Turecek Frantisek, Yao Chunxiang, Fung Y M Eva, Hayakawa Shigeo, Hashimoto Mami, Matsubara Hiroshi
Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, Washington 98195, USA.
J Phys Chem B. 2009 May 21;113(20):7347-66. doi: 10.1021/jp900719n.
Radicals containing the histidine residue have been generated in the gas phase by femtosecond electron transfer to protonated histidine-N-methylamide (1H+), Nalpha-acetylhistidine-N-methylamide (2H+), Nalpha-glycylhistidine (3H+), and Nalpha-histidylglycine (4H+). Radicals generated by collisional electron transfer from dimethyldisulfide to ions 1H+ and 2H+ at 7 keV collision energies were found to dissociate completely on the microsecond time scale, as probed by reionization to cations. The main dissociations produced fragments from the imidazole side chain and the cleavage of the C(alpha)CO bond, whereas products of NCalpha bond cleavage were not observed. Electron transfer from gaseous potassium atoms to ions 3H+ and 4H+ at 2.97 keV collision energies not only caused backbone NCalpha bond dissociations but also furnished fractions of stable radicals that were detected after conversion to anions. Ion structures, ion-electron recombination energies, radical structures, electron affinities, and dissociation and transition-state energies were obtained by combined density functional theory and Møller-Plesset perturbational calculations (B3-PMP2) and basis sets ranging from 6-311+G(2d,p) to aug-cc-pVTZ. The Rice-Ramsperger-Kassel-Marcus theory was used to calculate rate constants on the B3-PMP2 potential energy surfaces to aid interpretation of the mass spectrometric data. The stability of Nalpha-histidylglycine-derived radicals is attributed to an exothermic isomerization in the imidazole ring, which is internally catalyzed by reversible proton transfer from the carboxyl group. The isomerization depends on the steric accessibility of the histidine side chain and the carboxyl group and involves a novel cation radical-COO salt-bridge intermediate.
通过飞秒电子转移至质子化的组氨酸 - N - 甲基酰胺（1H⁺）、Nα - 乙酰组氨酸 - N - 甲基酰胺（2H⁺）、Nα - 甘氨酰组氨酸（3H⁺）和Nα - 组氨酰甘氨酸（4H⁺），已在气相中生成含有组氨酸残基的自由基。在7 keV碰撞能量下，通过二甲基二硫化物与离子1H⁺和2H⁺的碰撞电子转移产生的自由基，经再电离为阳离子探测发现，在微秒时间尺度上完全解离。主要解离产生咪唑侧链的碎片以及C(α)CO键的断裂，而未观察到N - Cα键断裂的产物。在2.97 keV碰撞能量下，气态钾原子与离子3H⁺和4H⁺的电子转移不仅导致主链N - Cα键解离，还产生了一部分稳定自由基，这些自由基在转化为阴离子后被检测到。通过结合密度泛函理论和莫勒 - 普莱塞特微扰计算（B3 - PMP2）以及从6 - 311 + G(2d,p)到aug - cc - pVTZ的基组，获得了离子结构、离子 - 电子复合能、自由基结构、电子亲和能以及解离和过渡态能量。使用赖斯 - 兰斯伯格 - 卡塞尔 - 马库斯理论计算B3 - PMP2势能面上的速率常数，以辅助解释质谱数据。Nα - 组氨酰甘氨酸衍生自由基的稳定性归因于咪唑环中的放热异构化，该异构化由羧基的可逆质子转移内部催化。异构化取决于组氨酸侧链和羧基的空间可及性，并且涉及一种新型阳离子自由基 - COO盐桥中间体。
