Deprotonation and acidity characterization of biomass sugars: a first-principles study.

Deprotonation of biomass sugars is investigated by high-level electronic structure calculations, considering α- and β-anomers as well as all O and C sites. Structural alterations are generally focused on the deprotonated sites. Proton migration or/and ring opening occurs in 15 of 42 deprotonation processes and more frequently for D-fructofuranose than D-glucopyranose conformers. Other regular patterns of structural changes are observed; e.g., deprotonation at anomeric sites often cause larger structural perturbations, strong H-bonds of O(i)H(i)•••C(i) type are constructed by deprotonation at C sites. Then the enthalpy changes and Gibbs free energies for deprotonation at the various O/C sites are calculated by an economical and sufficiently testified composite method, which are consistent with previous results available. Deprotonation of anomeric groups is always preferred, while not the case for C sites. D-fructofuranose is more acidic than D-glucopyranose, whether for O or C sites. Although O sites are known to deprotonate, some C sites are found to be comparable, or even preferential. Whether for O or C sites, deprotonation is highly regioselective and this clearly indicates the potential utilization for selective conversion of biomass sugars.