Electrochemical and spectroscopic studies on electron-transfer reaction between novel water-soluble tetrazolium salts and a superoxide ion

The electrochemical behavior of water-soluble tetrazoliums (WST) has been studied by cyclic voltammetry. WST was reduced in a two-step process. The first reduction peak at −0.20 V versus Ag/AgCl corresponds to one-electron reduction reaction and is independent of pH. The second reduction peak at −0.47 V corresponds to one-electron/one-proton process. Since the first reduction peak potential is more positive than the formal potential of O2/O2 − redox couple, WST can be reduced by O2 −. A possible mechanism is proposed for the reduction of WST dyes by O2 −. Their reduced forms, which are called formazan, exhibited the absorbance maxima in the range of 435–537 nm with large molar extinction coefficients ((1–2) × 104 M−1 cm−1). The electron-transfer reactions between O2 − and WST dyes were quantitatively examined by stopped-flow spectroscopy using KO2/DMSO as O2 − generating system and the second-order rate constants of the order of 104 M−1 s−1 were obtained. These values are comparable to that obtained for the conventional nitroblue tetrazolium (NBT).