Protonated members of a homologous series of biologically significant α,ω-diamino carboxylic acids were subjected to collision induced dissociation (CID). The resulting fragmentation patterns were studied using isotopic labeling, quantum mechanical computations, and pseudo MS³ experiments conducted primarily on an ion trap mass spectrometer. Each protonated α,ω-diamino acid showed a primary neutral loss of either ammonia or water; a clear explanation was developed for the observed variation of the two losses within the series. Protonated 2,3-diaminopropanoic acid, 2,4-diaminobutanoic acid, and 2,7-diaminoheptanoic acid gave secondary losses of water, carbon monoxide, and a loss of water plus carbon monoxide, respectively. In the parallel pathways characterized for the fragmentations of protonated ornithine and lysine, the α-nitrogen of the diamino acid was maintained in the cyclic iminium product formed by successive losses of NH₃ and (H₂O + CO), whereas the side-chain nitrogen was retained by consecutive losses of H₂O and (CO, NH₃). The 1-piperideine ion from protonated lysine was fragmented further, losing ethylene from carbons 4 and 5. Protonated 2,6-diaminopimelic acid fragmented by analogous reactions. Detailed mechanistic schemes for the fragmentation of both protonated 2,3-diaminopropanoic and ornithine were generated from MP2/DFT computations. This work highlights the participation of the side-chain amino group, which distinguishes the gas-phase chemistry of protonated α,ω-diamino acids from the well-documented fragmentation reactions of protonated α-amino acids bearing a hydrogen atom or an alkyl side chain. In general, the results further illustrate the importance of intramolecular separations affecting the specific interactions between functional groups leading to the fragmentation of multifunctional ions.

中文翻译：

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质子化 α,ω-二氨基羧酸的断裂反应：官能团相互作用的重要性

对具有生物学意义的 α,ω-二氨基羧酸的同源系列的质子化成员进行碰撞诱导解离 (CID)。使用同位素标记、量子力学计算和伪 MS ³研究了所得的碎裂模式实验主要在离子阱质谱仪上进行。每个质子化的 α,ω-二氨基酸都表现出氨或水的主要中性损失；为观察到的系列中两个损失的变化制定了明确的解释。质子化的 2,3-二氨基丙酸、2,4-二氨基丁酸和 2,7-二氨基庚酸分别导致水、一氧化碳和水加一氧化碳的二次损失。在表征为质子化鸟氨酸和赖氨酸断裂的平行途径中，二氨基酸的 α-氮保留在由 NH ₃和 (H ₂ O + CO)连续损失形成的环状亚胺产物中，而侧链通过连续损失 H ₂ O 和 (CO, NH₃）。来自质子化赖氨酸的 1-哌啶离子进一步碎裂，从碳 4 和 5 失去乙烯。质子化 2,6-二氨基庚二酸通过类似反应碎裂。MP2/DFT 计算生成了质子化 2,3-二氨基丙酸和鸟氨酸裂解的详细机制方案。这项工作突出了侧链氨基的参与，它将质子化 α,ω-二氨基酸的气相化学与有据可查的带有氢原子或烷基侧链的质子化 α-氨基酸的断裂反应区分开来. 一般而言，结果进一步说明了分子内分离对导致多功能离子碎裂的官能团之间特定相互作用的影响的重要性。