Oxidation processes of N,S-diacetyl-L-cysteine ethyl ester: Influence of S-acetylation

The mechanism of the . OH-induced oxidation of NS-diacetyl-L-Cysteine ethyl ester (SNACET) was investigated in aqueous solutions using pulse radiolysi s and steady-state gamma -radiolysis combined with chromatographic and ESR techniques. The reaction of hydroxyl radicals with SNACET at slightly acidi c to neutral pH results in the degradation into acetaldehyde. The underlyin g mechanism involves a very fast fragmentation of an initially formed hydro xysulfuranyl radical (k(fragm) greater than or equal to 7.9 x 10(7) s(-1)) into acyl radicals (H3C-CO .) and the respective sulfenic acid (RSOH). Subs equently, these intermediates react via a hydrogen abstraction reaction tha t yields acetaldehyde and the respective sulfinyl radical (RSO .). In contr ast, in very acidic solutions (pH < 2), the . OH-induced oxidation results in the formation of the monomeric sulfur radical cation (SNACET >S . (+)) w hich absorbs at lambda (max) = 420 nm. This intermediate is formed with the absolute bimolecular rate constant k = 3.9 x 10(9) M-1 s(-1). It decays in a SNACET concentration independent process (k(d) = 2.5 x 10(5) s(-1)) and in, a SNACET concentration dependent process (k(s-s) = 2.2 x 10(7) M-1 s(-1 )). The first process involves mainly fragmentation of the carbon-sulfur bo nd and yields acetylthiyl radical (CH2=C(OH)-S .). The latter intermediate was identified via its reaction with oxygen as the thiylperoxyl radical (RS OO .), characterized by a transient absorption band with lambda (max) = 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 therefore S <)(+) dime ric radical cation. The low rate constant (k(s-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 pa per provides further evidence that the nature of the neighboring group affe cts the ultimate course of the sulfide oxidation.