Concerted two-proton-coupled electron transfer from catechols to superoxide via hydrogen bonds

The reaction mechanism between electrogenerated superoxide (O-2(center dot-)) and catechols as an important functional group of polyphenolic antioxidants was investigated based on electrochemical and electron spin resonance (ESR) spectral measurements and supported by density functional theory (DFT) calculations. The quasi-reversible O-2/O-2(center dot-) redox is significantly affected by the presence of a catechol (CatH(2)), showing the associated reaction, O-2(center dot-) + CatH(2) -> H2O2 + Cat(center dot-). We proposed that this reaction is governed by one-step one-electron transfer concerted with two phenolic proton movement after initial formation of the hydrogen-bonded complexes between O-2(center dot-) and CatH(2), which we refer to as concerted two-proton-coupled electron transfer (2PCET) reactions, based on the energetics and kinetics of the reaction obtained by B3P86/6-31 + G(d,p) and B3LYP/6-31 + G(d,p) calculations, respectively. The concerted 2PCET mechanism for the reaction between O-2(center dot-) and CatH(2) is demonstrated by the finding that a signal of the O-2(center dot-) and HO2 center dot adducts of 5,5-dimethyl-1-pyrroline-N-oxide as a spin trap completely disappears in the ESR spectra of the electrogenerated O-2(center dot-) solution containing CatH(2). This indicates that the concerted 2PCET reaction is important for the analysis of polyphenolic antioxidants containing an o-diphenol ring. (C) 2016 Elsevier Ltd. All rights reserved.