Scavenging of hydroxyl, methoxy, and nitrogen dioxide free radicals by some methylated isoflavones

Free radicals can be scavenged from biological systems by genistein, daidzein, and their methyl derivatives through hydrogen atom transfer (HAT), single-electron transfer (SET), and sequential proton-loss electron-transfer (SPLET) mechanisms. Reactions between these derivatives and the free radicals OH^., OCH₃^., and NO₂^. via the HAT mechanism in the gas phase were studied using the transition state theory within the framework of DFT. Solvation of all the species and complexes involved in the HAT reactions in aqueous media was treated by performing single point energy calculations using the polarizable continuum model (PCM). The SET and SPLET mechanisms for the above reactions were also considered by applying the Marcus theory of electron transfer, and were found to be quite sensitive to geometry and solvation. Therefore, the geometries of all the species involved in the SET and SPLET mechanisms were fully optimized in aqueous media. The calculated barrier energies and rate constants of the HAT-based scavenging reactions showed that the OH group of the B ring in genistein, daidzein, and their methyl derivatives plays a major role in the scavenging of free radicals, and the role of this OH group in the HAT-based free-radical scavenging decreases in the following order: OH^. > OCH₃^. > NO₂^.. The SPLET mechanism was found to be an important mechanism in these free-radical scavenging reactions, whereas the SET mechanism was not important in this context.