where, E298 represents the calculated energy including thermal vibrational corrections. The HA andA represent acidic sites of sugar-OH and its conjugated base of sugars-O-, respectively. The(5/2) RT term includes the translational energy of the proton and the ∆(PV) term. The gas phase values are positive and represent enthalpy requirement for deprotonating the neutral species at 298 K. The gas phase dissociation is endothermic due to the absence of solvent to stabilize the generated ions. Therefore, the lower values of∆Hacid refer to stronger acidic (i.e., easier deprotonation). Table 2 summarizes the calculated gas-phase acidity (∆Hacid ) values in kcal.mol-1for different OH groups for the most stable conformers of L-fucose, L-rhamnose, D-xylose, L-lyxose, D-ribose and L-arabinose sugars (Figure 1) according to Eq.1 at the B3LYP/6-311++G (d,p) level.