Dipositively Charged Protonated a3 and a2 Ions: Generation by Fragmentation of [La(GGG)(CH3CN)2]~(3+)

摘要

Iminium ions a2 and a3 are fragment ions in the gas-phase microsequencing of protonated peptides in proteomics. Protonation of these small a_n ions as they are being formed during peptide fragmentation is, in theory, possible but has not hitherto been reported. This absence is presumably because of the Coulombic repulsion encountered by any given mobile proton in the vicinity of the incipient a2 and a3 ions, just prior to dissociation. Herein, we report the first observation of a small, dipositive (and hence high-charge-density) protonated a3 ion, (a3 + H)~(2+), and a protonated a2 ion, (a2 + H)~(2+), produced through the tandem mass spectrometry of a triply charged lanthanum complex of triglycine (GGG).