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.