Figure 1. General structure of Deoxy-hexose sugars; (a) represents CH3-pentose and (b) represents Aldo-pentose sugars. (c) For the atom label’s we used Mamony et al. [17] and IUPAC [18] . β-D-xylose is an example of atom labels.
Non-covalent interactions, especially hydrogen bonding, are dominant factor in the machinery of carbohydrate molecular recognition [19, 20]; and more generally in the maintenance of their preferred conformational structures. The H-bond occurs between a proton donor (attached covalently to a highly electronegative atom such as O, N and F) and one of these heteroatoms. Also, during the last decade, the existence of weak CH…O hydrogen bonds in many biological structures was clearly established [21]. All OH groups in saccharides sugars can be participated in the H-bond donors and acceptors and make the stabilized favored conformations. In general, the minimum energy value for weak H-bond is as low as 0.24-0.28 kcal.mol-1, whereas, it can be reached to maximum 38 kcal.mol-1 for strong H-bonds. Also, On average, most H-bond energy values in sugars fall in the range of 1.2-7.2 kcal.mol-1 [22-24]. Moreover, according to numerical theoretical studies, the energy of CH…O bond is estimated between 0.1-1 kcal.mol-1 [25, 26]. The H-bond with the terms such as regular (two-center), bifurcated (three-center), or trifurcated (four-center) are defined by Jeffery and Saenger [27, 28]. It is ambiguous to access directly the presence and configuration of intramolecular H–bonds along with their relative strength by experimental methods. Therefore, the only reference data are theoretical results to supply reliable information and to characterize intramolecular O-H…O and C-H…O bonds. Gorbiz and Etter [29] studied the existence of the three-centered H–bonds with carboxylate groups based mainly on geometric parameters. The existence of bifurcated H-bond in rare sugar and also C-H…O bonds in Guanosine was investigated using computational calculations [30, 31].
Moreover, H-bonds could organize three-dimensional structures in compounds containing of O-H and N-H bonds. Also, the H-bonds could lead to enhanced acidity. In our previous studies, the role of H-bonds on the acidity of a series of polyols such as 2,3 (HOCH2CH2CH(OH)CH2)3COH was investigated. It was found that multiple intramolecular H-bonds can dramatically increase gas phase acidity in these alcohols [32].
In this study, we provide a comprehensive theoretical examination of the gas phase thermochemical properties of deoxy-hexose monosaccharide sugars by employing density functional theory (DFT, B3LYP) with the 6-311++G (d, p) basis set. The goal of this study is to provide insight into the electronic properties, H-bond pattern and influence of H-bonds on the gas phase acidity of L-fucose, L-rhamnose, D-xylose, L-lyxose, D-ribose and L-arabinose. Furthermore, we use topological parameters such as electron density and Laplacian of electron density from Bader’s atoms in molecules (AIM) theory and natural bond orbital (NBO) analysis to interpret different types of intramolecular hydrogen bonds in Deoxy-hexose sugars.