Anaerobic oxidation of cysteine to cystine by iron(III). Part 1. The reaction in acidic solution

The anaerobic oxidation of cysteine (H2L) by iron(III) has been followed in acidic media by use of a stopped-flow high-speed spectrophotometric method. It is suggested that the species responsible is [FeL]+ which is observable as a transient blue colour on the addition of iron(III) solutions to acidic cysteine solutions. This species exhibits an absorption maximum at 614 nm with a molar absorption coefficient of 1 030 dm3 mol–1 cm–1. The second-order rate constant for its formation from [Fe(OH)]2+ and H2L was found to be 1.14 × 104 dm3 mol–1 s–1 and the value of log KFeOH{for the protonation of [Fe(OH)]2+} required to fit the data is 2.82. Decomposition to iron(II) and subsequently cystine follows protonation, with an apparent third-order rate constant (including an unknown stability constant) of 1.63 × 1011 dm6 mol–2 s–1. All measurements were carried out at 25 °C in solutions of ionic strength 0.10 mol dm–3(KCl).