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

The reaction mechanism between electrogenerated superoxide (O2−) 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 O2/O2− redox is significantly affected by the presence of a catechol (CatH2), showing the associated reaction, O2−+CatH2→H2O2+Cat−. 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 O2− and CatH2, 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 O2− and CatH2 is demonstrated by the finding that a signal of the O2− and HO2 adducts of 5,5-dimethyl-1-pyrroline-N-oxide as a spin trap completely disappears in the ESR spectra of the electrogenerated O2− solution containing CatH2. This indicates that the concerted 2PCET reaction is important for the analysis of polyphenolic antioxidants containing an o-diphenol ring.