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Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis

机译：模型肽中半胱氨酸巯基与甘氨酸和丙氨酸之间的可逆分子内氢转移：源自脉冲辐射分解和激光闪光光解的绝对速率常数

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摘要

The intramolecular reaction of cysteine thiyl radicals with peptide and protein αC-H bonds represents a potential mechanism for irreversible protein oxidation. Here, we have measured absolute rate constants for these reversible hydrogen transfer reactions by means of pulse radiolysis and laser flash photolysis of model peptides. For N-Ac-CysGly6 and N-Ac-CysGly2AspGly3, Cys thiyl radicals abstract hydrogen atoms from Gly with kf = (1.0-1.1)×105 s-1, generating carbon-centered radicals, while the reverse reaction proceeds with kr = (8.0-8.9)×105 s-1. The forward reaction shows a normal kinetic isotope effect of kH/kD = 6.9, while the reverse reaction shows a significantly higher normal kinetic isotope effect of 17.6, suggesting a contribution of tunneling. For N-Ac-CysAla2AspAla3, cysteine thiyl radicals abstract hydrogen atoms from Ala with kf =(0.9-1.0)×104 s-1, while the reverse reaction proceeds with kr = 1.0×105 s-1. The order of reactivity, Gly > Ala, is in accord with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that kf < kr suggests some secondary structure of the model peptides, which prevents the adoption of extended conformations, for which calculations of homolytic bond dissociation energies would have predicted kf > kr. Despite kf < kr, model calculations show that intramolecular hydrogen abstraction by Cys thiyl radicals can lead to significant oxidation of other amino acids in the presence of physiologic oxygen concentrations.
机译：半胱氨酸巯基与肽和蛋白质α C-H键的分子内反应代表了不可逆蛋白质氧化的潜在机制。在这里，我们通过脉冲辐射分解和模型肽的激光闪光光解测量了这些可逆氢转移反应的绝对速率常数。对于N-Ac-CysGly6和N-Ac-CysGly2AspGly3，Cys硫基从kly =（1.0-1.1）×10 5 s -1 的Gly中提取氢原子，生成以碳为中心的自由基，而逆反应以kr =（8.0-8.9）×10 5 s -1 进行。正向反应显示出正常的动力学同位素效应，kH / kD = 6.9，而逆向反应则显示出明显更高的正常动力学同位素效应，为17.6，表明有隧穿作用。对于N-Ac-CysAla2AspAla3，半胱氨酸硫基自由基从Ala提取氢原子，其kf =（0.9-1.0）×10 4 s -1 ，而反向反应随着kr = 1.0×10 5 s -1 。反应顺序为Gly> Ala，与先前有关硫基自由基与这些氨基酸的分子间反应的研究一致。 kf kr。尽管有kf r ，模型计算表明，在存在生理性氧气浓度的情况下，Cys巯基自由基对分子内氢的吸收可导致其他氨基酸的显着氧化。

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