Electron-capture-induced dissociation of microsolvated Di- and tripeptide monocations: Elucidation of fragmentation channels from measurements of negative ions

The results from an experimental study of bare and microsolvated peptide monocations in high-energy collisions with cesium vapor are reported. Neutral radicals form after electron capture from cesium, which decay by H loss, NH ₃ loss, or N - C_a bond cleavage into characteristic ż and c fragments. The neutral fragments are converted into negatively charged species in a second collision with cesium and are identified by means of mass spectrometry. For protonated GA (G=glycine, A = alanine), the branching ratio between NH₃ loss and N - C_a bond cleavage is i found to strongly depend on the molecule attached (H₂O, CH₃CN, CH₃OH, and 18-crown-6 ether (CE)). Addition of H₂O and CH₃OH increases this ratio whereas CH₃CN and CE decrease it. For protonated AAA ([AAA + H]⁺), a similar effect is observed with methanol, while the ratio between the z₁ and z₂ fragment peaks remains unchanged for the bare and microsolvated species. Density functional theory calculations reveal that in the case of [GA + H] ⁺(CE), the singly occupied molecular orbital is located mainly on the amide group in accordance with the experimental results.