Electron capture induced dissociation of peptide ions: Identification of neutral fragments from secondary collisions with cesium vapor

In high-energy collisions between peptide dications and cesium vapor, cations picked up an electron, which led to N-C_α bond breakage to give the characteristic c⁺ and z{radical dot}⁺ type fragments together with their complementary neutral fragments. Neutrals were converted to anions in secondary collisions with cesium. Peaks corresponding to z^- anions dominate the product ion spectra, whereas c{radical dot}^- anions could not with certainty be identified, instead O^-, OH^-, CN^-, CNH^-, C₂O^-, and OCN^- were formed. A fragment ion is also assigned to HCOO^- formed from dissociation of z^-. We ascribe the outcome of secondary collisions to the fact that a z{radical dot} fragment is a radical that when accepts an electron becomes a stable carbanion whereas electron capture to an even-electron c fragment produces a reactive radical anion that dissociates within the timescale of the experiment (few microseconds). Such an experimental scheme in which anions are measured implies that we are not limited to the study of dications; indeed, selective N-C_α bond cleavages of peptide monocations were identified from negative fragment ion spectra that were again dominated by z^- type ions. Neutral y fragments from collision-induced dissociation are also easily detected as y^- ions after electron pickup in secondary collisions.