Formation of Superoxide Ion via One-Electron Transfer from Electron Donors to Singlet Oxygen

Article abstract of DOI:10.1021/ja00348a040

The formation of superoxide ion (O2^-.) via one-electron transfer from substituted N,N-dimethylanilines to singlet oxygen (¹O2) was examined in phosphate buffer by employing a water-soluble oxygen source and a combination of p-nitrotetrazolium blue (NBT) and superoxide dismutase (SOD) as a detecting reagent for O2^-..When a solution of an electron-donor-substituted N,N-dimethylaniline, NBT, and 3-(1,4-epidioxy-4-methyl-1,4-dihydro-1-naphthyl)propionic acid in phosphate buffer at pH 7.5 was incubated at 35 deg C, NBT was reduced to formazan.The inhibition of the reduction of NBT to formazan by SOD indicated the formation of O2^-..Control experiments demonstrated that O2^-. is produced by a direct reaction between ¹O2 and the amine.Both the yield of O2^-. and the quenching rate constants of ¹O2 are well correlated with the oxidation potentials of these amines.Tetramethyl-p-phenylenediamine having a quenching rate constant close to a diffusion-controlled limit is the most effective for the generation of O2^-. A plot of the log of the quenching rate constant of ¹O2 by the amines against the calculated free-energy change (ΔG) for full electron transfer showed a linear relationship with a slope of -0.19 +/- 0.05 mol/kcal.The correlation of the log of the relative yield of O2^-. against ΔG strongly supports the electron-transfer mechanism for the formation of O2^-..It was demonstrated that an electron-transfer reaction giving rise to O2^-. is only possible for aromatic amines with oxidation potentials less than ca. 0.5 V vs.SCE in aqueous media.