The capture of ˙H and ˙OH radicals by vitamin C and implications for the new source for the formation of the anion free radical

Abstract

To gain a better understanding of the antioxidation behaviors of vitamin C, the reactions between vitamin C (monoanionic form, AAH⁻) and two radicals, ˙H and ˙OH, have been investigated employing the B3LYP and BHandHLYP methods in combination with the atoms in molecules (AIM) theory and energy decomposition analyses (EDA). Both the radical additions to the five-membered ring of AAH⁻ and H-abstraction reactions are explored. The reaction profiles of various reactions have been obtained. The most favorable active site to be attacked by radical addition has been confirmed to be the C2 site of AAH⁻, which is different from that of the C3 site in the neutral vitamin C. The ˙OH addition reactions are essentially diffusion-controlled processes, which is in contrast to the previous reports. A new source for the formation of the principal anion free radical (AFR) of AAH⁻ has been observed in the ˙OH attack process, i.e., AFR can be formed mainly from the H13 abstraction reaction involving two types of concerted proton–electron transfer (CPET) mechanisms. Moreover, the binding characters and formation mechanisms of the stable reaction complex formed during the formation of AFR have been systematically investigated.