Computational details
To calculate the acidity of the CH3-pentose and
aldo-pentose sugars, the lowest–energy conformers of neutral sugar and
corresponding conjugate bases were first explored and selected at the
relative energy range of 0–10 kcal.mol-1 by using the
Merck Molecular Force Field (MMFF) in Spartan 14 software [33].
Then, the most stable conformers were optimized based on the B3LYP
(Becke-Lee-Yang-Parr) version [34, 35] of the density functional
theory (DFT) method with the 6-311++G(d, p) basis set [36, 37].
Using the DFT method, the lowest energy conformations were achieved by
full geometrical optimization of each sugar and its anion. The absence
of negative imaginary frequencies proved that energy-minimized
structures correspond well to the local minima of the energy landscape
(local minima were verified by establishing that the matrix of energy
second derivatives has only positive eigenvalues). The gas phase acidity
values of OH groups of methyl-pentose and aldo-pentose sugars were
calculated at B3LY/6-311++G (d, p) level. Quantum theory of atoms in
molecules (QTAIM) of Bader [38, 39] was employed for the structures
optimized at the B3LYP/6-311++G (d,p) using the AIM2000 software
[40] package to obtain the non-covalent interactions information
including hydrogen bond in neutral Deoxy-hexose sugars and their
conjugate bases. For this purpose, the relevant bond critical points
(BCPs) were located and the charge density for each BCP was evaluated.
Furthermore, natural bond orbital (NBO) analysis [41] was carried
out using the same method and basis set to gain insight into
intramolecular hydrogen bonding interaction of these compounds.