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

Tatsushi Nakayama,Bunji Uno,Kazuo Kuwata,Shigeyuki Usui,Ryo Honda

doi:10.3390/antiox11010009

Abstract

Scavenging of superoxide radical anion (O2•−) 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/O2•− redox was modified by the presence of TOH, suggesting that the electrogenerated O2•− was scavenged by α-, β-, γ-TOH through proton-coupled electron transfer (PCET), but not by δ-TOH. The reactivities of α-, β-, γ-, and δ-TOH toward O2•− 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 O2•− 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 O2•− scavenging by TOH proceeds efficiently along the PCET mechanism involving one ET and two PTs.

Highlights

IntroductionCharacterized by the methyl group on the 6-chromanol ring was experimentally confirmed, where the methyl group promotes the proton-coupled electron transfer (PCET) mechanism
The reversible cyclic voltammograms (CVs) investigated here were all modified to irreversible ones by the presence of any compounds (a–c, e–h), except (d) δ-TOH, with concentration dependency (0 to 5.0, 10.0 × 10−3 mol dm−3 ), which is similar to general phenolic compounds [17,18,19,20,25,26], demonstrating that CVs of bubbled N2 showed no peak over the potential range
The effect of tertiary butyl (t-Bu) groups at positions 2 and 6 on the O2 − scavenging is expected to be similar to that of the methyl group. These results suggested that the para-oxygen-atom in the 6-chromanol ring of TOH, together with the methyl groups, is essential for successful O2 − scavenging through the proton-coupled electron transfer (PCET), increasing electron density in the benzene ring, and increasing its reactivity to electrophilic attack

Summary

Introduction

Characterized by the methyl group on the 6-chromanol ring was experimentally confirmed, where the methyl group promotes the PCET mechanism. 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 O2 − through the PCET. The electrochemical and DFT results in dehydrated DMF suggested that the PCET mechanism involves the preceding proton transfer (PT). The O2 − scavenging by TOH proceeds efficiently along the PCET mechanism involving one ET and two PTs. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Four structurally related compounds (α-, β-, γ-, and δ-TOH) with saturated phytyl chains are designated as TOH, while tocotrienols are differentiated by the presence of double bonds in the 30 , 70 , and 110 positions of the alkyl side chain.

Methods

Results

Conclusion