The role of tyrosine radicals as mediators of electron transfer reactions in enzymes is well established, as is the involvement of histidine as a binding partner. But how environmental factors affect these reactions remains poorly explored. In the study presented here, kinetic data on the influence of the protonation state of the reactants on the reduction of transient histidine radicals by tyrosine were obtained in neutral and basic aqueous solution (pH 6–12) using time‐resolved chemically induced dynamic nuclear polarization (CIDNP). The histidine radicals were generated in the photo‐induced reaction with the photosensitizer 3,3′,4,4′‐tetracarboxy benzophenone. From model simulations of the detected CIDNP kinetics, pH dependent second‐order rate constants of the reduction of histidine radicals were obtained for four possible combinations of the amino acids and their N‐acetyl derivatives, and also for the systems histidine‐phenylalanine dipeptide/N‐acetyl tyrosine, and N‐acetyl histidine/tyrosine‐glutamine dipeptide. The pH dependences of the rate constant of the reduction reaction are explained accounting for the protonation states of reactants, and also protonation state of the equilibrium form of the product ‐ reduced form of histidine radical, which is histidine with neutral or a positively charged imidazole.

中文翻译：

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酪氨酸对瞬时组氨酸自由基的还原作用：反应物质子化状态的影响。

酪氨酸自由基作为酶中电子转移反应的介质的作用已被充分确立，组氨酸作为结合配偶体也参与其中。但是，环境因素如何影响这些反应的研究仍很少。在此处介绍的研究中，使用时间分辨化学诱导的动态核极化方法，在中性和碱性水溶液（pH 6-12）中获得了动力学数据，该数据涉及酪氨酸对反应物的质子化状态的还原，从而对瞬时组氨酸自由基的还原。 （CIDNP）。组氨酸自由基是在与光敏剂3,3'，4,4'-四羧基二苯甲酮的光诱导反应中产生的。根据检测到的CIDNP动力学的模型模拟，氨基酸及其N-乙酰基衍生物的四种可能组合以及系统组氨酸-苯丙氨酸二肽/ N-乙酰基酪氨酸和N-乙酰基组氨酸的组合，获得了pH依赖性的组氨酸自由基还原的二级速率常数。 /酪氨酸-谷氨酰胺二肽。解释了还原反应速率常数的pH依赖性，其中考虑了反应物的质子化状态，还考虑了产物还原型组氨酸自由基的平衡形式的质子态，组氨酸自由基是具有中性或带正电荷的咪唑的组氨酸。