Fragmentation of peptide radical cations [M]^.+ has been examined using matrix‐assisted laser desorption/ionization (MALDI) in‐source decay (ISD) with hydrogen‐abstracting nitro‐substituted matrices. The ISD spectra of peptides containing an arginine (Arg) residue at carboxyl (C)‐termini showed preferential [w]⁺ ions when 4‐nitro‐1‐naphthol (4,1‐NNL) matrix was used, whereas the use of 3,5‐dinitrosalicylic acid (3,5‐DNSA) resulted in preferential [x]⁺ ions. Minor or some [d]⁺, [x]⁺, [y]⁺, and [z]⁺ ions were also observed. For peptides containing Arg residue at amino (N)‐termini, the ISD spectra showed preferential [a]⁺ ions independent of matrix used. The observed [a]⁺, [w]⁺, [x]⁺, [y]⁺, and [z]⁺ ions can be rationally explained by radical‐directed dissociation (RDD) of the peptide radical cations [M]^.+, although [d]⁺ ions may be formed via Norrish Type I cleavage and/or by RDD of [M]^.+ ions. The formation of overdegraded [d]⁺, [w]⁺, [y]⁺, and [z]⁺ ions is discussed from the standpoint of the internal energy of radical cations [M]^.+ and radical fragment ions [a + H]^.+ and [x + H]^.+ deposited via collisional interactions with excited matrix molecules in the MALDI plume. The radical site of the peptide cations [M]^.+ was presumed to be backbone amide nitrogen, from MALDI‐ISD data with three different deuterated amino acids.

中文翻译：

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基质辅助的激光解吸/电离源内衰变中肽自由基阳离子[M] +的自由基定向解离产生的依赖于基质的a / x对和过度降解的w / y / z离子

肽自由基阳离子的断裂[M] ^。+已使用具有氢吸收性硝基取代基质的基质辅助激光解吸/电离（MALDI）源衰减（ISD）进行了检验。当使用4-硝基-1-萘酚（4,1-NNL）基质时，在羧基（C）-末端含有精氨酸（Arg）残基的肽的ISD谱显示优先[ w ] ⁺离子，而使用3 ，5-二硝基水杨酸（3,5-DNSA）产生优先[ x ] ⁺离子。次要或某些[ d ] ⁺，[ x ] ⁺，[ y ] ⁺和[ z ] ⁺还观察到离子。对于在氨基（N）-末端含有Arg残基的肽，ISD光谱显示优先[ a ] ⁺离子，与所用基质无关。观察到的[ a ] ⁺，[ w ] ⁺，[ x ] ⁺，[ y ] ⁺和[ z ] ⁺离子可以通过肽自由基阳离子[M]的自由基定向解离（RDD）进行合理解释^。+，尽管[ d ] ⁺离子可通过Norrish I型裂解和/或[M]的RDD形成^。+离子。从自由基阳离子的内能角度探讨了[ d ] ⁺，[ w ] ⁺，[ y ] ⁺和[ z ] ⁺离子的降解[M] ^。+和自由基碎片离子[ a + H] ^。+和[ x + H] ^。+通过与MALDI羽中的激发基质分子的碰撞相互作用而沉积。肽阳离子的自由基位点[M] ^。+ 根据MALDI-ISD数据中的三种不同的氘代氨基酸，推测其为骨架酰胺氮。