Frontier orbitals and transition states in the oxidation and degradation of l-ascorbic acid: a DFT study

Abstract

DFT calculations were carried out to investigate reaction paths of L-ascorbic acid (AAH₂), hydroxyl radicals and water clusters. Frontier-orbital analyses were also performed to examine the regioselectivity of the OH˙ addition. Transition states of the electrolytic dissociation of AAH₂ and intermediate carboxylic acids were found to have very small activation energies through proton transfers along hydrogen bonds. The ionized species (anions) are subject to the electrophilic attack of OH˙. The elementary processes of AAH₂ → A˙⁻ → dehydroascorbic acid → diketogulonic acid → threonic, oxalic, xylonic and lyxonic acids were investigated and discussed. The processes involved in the conversion of dehydroascorbic acid into a bicyclic hemiketal were also examined as a side-chain participating reaction. The oxidation and degradation of vitamin C up to threonic acid were described mainly as a donor (AAH₂)–acceptor (OH˙) reaction.