Analysis of the Oxidative Destruction Routes of Trityl Radicals Caused by Superoxide

Triarylmethyl radicals (TAM, trityls) presented in the form of water-soluble tricarboxylic acids are widely used in biomedical, chemical and spectroscopic studies. In most applications, trityl radicals demonstrate high stability against various redox agents. An exception is the oxidative degradation of TAM to diamagnetic quinone methides. A typical active form of oxygen in biological environments, superoxide (O2· - or HO2·) is involved in this reaction. The details of this process have been studied in order to identify the factors controlling the channel of TAM consumption. The study has been performed using quantum chemical calculations (by means of the density functional theory) on a model trityl - triphenylmethyl radical presented in the form of a monocarboxylic acid. Alternative routes differing in the nature of protonation of key reagents in the aqueous medium have been considered. It has been established that protonation/deprotonation of reagents controls the barrier of the rate-limiting stage and, thus, the rate of the observed decarboxylation. This conclusion is in good agreement with the experimental data on the dependence of the reaction rate on pH. Among the reaction pathways considered, the most favourable one is the interaction of TAM anion, containing deprotonated carboxyl group, with protonated superoxide (neutral hydroperoxide radical HO2·). Taking into account the identified features, an analysis of alternative oxidation routes of another TAM derivative was carried out, for which the role of the key stage is assigned to proton elimination.