Kinetic studies on oxidations of leucomethylene blue and leucothionine by iron (III) in aqueous solution

✍ Scribed by Yingjin Liu; Shunzo Yamamoto; Yoshimi Sueishi

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 92 KB
• Volume: 12
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(199903)12:3<194::aid-poc116>3.0.co;2-q

✦ Synopsis

The reaction kinetics and mechanism of the oxidations of leucomethylene blue (MBH) and leucothionine (TH) by iron (III) sulfate in aqueous solution were studied spectrophotometrically by the stopped-flow technique. MBH and TH, which were freshly prepared by photoreduction of methylene blue and thionine, respectively, with ascorbic acid were used in the kinetic measurements. The pseudo-first-order rate constants (k obsd ) show kinetic saturation at high initial concentrations of iron (III) sulfate for MBH and TH. It was found that the reciprocal of k obsd increases linearly with increase in the reciprocal of [Fe 3 ] 0 . A broad absorption band was observed on mixing MBH and Fe 3 solutions at low temperatures, and this was attributed to a charge-transfer complex between MBH and Fe 3 . The effects of Fe 2 ion and L-()-ascorbic acid on the rates of oxidation were also investigated. A small kinetic isotope effect on the oxidation rate for MBH was observed. The results can be explained by a general mechanism with stepwise electron-proton-electron transfers with the formation of a complex between reactants.