In-Situ ESR Detection of Radical Species of p-Benzoquinone in Aqueous Media

An in situ electrochemical ESR experiment was carried out to separately detect each reduction product of benzoquinone (BQ), anion radical (BQ−.) and radical (BQH.), in aqueous media. ESR spectra were obtained during the reduction of BQ at −0.3 V vs. Ag/AgCl in a pH range from 2.5 to 6.8. In situ ESR spectra exhibit two kinds of five narrowly separated hyperfine lines, which differ from the hyperfine structure in the line width (ΔH), the g value, and the line intensity ratio. At pHs greater than 3.1, a hyperfine structure corresponding to BQ−.was observed. On the other hands, a hyperfine structure corresponding to BQH. was observed at pHs between pH 3.0 and 3.1. The decomposition rate constants of BQ−. were also determined, and increased as the pH values of the media increased. In addition, in situ ESR spectra for the reduction reaction of BQ on phosphatidylcholine (PC) derivatives, such as dilauroyl-PC, dipalmitoyl-PC, and diarachidoyl-PC, were investigated in well-buffered solutions. The results showed that BQ−. was generated and stabilized in the lipid layers.