Electrochemical and Mechanistic Study of Reactivities of α-, β-, γ-, and δ-Tocopherol toward Electrogenerated Superoxide in N,N-Dimethylformamide through Proton-Coupled Electron Transfer

Scavenging of superoxide radical anion (O-2(center dot-)) by tocopherols (TOH) and related compounds was investigated on the basis of cyclic voltammetry and in situ electrolytic electron spin resonance spectrum in N,N-dimethylformamide (DMF) with the aid of density functional theory (DFT) calculations. Quasi-reversible dioxygen/O-2(center dot-) redox was modified by the presence of TOH, suggesting that the electrogenerated O-2(center dot-) was scavenged by alpha-, beta-, gamma-TOH through proton-coupled electron transfer (PCET), but not by delta-TOH. The reactivities of alpha-, beta-, gamma-, and delta-TOH toward O-2(center dot-) characterized by the methyl group on the 6-chromanol ring was experimentally confirmed, where the methyl group promotes the PCET mechanism. Furthermore, comparative analyses using some related compounds suggested that the para-oxygen-atom in the 6-chromanol ring is required for a successful electron transfer (ET) to O-2(center dot-) through the PCET. The electrochemical and DFT results in dehydrated DMF suggested that the PCET mechanism involves the preceding proton transfer (PT) forming a hydroperoxyl radical, followed by a PCET (intermolecular ET-PT). The O-2(center dot-) scavenging by TOH proceeds efficiently along the PCET mechanism involving one ET and two PTs.

Automated summary

The scavenging of superoxide radical by tocopherols was investigated using cyclic voltammetry and in situ electrolytic electron spin resonance spectrum. The results showed that tocopherols can efficiently scavenge superoxide radical through proton-coupled electron transfer mechanism.