A Computational Study on the Acidity Dependence of Radical-Scavenging Mechanisms of Anthocyanidins

On the basis of quantum chemical calculations, the radical-scavenging property attributed to anthocyanidins was analyzed considering three mechanisms: hydrogen atom transfer (HAT), stepwise electron-transfer-proton-transfer (ET-PT), and sequential proton loss electron transfer (SPLET). We found that the activity of anthocyanidins and the mechanism through which they react are pH-dependent, because the diverse colorful forms in which anthocyanidins may exist in prototropic equilibria (cationic, neutral, anionic) are susceptible to experience each of the mechanisms proposed. According to redox parameters calculated, we can conclude that HAT is always the most favored of the generally accepted mechanisms to scavenge reactive oxygen species (ROS) by the three colored forms. Nevertheless, only neutral and anionic forms are found to be able to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH center dot) radical through HAT and SPLET mechanisms from a thermodynamical point of view, whereas ET-PT is only feasible for anions. Sequential proton loss hydrogen atom transfer (SPL-HAT) is proposed as the only pathway for the reaction between anthocyanidin cations and the DPPH center dot radical. It should be viable according to our quantum mechanical calculations and even competitive with typical HAT, ET-PT, and SPLET.