Electrochemical and Mechanistic Study of Superoxide Scavenging by Pyrogallol in N,N-Dimethylformamide through Proton-Coupled Electron Transfer

Abstract

Scavenging of electrogenerated superoxide radical anion (O2•−) by pyrogallol (PyH3) was investigated on the basis of cyclic voltammetry and in situ electrolytic electron spin resonance spectrum in N,N-dimethylformamide with the aid of density functional theory (DFT) calculations. Quasi-reversible dioxygen/O2•− redox couple was modified by the presence of PyH3, suggesting that O2•− was scavenged by PyH3 through proton-coupled electron transfer (PCET) involving two proton transfer and one electron transfer. DFT calculation suggested that the pre-reactive formation of a hydrogen-bond (HB) complex and the subsequent concerted two-proton-coupled electron transfer characterized by catechol moiety in PyH3 is plausible mechanism that embodies the superior kinetics of the O2•− scavenging by PyH3 as shown in the electrochemical results. Furthermore, it was clarified that the three hydroxyl groups of PyH3 promote the formation of HB complex, in comparative analyses using related compounds, resulting in the promotion of the O2•− scavenging.

Reactivity of trans-Resveratrol toward Electrogenerated Superoxide in N,N-Dimethylformamide

“…Otherwise, one-step one ET concerted with two PT without generating high-energy intermediates is a feasible pathway. We refer to this pathway as the concerted two-proton-coupled electron transfer (2PCET) reaction. ,, Notably, PT4 is exergonic (−45.8, −70.6), meaning that the second PT must be coupled to ET via a HB. Considering together that 4′OH in the para-position plays a significant role in resonance for a successful ET, − the second PT coupled to ET feasibly occurs in 4′OH.…”

Concerted Two-Proton-Coupled Electron Transfer from Piceatannol to Electrogenerated Superoxide in N,N-Dimethylformamide

“…We refer to this pathway as the 2PCET reaction. [23][24][25]36 Notably, 2PCET must occur in a one-kinetic process among an HB complex formed from the catechol moiety of PiceH 4 to form O 2 •− . The ΔG 0 values of the PCET pathways of PiceH 4 , RsvH 3 , and pinosylvin were calculated by using B3LYP, M06-2X, and TPSSh functionals for comparison (Table 1).…”

“…We previously analyzed the PCET reaction between electrogenerated O 2 •– and phenolic antioxidants including catechols, − then, trans -resveratrol, in N , N -dimethylformamide (DMF). Here, the O 2 •– cannot accept electrons from phenolic substrates because the O 2 •– is not so electrophilic; however, the HO 2 • (a protonated form of the O 2 •– ) is a strong oxidant.…”

Reactivity of trans-Resveratrol toward Electrogenerated Superoxide in N,N-Dimethylformamide

“…We refer to this pathway as the concerted two-proton-coupled electron transfer (2PCET) reaction. 25,33,37 Notably, PT4 is exergonic (−45.8, −70.6), meaning that the second PT must be coupled to ET via a HB. Considering together that 4'OH in the para-position plays a significant role in resonance for a successful ET, [12][13][14][15] the second PT coupled to ET feasibly occur in 4'OH.…”