Mardis Kristy L, Niklas Jens, Omodayo Harriet, Odella Emmanuel, Moore Thomas A, Moore Ana L, Poluektov Oleg G
Department of Chemistry, Physics, and Engineering Studies, Chicago State University, Chicago, Illinois 60628, USA.
Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, USA.
Appl Magn Reson. 2020 Oct;51(9-10):977-991. doi: 10.1007/s00723-020-01252-8. Epub 2020 Oct 1.
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 analogue, 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 g-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 g-value of up to 0.003, but are not sensitive to the second proton transfer (E2PT product). In the latter case the change of the g-value is much smaller ( 0.0001), which is too small to be distinguished even by high frequency EPR. The N 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 g-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 N and N positions of the imidazole.
电子顺磁共振光谱法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。酪氨酸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中的水氧化反应。由于生物过程通常极其复杂且难以控制，因此常常使用分子仿生模型复合物来阐明生物反应的机制。在此，我们进行理论计算，以研究磁共振参数对质子耦合电子转移事件的敏感性，以及模拟多种蛋白质中发现的酪氨酸 - 组氨酸（Tyr - His）对的分子结构的构象亚态。当酚类（酪氨酸类似物）被氧化时，这些复合物不仅可以进行单电子单质子转移（EPT），还可以进行单电子双质子转移（E2PT）。结果表明，在非质子环境中，电子g张量的g分量对从苯氧基氧到咪唑氮的首次质子转移（EPT产物）极为敏感，导致g值显著增加，高达0.003，但对第二次质子转移（E2PT产物）不敏感。在后一种情况下，g值的变化要小得多（0.0001），甚至通过高频电子顺磁共振也难以区分。N超精细值也非常相似，无法区分EPT和E2PT中不同的质子化状态。还计算了磁共振参数作为围绕单键旋转角度的函数。结果表明，苯氧基的旋转会导致g值出现大幅正向变化（>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N和N位置上的质子之间的氢键强度有关。
