Theoretical studies of l-ascorbic acid (vitamin C) and selected oxidised, anionic and free-radical forms

Calculations were carried out at the restricted and unrestricted B3LYP/6-311++G(d,p) and B3LYP/EPR-II levels for two conformers (1 and 2) of l-ascorbic acid and their respective oxidation products to di- and monodehydroascorbates. For the didehydroascorbates, corresponding to Conformer 1, free radical properties are compared with previously published calculations in the gaseous and aqueous solution states and with experimental EPR values. Calculated molecular structures, EPR and vibrational spectral and energetic properties are reported including some proposed changes to previous EPR assignments. Conformer 2 of l-ascorbic acid is predicted to have lower energy than Conformer 1, under the method and basis sets used, by between 11 and 26 kJ mol−1 and is stabilised by internal hydrogen bonding. For particular monodehydroascorbates, formed by loss of H from the chain CH(OH) group, calculated properties of conformers of five neutral mono-radicals and the corresponding five anionic di-radicals are reported. Relaxed potential energy surface (PES) scans were carried out for two proton transfer processes and relative energies of stable minima and barriers between them determined. The physiological significance of the protective oxidation of l-ascorbic acid by free radicals is briefly described.