Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, USA.
J Am Chem Soc. 2011 Oct 26;133(42):16790-3. doi: 10.1021/ja207736y. Epub 2011 Oct 5.
The generation of gaseous polyanions with a Coulomb barrier has attracted attention as exemplified by previous studies of fullerene dianions. However, this phenomenon has not been reported for biological anions. By contrast, electron attachment to multiply charged peptide and protein cations has seen a surge of interest due to the high utility for tandem mass spectrometry (MS/MS). Electron capture dissociation (ECD) and electron transfer dissociation (ETD) involve radical-driven fragmentation of charge-reduced peptide/protein cations to yield N-C(α) backbone bond cleavage, resulting in predictable c'/z(•)-type product ions without loss of labile post-translational modifications (PTMs). However, acidic peptides, e.g., with biologically important PTMs such as phosphorylation and sulfonation, are difficult to multiply charge in positive ion mode and show improved ionization in negative-ion mode. We found that peptide anions (M - nH, n ≥ 1) can capture electrons within a rather narrow energy range (~3.5-6.5 eV), resulting in charge-increased radical intermediates that undergo dissociation analogous to that in ECD/ETD. Gas-phase zwitterionic structures appear to play an important role in this novel MS/MS technique, negative-ion electron capture dissociation (niECD).
具有库仑势垒的气态多阴离子的生成引起了人们的关注，例如先前对富勒烯二阴离子的研究。然而，这种现象尚未在生物阴离子中报道。相比之下，由于串联质谱（MS/MS）的高实用性，多电荷肽和蛋白质阳离子的电子捕获受到了极大的关注。电子俘获解离（ECD）和电子转移解离（ETD）涉及电荷减少的肽/蛋白质阳离子的自由基驱动的碎裂，产生 N-C（α）骨架键的裂解，导致可预测的 c'/z(•)-型产物离子，而不会失去不稳定的翻译后修饰（PTMs）。然而，酸性肽，例如具有生物学重要的 PTMs，如磷酸化和磺化，在正离子模式下很难多次荷电，并在负离子模式下显示出改善的电离。我们发现肽阴离子 (M - nH, n ≥ 1) 可以在相当窄的能量范围内（~3.5-6.5 eV）捕获电子，导致电荷增加的自由基中间体经历类似于 ECD/ETD 的解离。气相两性离子结构似乎在这种新型 MS/MS 技术，负离子电子捕获解离（niECD）中发挥重要作用。
