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 O-2/O-2(-) redox couple, WST can be reduced by O-2(-). A possible mechanism is proposed for the reduction of WST dyes by O-2(-). 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) x 10(4) M-1 cm(-1)). The electrontransfer reactions between O-2(-) and WST dyes were quantitatively examined by stopped-flow spectroscopy using KO2/DMSO as O-2(-) generating system and the second-order rate constants of the order of 10(4) M-1 s(-1) were obtained. These values are comparable to that obtained for the conventional nitroblue tetrazolium (NBT). (C) 2003 Elsevier B.V. All rights reserved.