Absolute rate constants for the formation of nitrogen-centred radicals from chloramines/amides and their reactions with antioxidants
作者:David I. Pattison、Michael J. Davies、Klaus-Dieter Asmus
DOI:10.1039/b202526d
日期:——
Pulse radiolysis techniques have been employed to investigate the one-electron reduction of a variety of chloramines and chloramides. These include models for the side-chain of Lys (6-aminohexanoic acid chloramine and α-N-acetyl-Lys chloramine), Gly chloramine, β-alanine chloramine and two models of protein backbone amides, the chloramides of cyclo-(Gly)2 and cyclo-(Ala)2. The molar absorption coefficients and stabilities of these chloramines/amides have been determined. The one-electron reduction of these chloramine/amide species by hydrated electrons occurs with second-order rate constants of the order of 109–1010 M−1 s−1, and results in cleavage of the N–Cl bonds to yield nitrogen-centred radicals and chloride ions (as measured by high performance ion chromatography). The reactivities of the nitrogen-centred radicals have been investigated with the readily
oxidisable quenchers, hydroquinone and Trolox. These quenchers were used as models of the in vivo antioxidants, ubiquinol-10 and α-tocopherol, and react with second-order rate constants between 2 × 107 and 1 × 108 M−1 s−1. No evidence was obtained in these pulse radiolysis experiments for a rapid rearrangement of the oxidising nitrogen-centred radicals to reducing carbon-centred radicals, though such reactions have been indicated in previous EPR studies.
利用脉冲辐射分解技术研究了多种氯胺和氯化物的单电子还原。其中包括 Lys 侧链模型(6-氨基己酸氯胺和α-N-乙酰-Lys 氯胺)、Gly 氯胺、β-丙氨酸氯胺和两种蛋白质骨架酰胺模型,即环-(Gly)2 和环-(Ala)2 的氯酰胺。测定了这些氯胺/酰胺的摩尔吸收系数和稳定性。水合电子对这些氯胺/酰胺物种的单电子还原以 109-1010 M-1 s-1 量级的二阶速率常数发生,并导致 N-Cl 键裂解,生成以氮为中心的自由基和氯离子(通过高效离子色谱法测量)。研究人员用容易氧化的淬灭剂对苯二酚和三氯氧烷研究了以氮为中心的自由基的反应活性。这些淬灭剂被用作体内抗氧化剂泛醌-10 和 α-生育酚的模型,其反应的二阶速率常数在 2 × 107 和 1 × 108 M-1 s-1 之间。在这些脉冲辐射分解实验中,没有证据表明氧化型氮中心自由基会迅速重新排列为还原型碳中心自由基,尽管在以前的 EPR 研究中曾出现过此类反应。