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 (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∴S<)+ dimeric 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 paper provides further evidence that the nature of the neighboring group affects the ultimate course of the sulfide oxidation.

Spectral, Kinetic, and Theoretical Studies of Sulfur-Centered Reactive Intermediates Derived from Thioethers Containing an Acetyl Group

“…Contrary to the observations in SNACET solutions, 34 there is no buildup of normalized absorption in the regions of λ ) 330-390 nm and 470-500 nm, which was indicative of the existence of the acetyl thiyl radicals in enolic form and intermolecularly threeelectron-bonded radical cations, respectively. This observation clearly indicates that the formation of SETAc(>S∴S<) + has to be very inefficient over the concentration studied from 10 -3 up to 5 × 10 -2 M and is compatible with the lack of influence of SETAc concentration on the decay rate of 350-nm absorption.…”

“…The last observation also differs from that found in SNACET solutions where the normalized G × 420 (G rel ) yields measured on the plateau between pH 3.5 and 6 account for less than 5% of the normalized G × 420 (G rel ) yield measured at pH 1. 34 The 420-nm species is long lived, and its lifetime is substantially enhanced (t 1/2 ≈ 50 µs) (bottom right inset in Figure 1A) compared to the lifetime of the monomeric sulfur radical cation (SNACET>S •+ ) (t 1/2 ≈ 2.5 µs) measured at the same pH and concentration conditions. 34 Therefore, at this point of the exposition, some doubts arise as to whether the 420-nm absorption band can be assigned to (SETAc)>S •+ .…”

Radioprotectant and Radiosensitizer Effects on Sterility of γ-irradiated Bone

“…LC and LCEE did not elicit radioprotection. These four scavengers have comparable affinities to OH radical [7,28,41,44,49] and therefore the absence of radioprotection of LC and LCEE can be explained by their inability to get within or around the spores. Our initial expectation was that charge-based differences could explain the differences in the radioprotective potential of these agents.…”

“…Thus, N-acetyl-Lcysteine (NAC; negative), L-cysteine-ethyl-ester (LCEE; positive), and L-cysteine-methyl-ester (LCME; positive) derivatives of L-cysteine (LC, zwitterionic) with varying charges were evaluated as radioprotectants. LC, LCME, LCEE and NAC (Acros Organics, Geel, Belgium; Sigma Aldrich, St Louis, MO) have comparable reaction rate constants with the hydroxyl radical ranging from 7.9 to 35 9 10 9 M -1 s -1 [7,28,41,44,49]. NLP is a bifunctional agent used in chemotherapy, which has a DNA/RNA intercalating phenanthridine group and an oxygen singlet generating nitroimidazole group on radiation [22-24, 37, 43].…”

Chloride anion effect on the advanced oxidation processes of methidathion and dimethoate: Role of Cl2− radical

“…In fact, it was early reported by Asmus and Hiller (Asmus et al, 1977;Asmus and Bonifa ci c, 1980;Hiller and Asmus, 1981) that Cl À 2 radicals are involved in an exchange equilibrium with sulfides yielding chloride anions and neutral radicals comprising a three electron sulfurechlorine bond (represented as SrCl) and absorbing around 390 nm. The latter radicals may further dissociate to Cl À anions and sulphenyl radical cations, which exhibit absorption in the 300e400 nm range depending on the substituents (Jonah and Madhava Rao, 2001;Asmus et al, 1977;Varmenot et al, 2001). Formation of sulphenyl radical cations of MT and DM is supported by the nature of the reaction products and the fact that the observed organic transients do not react with O 2 (Glass, 1999).…”