Theoretical and electrochemical study of the mechanism of anthraquinone-mediated one-electron reduction of oxygen: the involvement of adducts of dioxygen species to anthraquinones

Anthraquinone derivatives, which are an important class of anticancer drugs, possess the ability to mediate the transfer of one electron to molecular oxygen to form the superoxide anion radical, which results in their undesirable peroxidating and, further, cardiotoxic properties. In this paper one-electron reduction of dioxygen–anthraquinone systems was studied using electrochemical and theoretical methods. Cyclic voltammetry (CV) experiments performed on dimethyl sulfoxide and dimethylformamide solutions of selected anthraquinones suggest that anthraquinones bearing hydroxy groups or their semiquinones interact remarkably with molecular oxygen; this is manifested as a shift of anthraquinone-reduction potential towards more positive values in the presence of oxygen. This phenomenon can be explained by the assumption that anthraquinone reduction is accompanied with oxygen addition to form hydroperoxide anion radicals, which can be formed by anthraquinones possessing proton-donor (e.g. hydroxy) groups only; the calculated (using the ab initio, DFT and semiempirical PM3 methods, with 1-hydroxynaphthoquinone and model anthraquinone derivatives as model compounds) enthalpies of this process are greater than those of anthraquinone reduction alone.