH2OH group on C5; conversely, methyl‐pentose has a CH3 group on C5. The aim of the present study is to investigate the role and nature of intramolecular H‐bonds on acidity of CH3‐pentose sugars (L‐fucose and L‐rhamnose) and aldo‐pentose sugars (D‐xylose, L‐lyxose, D‐ribose, and L‐arabinose) using B3LYP/6‐311++G (d, p) level. The calculated acidity values (ΔHacid) of these Dexoy‐hexose were found to be from 343 to 369 kcal.mol−1, indicating they are stronger acid than ethanol and 2‐propanol with the acidity values of 378.3 and 375.1 kcal.mol−1, respectively. This is related to the stabilization of the conjugate bases of these sugar through intramolecular H‐bonds, which were analyzed in this study using atoms in molecules (AIM) and natural bonding orbital (NBO) methods. AIM and NBO analyses indicate the presence of one bifurcated intramolecular H‐bond in the conjugate bases of L‐lyxose and L‐arabinose and two bifurcated H‐bonds in conjugate base of D‐ribose, whereas the conjugate bases of L‐fucose, L‐rhamnose, and D‐xylose present one normal intramolecular H‐bond. According to the topological parameters and charge transfer data, existence of normal and bifurcated intramolecular H‐bonds could greatly increase acidity of deoxy‐hexose sugars. The H‐bond strength in the conjugate bases of aldo‐pentose sugars is higher than that in the conjugate bases of methyl‐pentose sugars including CH3 group on C5, making aldo‐pentose sugars stronger acid than methyl‐pentose sugars.

Theoretical Study of Hydrogen Bond Formation in Four Rare Sugars: Gulose, Allose, Altrose, and Talose

“…for deoxy-hexose sugars [26]; 2.17-2.19 (Å) for α/β-D-mannose [24]; and 1.85-2.11 (Å) for α/β-Dglucopyranose and α/β-D-galactopyranose [45]. Accordingly, our calculated results are comparable with those reported in the references [24,26,[44][45][46].…”

“…Several theoretical methods have been used to investigate the structural and electronic properties of carbohydrate sugars to understand the nature of H-bonds interactions [24][25][26][27][28]. The present study aims to investigate the H-bonds formation and the orientation effects of five hydroxyl groups in the four rare sugars theoretically.…”

Computational insight into networking H‐bonds in open and cyclic forms of glucose

“…These results are in good agreement with our previous works for other similar systems, where the results indicated that the intramolecular H‐bonds dramatically affect the stability of the deprotonated sugar forms. [ 61,62 ]…”