Oxidation Processes of N,S-Diacetyl-l-cysteine Ethyl Ester:  Influence of S-Acetylation

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2001-07, Vol.105 (28), p.6867-6875
• Main Authors: Varmenot, Nicolas, Remita, Samy, Abedinzadeh, Zohreh, Wisniowski, Pawel, Strzelczak, Grazyna, Bobrowski, Krzysztof
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp010341e

The mechanism of the •OH-induced oxidation of N,S-diacetyl-l-cysteine ethyl ester (SNACET) was investigated in aqueous solutions using pulse radiolysis and steady-state γ-radiolysis combined with chromatographic and ESR techniques. The reaction of hydroxyl radicals with SNACET at slightly acidic to neutral pH results in the degradation into acetaldehyde. The underlying mechanism involves a very fast fragmentation of an initially formed hydroxysulfuranyl radical (k fragm ≥ 7.9 × 107 s-1) into acyl radicals (H3C−CO•) and the respective sulfenic acid (RSOH). Subsequently, these intermediates react via a hydrogen abstraction reaction that yields acetaldehyde and the respective sulfinyl radical (RSO•). In contrast, in very acidic solutions (pH < 2), the •OH-induced oxidation results in the formation of the monomeric sulfur radical cation (SNACET>S•+) which absorbs at λmax = 420 nm. This intermediate is formed with the absolute bimolecular rate constant k = 3.9 × 109 M-1 s-1. It decays in a SNACET concentration independent process (k d = 2.5 × 105 s-1) and in a SNACET concentration dependent process (k S - S = 2.2 × 107 M-1 s-1). The first process involves mainly fragmentation of the carbon−sulfur bond and yields acetylthiyl radical (CH2C(OH)−S•). The latter intermediate was identified via its reaction with oxygen as the thiylperoxyl radical (RSOO•), characterized by a transient absorption band with λmax = 540 nm. The second process represents the association of the monomeric sulfur radical cation (SNACET>S•+) with a second nonoxidized SNACET molecule to form intermolecularly three-electron-bonded (>S∴SS•+) because of spin delocalization in the carbonyl group. The monomeric radical cation (SNACET>S•+) is alternatively produced in slightly acidic solutions using the sulfate radical anion, SO4 •-, as an one-electron oxidant. This paper provides further evidence that the nature of the neighboring group affects the ultimate course of the sulfide oxidation.