Abstract Mono- and di-lithiation of small amino acids (AA) including alanine, proline, histidine and arginine were studied by laser desorption/ionization-time of flight (TOF) mass spectrometry. The mass spectra showed that two adduct cations of amino acids [AA + Li]+ and [AA+2Li-H]+ are formed. The relative intensities of the [AA + Li]+ and [AA+2Li-H]+ peaks in mass spectrum depend on the nature of side chain of the amino acids and the pH of the solution containing AA and lithium salt. In basic solution, the [AA+2Li-H]+ peak was more intense than that of [AA + Li]+ and a reverse trend was observed in the acidic solution. Furthermore, it was found that the di-lithiation of arginine and histidine takes place more favorably than alanine and proline which was attributed to presence of strong basic sites in the side chains of the formers. The theoretical calculations showed that attachment of the first Li+ cation to amino acids enhances their acidity so that [AA + Li]+ is more easily releases it proton compared to AA. Hence, Li+ attachment, basicity of the side chain of AA, and solution pH influence the formation of [AA+2Li-H]+.

中文翻译：

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水溶液中氨基酸锂化的机理：飞行时间质谱和理论研究

摘要 通过激光解吸/电离飞行时间 (TOF) 质谱法研究了包括丙氨酸、脯氨酸、组氨酸和精氨酸在内的小氨基酸 (AA) 的单锂化和双锂化。质谱表明，形成了氨基酸 [AA + Li]+ 和 [AA+2Li-H]+ 的两种加合物阳离子。质谱中[AA + Li]+ 和[AA+2Li-H]+ 峰的相对强度取决于氨基酸侧链的性质以及含有AA 和锂盐的溶液的pH。在碱性溶液中，[AA+2Li-H]+峰比[AA+Li]+峰更强烈，而在酸性溶液中则观察到相反的趋势。此外，发现精氨酸和组氨酸的二锂化比丙氨酸和脯氨酸更容易发生，这归因于前者侧链中存在强碱性位点。理论计算表明，第一个 Li+ 阳离子与氨基酸的连接增强了它们的酸度，因此与 AA 相比，[AA + Li]+ 更容易释放质子。因此，Li+ 的附着、AA 侧链的碱性和溶液 pH 会影响 [AA+2Li-H]+ 的形成。