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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Oxidation Processes of N,S-Diacetyl-L-cysteine Ethyl Ester: Influence of S-Acetylation
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Oxidation Processes of N,S-Diacetyl-L-cysteine Ethyl Ester: Influence of S-Acetylation

机译：N，S-二乙酰基-L-半胱氨酸乙酯的氧化过程：S-乙酰化的影响

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摘要

The mechanism of the centre dotOH-induced oxidation of N,S-diacetyl-L-cysteine ethyl ester (SNACET) was investigated in aqueous solutions using pulse radiolysis and steady-state #gamma#-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)>=7.9X 10~7 s~(-1))into acyl radicals (H_3C-Cocentre dot) and the respective sulfenic acid (RSOH). Subsequently, these intermediates react via a hydrogen abstraction reaction that yields acetaldehyde and the respective sulfinyl radical (RSOcentre dot). In contrast, in very acidic solutions (pH<2), the centre dotOH-induced oxidation results in the formation of the monomeric sulfur radical cation (SNACET>Scentre dot~+) which absorbs at #lambda#_(max)=420 nm. This intermediate is formed with the absolute bimolecular rate constant k=3.9 X10~9 M~(-1) s~(-1). It decays in a SNACET concentration independent process (k_d=2.5X10~5 s~(-1)) and in a SNACET concentration depe4ndent process (k_s-s)=2.2X10~7 M~(-1) s~(-1)). The first process involves mainly fragmentation of the carbon-sulfur bond and yields acetylthiyl radical (CH_2=C(OH)-Scentre dot). The latter intermediate was identified via its reaction with oxygen as the thiylperoxyl radical (RSOOcentre dot), characterized by a transien absorption band with #lambda#_(max)=450 nm. The second process represents the association of the monomeric sulfur radical cation (SNACET>Scentre dot~+) with a second nonoxidized SNACET molecule to form intermolecularly three-electron-bonded(>S thereforeS<)~+ dimeric radical cation. The low rate constant(k_(s-s)) is in line with the high stability of the monomeric sulfur radical cation (SNACET>scentre dot~+) because of spin delocalization in the carbonyl group. The monomeric radical cation (SNACET>centre dot~+) is alternatively produced in slightly acidic solutions using the sulfate radical anion, SO_4~(centre dot-), 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.
机译：研究了中心点OH诱导的水溶液中N，S-二乙酰基-L-半胱氨酸乙酯（SNACET）氧化的机理，采用脉冲辐射分解和稳态＃γ＃辐射分解结合色谱和ESR技术。在弱酸性至中性pH下，羟基自由基与SNACET的反应导致降解为乙醛。潜在的机制涉及将最初形成的羟基硫磺基自由基（k_（fragm）> = 7.9X 10〜7 s〜（-1））非常快地裂解为酰基基团（H_3C-Cocentre点）和相应的亚硫酸（RSOH） 。随后，这些中间体通过氢提取反应进行反应，生成乙醛和相应的亚磺酰基基团（RSO中心点）。相比之下，在强酸性溶液（pH <2）中，中心点OH诱导的氧化导致形成单体硫自由基阳离子（SNACET>中心点〜+），该单体在#lambda #_（max）= 420 nm处吸收。该中间体以绝对双分子速率常数k ＝ 3.9×X 10〜9M〜（-1）s〜（-1）形成。它在SNACET浓度无关过程（k_d = 2.5X10〜5 s〜（-1））和SNACET浓度依赖性过程（k_s-s）= 2.2X10〜7 M〜（-1）s〜（-1）中衰减。 ））。第一个过程主要涉及碳硫键的断裂，并产生乙酰基噻吩基（CH_2 = C（OH）-Scentre dot）。通过与氧反应作为硫代过氧自由基（RSOOcentre dot）鉴定了后者的中间体，其特征在于＃λ（max）= 450 nm的瞬态吸收带。第二个过程表示单体硫自由基阳离子（SNACET> Scentre dot +）与第二个未氧化的SNACET分子缔合形成分子间三电子键合（> S因此S <）〜+二聚自由基阳离子。由于羰基中的自旋离域，低速率常数（k_（s-s））与单体硫自由基阳离子的高稳定性（SNACET> scentre〜+）相一致。或者，使用硫酸根阴离子SO_4-（中心点-）作为单电子氧化剂，在弱酸性溶液中产生单体自由基阳离子（SNACET>中心点+）。本文提供了进一步的证据，表明邻近基团的性质影响了硫化物氧化的最终过程。

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