Although the N-H bond in peptide backbones is stronger than the C-H bond, hydrogen abstraction from the amide nitrogen is considered to be the initial step in the Cα-C bond cleavage of peptide backbones by matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) when using an oxidizing matrix. MALDI-ISD induces Cα-C bond cleavage in most amino acid residues, whereas the N-terminal sides of proline (Pro) residues preferentially undergo peptide bond cleavage, which cannot be explained by the previously proposed mechanism involving hydrogen abstraction from peptides. To explain the whole MALDI-ISD process, electron abstraction from peptides by the oxidizing matrix is proposed as the initial step in the MALDI-ISD process. The electron abstraction occurs from either nitrogen or oxygen in the peptide backbone and induces the cleavage of both Cα-C and N-H bonds in most amino acid residues, except for those on the N-terminal sides of Pro residues. Electron abstraction from the Pro residues induces the cleavage of both peptide and Cα-C bonds, which is consistent with MALDI-ISD experimental results. The electron transfer from the peptide to the oxidizing matrix occurs simultaneously with the formation of matrix ions, which is considered to be the initial ion formation process in MALDI. The resultant peptide radical cation produces protonated and neutral molecules/radicals, which undergo subsequent ion-molecule reactions in the MALDI plume, finally yielding the ions that are observed in MALDI-ISD spectrum. As a result, the fragment ions formed by MALDI-ISD are observed as both positive and negative ions.

中文翻译：

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紫外激光诱导从肽到氧化基质的电子转移：MALDI 源内衰变质谱的第一步研究。

尽管肽主链中的 NH 键比 CH 键强，但从酰胺氮中夺取氢被认为是通过基质辅助激光解吸/源内衰变对肽主链进行 Cα-C 键裂解的第一步。 MALDI-ISD) 当使用氧化基质时。MALDI-ISD 在大多数氨基酸残基中诱导 Cα-C 键裂解，而脯氨酸 (Pro) 残基的 N 端侧优先进行肽键裂解，这无法用先前提出的涉及从肽中提取氢的机制来解释。为了解释整个 MALDI-ISD 过程，建议通过氧化基质从肽中提取电子作为 MALDI-ISD 过程的初始步骤。吸电子发生在肽骨架中的氮或氧，并诱导大多数氨基酸残基中的 Cα-C 和 NH 键断裂，除了 Pro 残基 N 端侧的那些。Pro 残基的电子提取诱导肽和 Cα-C 键的裂解，这与 MALDI-ISD 实验结果一致。从肽到氧化基质的电子转移与基质离子的形成同时发生，这被认为是 MALDI 中的初始离子形成过程。所得肽自由基阳离子产生质子化和中性分子/自由基，它们在 MALDI 羽流中进行后续的离子-分子反应，最终产生在 MALDI-ISD 光谱中观察到的离子。因此，