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.