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

Abstract

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 K^FeOH{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).