Revisiting a proposed kinetic model for the reaction of cysteine and hydrogen peroxide via cysteine sulfenic acid

✍ Scribed by Michael T. Ashby; Péter Nagy

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 214 KB
• Volume: 39
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.20211

✦ Synopsis

Abstract

A mechanism for the oxidation of cysteine by hydrogen peroxide has been proposed by Luo et al. (J Pharm Sci, 2005, 94, 304–316): rapid equilibrium to form cysteinate (CS^−^), followed by irreversible oxidation of CS^−^ by H~2~O~2~ to give cysteine sulfenic acid (CSOH), and nucleophilic attack on CSOH by CS^−^ to yield cystine (CSSC). We have argued that the data that were published by Luo et al. afford no insight into the mechanism that follows the rate‐limiting step, oxidation of CS^−^ by H~2~O~2~ (J Pharm Sci, 2006, 95, 15–18), but Anderson and Luo contend that their model is sound (J Pharm Sci, 2006, 95, 19–24). Evidence is presented herein that discredits the model proposed by Luo et al.: (1) time‐resolved ^1^H NMR spectra show that no intermediate is produced, (2) time‐resolved electronic spectra exhibit multiple isosbestic points, and (3) kinetic measurements and models employ the initial rate method (which focus on that part of the kinetic traces for which the differences between the two models are most pronounced). All three experiments verify that a pseudo‐first‐order model with a single kinetic parameter completely describes the observed kinetics of the oxidation of CS^−^ by H~2~O~2~. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 32–38, 2007