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

Abstract

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 (kfragm ≥ 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 (kd = 2.5 × 105 s-1) and in a SNACET concentration dependent process (kS-S = 2.2 × 107 M-1 s-1). The first process involves mainly fragmentation of the carbon−sulfur bond and yields acetylthiyl radical (CH2C(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∴S<)+ dimeric radical cation. The low rate constant (kS-S) is in line with the high stability of the monomeric sulfur radical cation (SNACET>S•+) 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.