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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Structure of a Transient Neutral Histidine Radical in Solution: EPR Continuous-Flow Studies in a Ti~(3+)/EDTA-Fenton System and Density Functional Calculations
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Structure of a Transient Neutral Histidine Radical in Solution: EPR Continuous-Flow Studies in a Ti~(3+)/EDTA-Fenton System and Density Functional Calculations

机译：溶液中的瞬时中性组氨酸自由基的结构：Ti〜（3 +）/ EDTA-Fenton系统中的EPR连续流研究和密度泛函计算

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摘要

Oxidation of histidine by OH~* radicals has been studied at room temperature over a large range of pH values in a Ti~(3+)/EDTA/H_2O_2-Fenton system using a special EPR continuous-flow setup. At pH 7 to 8, during fast flow, and EPR spectrum from a new transient histidine radical with a well-resolved hyperfine structure (hfs) has been observed at g_(iso) = 2.0026. The hfs changed significantly in the cases of methylene-deuterated histidine in H_2O and of histidine in D_2O. ERP spectral simulations resulted in an assignment of two slightly different splittings of the two methylene protons (1.27 and 1.59 mT); one large splitting from an out-of-plane proton of the imidazole ring (2.47 mT); and splittings of a ring proton (0.99 mT); a NH (or OH) proton (0.14 mT), and two ring nitrogens (0.27 and 0.11 mT). The large proton-splitting indicates an addition of an OH~* radical to the neutral imidazole ring. Such an addition of an OH~* radical was already observed for the histidine cation radical in a Ti~(3+)/H_2O_2 Fenton system at pH 2. Density functional theory (DFT) calculations of hyperfine coupling constants were performed at the PCM/B3LYP/6-311G(2df, p) level for three isomeric forms of a histidine model (4-ethyl imidazole) in which an OH~* radical is added to different positions (C2, C4, and C5) of the neutral imidazole ring. Theoretical hyperfine data are in excellent agreement with the experiment and clearly support an assignment to a neutral histidine radicalk formed by addition of an OH~* radical at the C5 position of the imidazole ring. Extensive studies of histidine oxidation in a Ti~(3+)/EDTA-Fenton system, at acidic pH as well at neutral and basic pH values, have advanced our understanding of the Fenton chemistry of histidine and the electronic structure of involved paramagnetic species. The pH profile of the formation of histidine OH-addition radical shows that the cation radical dominates at pH 2 to 4, and the neutral radical at pH 5 to 9. At pH ≥ 7, in the presence of histidine, a titanium complex with g_(iso) = 1.9632 and a significant hfs from naturally occurring ~(47)Ti and ~(49)Ti nuclei with a_(iso)(Ti) = 1.54 mT has been observed. This complex, which dominates at pH 9 to 10, is also visible in the absence of H_2O_2 and without flow, and is thus assigned to a stable mixed-ligand Ti~(3+) complex containing EDTA and histidine. This is the first communication on a neutral histidine radical with a complete set of hyperfine coupling constants derived by EPR as well as by DFT calculations.
机译：在室温下，使用特殊的EPR连续流装置，在Ti〜（3 +）/ EDTA / H_2O_2-Fenton系统中，在较大pH值范围内，研究了OH〜*自由基对组氨酸的氧化作用。在pH值为7至8的过程中，在快速流动过程中，在g_（iso）= 2.0026处观察到来自新的具有良好解析的超精细结构（hfs）的瞬时组氨酸自由基的EPR光谱。在H_2O中亚甲基氘的组氨酸和D_2O中的组氨酸的情况下，hfs显着变化。 ERP光谱模拟导致分配了两个亚甲基质子（1.27和1.59 mT）的两个略有不同的分裂;从咪唑环的平面外质子（2.47 mT）分裂出一个大片段;环质子的分裂（0.99 mT）; NH（或OH）质子（0.14 mT）和两个环氮（0.27和0.11 mT）。大的质子分裂表明在中性咪唑环上有一个OH〜*基团。在pH 2的Ti〜（3 +）/ H_2O_2 Fenton系统中，组氨酸阳离子已经观察到这种OH〜*自由基的加成。在PCM / C上进行了超精细耦合常数的密度泛函理论（DFT）计算。组氨酸模型的三种异构形式（4-乙基咪唑）的B3LYP / 6-311G（2df，p）水平，其中将OH〜*基团添加到中性咪唑环的不同位置（C2，C4和C5） 。理论上的超精细数据与实验非常吻合，并且清楚地支持了通过在咪唑环的C5位添加OH〜*自由基形成的中性组氨酸自由基的分配。在Ti〜（3 +）/ EDTA-Fenton系统中，在酸性pH以及中性和碱性pH值下，对组氨酸氧化的广泛研究，使我们对组氨酸的Fenton化学和所涉及的顺磁性物质的电子结构有了更深入的了解。组氨酸OH加成基团形成的pH曲线表明，阳离子基团在pH 2至4占主导地位，中性基团在pH 5至9主导。在pH≥7时，在组氨酸存在下，具有g_的钛配合物（iso）= 1.9632，并且已观察到天然存在的〜（47）Ti和〜（49）Ti原子核的af（iso）（Ti）= 1.54 mT的明显hfs。在不存在H_2O_2且不流动的情况下，这种在pH为9至10时仍占优势的络合物也可见，因此被分配给含有EDTA和组氨酸的稳定的混合配体Ti〜（3+）络合物。这是在中性组氨酸基团上的首次通信，该基团具有通过EPR以及DFT计算得出的一整套超精细偶联常数。

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