A Mechanictic Insight into Superoxide Scavenging by Hydroxycinnamic Acid Derivatives Involving Caffeic Acid through Proton-coupled Electron Transfer

Abstract

This research clarifies a mechanistic insight into scavenging of superoxide radical anion (O₂^•-) by (2E)-3-(3,4-dihydroxyphenyl)prop-2-enoic acid (caffeic acid), classified as a hydroxycinnamic acid (HCA) derivatives. With the aid of density functional theory (DFT) calculations, the reactivity of caffeic acid toward electrogenerated O₂^•- was examined using cyclic voltammetry, in situ electrolytic electron spin resonance spectrometry, and in situ electrolytic ultraviolet-visible spectrometry in N,N-dimethylformamide (DMF). Caffeic acid appears to modify the quasi-reversible redox of dioxygen/ O₂^•-, indicating that it scavenges O₂^•-  by proton-coupled electron transfer.  Experimental evidence from comparison studies with other HCAs has shown that the ortho-diphenol (catechol) moiety rather than the acryloyl group mediates the reactivities of caffeic acid toward O₂^•-. The electrochemical and DFT measurements in DMF indicated that the catechol moiety serves as the intermediary in a coordinated two-proton-coupled electron transfer pathway. This method exemplifies the better kinetics of caffeic acid's O₂^•- scavenging.