Savitski Mikhail M, Nielsen Michael L, Zubarev Roman A
Laboratory for Biological and Medical Mass Spectrometry, Uppsala University, Uppsala, Sweden.
Anal Chem. 2007 Mar 15;79(6):2296-302. doi: 10.1021/ac0619332. Epub 2007 Feb 3.
Analysis of a database of some 20 000 conventional electron-capture dissociation (ECD) mass spectra of doubly charged ions belonging to tryptic peptides revealed widespread appearance of w ions and related u ions that are due to partial side chain losses from radical z* ions. Half of all z* ions that begin with Leu or Ile produce w ions in conventional one-scan ECD mass spectra, which differentiates these isomeric residues with >97% reliability. Other residues exhibiting equally frequent side chain losses are Gln, Glu, Asp, and Met (cysteine was not included in this work). Unexpectedly, Asp lost not a radical group like other amino acids but a molecule CO2, thus giving rise to a radical w* ion with the possibility of a radical cascade. Losses from amino acids as distant as seven residues away from the cleavage site were detected. The mechanism of such losses seems to be related to radical migration from the original site at the alphaCn atom in a zn* ion to other alphaC and betaC atoms. The side chain losses confirm sequence assignment, improve the database matching score, and can be useful in de novo sequencing.
对约20000个属于胰蛋白酶肽的双电荷离子的传统电子捕获解离（ECD）质谱数据库进行分析，发现w离子和相关u离子广泛出现，这是由于自由基z离子的部分侧链损失所致。在传统的单次扫描ECD质谱中，所有以亮氨酸（Leu）或异亮氨酸（Ile）开头的z离子中有一半会产生w离子，这能以>97%的可靠性区分这些同分异构残基。其他表现出同样频繁侧链损失的残基有谷氨酰胺（Gln）、谷氨酸（Glu）、天冬氨酸（Asp）和甲硫氨酸（Met）（本研究未包括半胱氨酸）。出乎意料的是，天冬氨酸不像其他氨基酸那样失去一个自由基基团，而是失去一个二氧化碳分子，从而产生一个自由基w离子，有可能引发自由基级联反应。检测到离裂解位点远至七个残基的氨基酸的损失。这种损失的机制似乎与自由基从zn离子中αCn原子的原始位点迁移到其他αC和βC原子有关。侧链损失可确认序列归属，提高数据库匹配分数，并且在从头测序中可能有用。
