Importance of Proton-Coupled Electron Transfer from Natural Phenolic Compounds in Superoxide Scavenging

The superoxide O2(·-) scavenging reaction of (+)-catechin (Cat), quercetin (Que), rutin, and α-tocopherol (α-TOH) as natural phenolic compounds is investigated on the basis of electrochemical and ESR spectral measurements with the aid of density functional theory (DFT) calculations. Reversibility of the O2/O2(·-) redox couple is significantly affected by the presence of the phenolic compounds. The catechol moiety of Cat, Que, and rutin plays an essential role in concerted proton-coupled electron transfer (PCET) to HO2(·) derived from O2(·-) to give H2O2 and the corresponding o-benzoquinone radical anions. On the other hand, the presence of α-TOH causes sequential electron and proton transfers to HO2(·) to give H2O2 and the α-tocopheroxyl radical. These electron transfers in the presence of the phenolic compounds are inferred from the ESR spectral measurements. The DFT calculation results suggest that the O2(·-) scavenging reaction of the natural phenolic compounds proceeds efficiently with the one-step concerted PCET or sequential PCET mechanism.