Methionine radical cation: structural studies as a function of pH using X- and Q-band time-resolved electron paramagnetic resonance spectroscopy.
Methionine radical cation: structural studies as a function of pH using X- and Q-band time-resolved electron paramagnetic resonance spectroscopy.
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蛋氨酸自由基阳离子：使用 X 和 Q 波段时间分辨电子顺磁共振波谱进行结构研究，作为 pH 函数。
DOI：
10.1021/jp051551k
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
M. Forbes
中科院分区：
文献类型：
--
作者：
H. Yashiro;R. C. White;A. Yurkovskaya;M. Forbes
提出了室温下液体溶液中 L-甲硫氨酸自由基阳离子及其 N-乙酰基衍生物的全面高分辨率电子顺磁共振 (EPR) 表征。通过水溶性染料蒽醌磺酸钠盐的准分子激光激发，以高产率光化学方式产生阳离子，其激发三重态通过来自甲硫氨酸或N-乙酰甲硫氨酸的侧链硫原子的电子转移而猝灭。通过 X 波段 (9.5 GHz) 和 Q 波段 (35 GHz) 微波频率的连续波 (CW) 时间分辨电子顺磁共振 (TREPR) 光谱检测自由基。当 pH 值远低于质子化胺氮的 pK(a) 时，阳离子通过 S-S 三电子键与另一个基态蛋氨酸分子形成二聚体。在碱性溶液中，氨基酸氮上的孤电子对可与侧链硫原子形成分子内S-N三电子键，形成阳离子的五元环结构。当氨基酸氮未被取代（蛋氨酸本身）时，在高 pH 值下会快速去质子化为氨基自由基。如果氮被取代（N-乙酰甲硫氨酸），则在其电子自旋弛豫时间内（约1微米）观察到环状结构。光谱模拟提供了所有结构的化学位移（g 因子）和超精细耦合常数，同位素标记实验有力地支持了这些分配。
A comprehensive high resolution electron paramagnetic resonance (EPR) characterization of the l-methionine radical cation and its N-acetyl derivative in liquid solution at room temperature is presented. The cations were generated photochemically in high yield by excimer laser excitation of a water soluble dye, anthraquinone sulfonate sodium salt, the excited triplet state of which is quenched by electron transfer from the side chain sulfur atom of methionine or N-acetylmethionine. The radicals were detected by continuous wave (CW) time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band (9.5 GHz) and Q-band (35 GHz) microwave frequencies. At pH values well below the pK(a) of the protonated amine nitrogen, the cation forms a dimer with another ground-state methionine molecule through a S-S three-electron bond. In basic solution, the lone pair on the nitrogen of the amino acid is available to make an intramolecular S-N three-electron bond with the side chain sulfur atom, leading to a five-membered ring structure for the cation. When the amino acid nitrogen is unsubstituted (methionine itself), rapid deprotonation to an aminyl radical takes place at high pH values. If the nitrogen is substituted (N-acetylmethionine), the cyclic structure is observed within its electron spin relaxation time at about 1 micros. Spectral simulation provides chemical shifts (g-factors) and hyperfine coupling constants for all structures, and isotopic labeling experiments strongly support the assignments.