Histidine is an aromatic amino acid crucial for the biological functioning of proteins and enzymes. When biological matter is exposed to ionising radiation, highly energetic particles interact with the surrounding tissue which leads to efficient formation of low-energy electrons. In the present study, the interaction of low-energy electrons with gas-phase histidine is studied at a molecular level in order to extend the knowledge of electron-induced reactions with amino acids. We report both on the formation of positive ions formed by electron ionisation and negative ions induced by electron attachment. The experimental data were complemented by quantum chemical calculations. Specifically, the free energies for possible fragmentation reactions were derived for the τ and the π tautomer of histidine to get insight into the structures of the formed ions and the corresponding neutrals. We report the experimental ionisation energy of (8.48 ± 0.03) eV for histidine which is in good agreement with the calculated vertical ionisation energy. In the case of negative ions, the dehydrogenated parent anion is the anion with the highest mass observed upon dissociative electron attachment. The comparison of experimental and computational results was also performed in view of a possible thermal decomposition of histidine during the experiments, since the sample was sublimated in the experiment by resistive heating of an oven. Overall, the present study demonstrates the effects of electrons as secondary particles in the chemical degradation of histidine. The reactions induced by those electrons differ when comparing positive and negative ion formation. While for negative ions, simple bond cleav ages prevail, the observed fragment cations exhibit partly restructuring of the molecule during the dissociation process.

中文翻译：

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在低能电子碰撞中形成氨基酸组氨酸的正离子和负离子。

组氨酸是一种芳香族氨基酸，对蛋白质和酶的生物功能至关重要。当生物物质暴露于电离辐射时，高能粒子与周围组织相互作用，从而有效形成低能电子。在本研究中，低能电子与气相组氨酸的相互作用在分子水平上进行了研究，以扩展电子诱导与氨基酸反应的知识。我们报告了由电子电离形成的正离子和由电子附着引起的负离子的形成。实验数据由量子化学计算补充。具体来说，为组氨酸的 τ 和 π 互变异构体推导了可能的裂解反应的自由能，以深入了解形成的离子和相应的中性子的结构。我们报告组氨酸的实验电离能为 (8.48 ± 0.03) eV，这与计算的垂直电离能非常吻合。在负离子的情况下，脱氢母体阴离子是在离解电子附着时观察到的具有最高质量的阴离子。由于样品在实验中通过烤箱的电阻加热升华，因此还考虑到实验过程中组氨酸可能发生热分解，从而对实验结果和计算结果进行了比较。全面的，本研究证明了电子作为二次粒子在组氨酸化学降解中的作用。比较正离子和负离子的形成时，这些电子引起的反应不同。虽然对于负离子，简单的键断裂占优势，但观察到的碎片阳离子在解离过程中表现出部分分子重组。