EPR spectroscopy is an important spectroscopic method for identification and characterization of radical species involved in many biological reactions. The tyrosyl radical is one of the most studied amino acid radical intermediates in biology. Often in conjunction with histidine residues, it is involved in many fundamental biological electron and proton transfer processes, such as in the water oxidation in photosystem II. As biological processes are typically extremely complicated and hard to control, molecular bio-mimetic model complexes are often used to clarify the mechanisms of the biological reactions. Here, we present theoretical calculations to investigate the sensitivity of magnetic resonance parameters to proton-coupled electron transfer events, as well as conformational substates of the molecular constructs which mimic the tyrosine–histidine (Tyr–His) pairs found in a large variety of proteins. Upon oxidation of the phenol, the Tyr analog, these complexes can perform not only one-electron one-proton transfer (EPT), but also one-electron two-proton transfers (E2PT). It is shown that in aprotic environment the gX-components of the electronic g-tensor are extremely sensitive to the first proton transfer from the phenoxyl oxygen to the imidazole nitrogen (EPT product), leading to a significant increase of the gX-value of up to 0.003, but are not sensitive to the second proton transfer (E2PT). In the latter case, the change of the gX-value is much smaller (ca. 0.0001), which is too small to be distinguished even by high-frequency EPR. The 14N hyperfine values are also too similar to allow differentiation between the different protonation states in EPT and E2PT. The magnetic resonance parameters were also calculated as a function of the rotation angles around single bonds. It was demonstrated that rotation of the phenoxyl group results in large positive changes (> 0.001) in the gX-values. Analysis of the data reveals that the main source of these changes is related to the strength of the H-bond between phenoxyl oxygen and the proton(s) on N1 and N2 positions of the imidazole.

中文翻译：

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模型有机供体-受体系统中的单电子多质子转移：对高频 EPR 的影响

EPR 光谱是一种重要的光谱方法，用于鉴定和表征参与许多生物反应的自由基物种。酪氨酰自由基是生物学中研究最多的氨基酸自由基中间体之一。它通常与组氨酸残基结合，参与许多基本的生物电子和质子转移过程，例如光系统 II 中的水氧化。由于生物过程通常极其复杂且难以控制，因此通常使用分子仿生模型复合物来阐明生物反应的机制。在这里，我们提出了理论计算来研究磁共振参数对质子耦合电子转移事件的敏感性，以及模拟在多种蛋白质中发现的酪氨酸-组氨酸（Tyr-His）对的分子构建体的构象亚态。在苯酚（Tyr 类似物）氧化后，这些配合物不仅可以进行单电子一质子转移 (EPT)，还可以进行单电子两质子转移 (E2PT)。结果表明，在非质子环境中，电子 g 张量的 gX 分量对从苯氧基氧到咪唑氮（EPT 产物）的第一次质子转移极为敏感，导致 gX 值显着增加至 0.003，但对第二次质子转移 (E2PT) 不敏感。在后一种情况下，gX 值的变化要小得多（约 0.0001），即使通过高频 EPR 也无法区分。14N 超精细值也太相似，无法区分 EPT 和 E2PT 中的不同质子化状态。磁共振参数也被计算为围绕单键的旋转角的函数。结果表明，苯氧基的旋转会导致 gX 值出现较大的正变化 (> 0.001)。数据分析表明，这些变化的主要来源与苯氧基氧与咪唑 N1 和 N2 位置上的质子之间的 H 键强度有关。