a2 ion derived from triglycine: an N1-protonated 4-imidazolidinone

Fragmentation of protonated peptides in the gas phase constitutes the basis for gas-phase sequencing of peptides using tandem mass spectrometry. Several mechanistic studies have indicated possible loss of b(n) ion sequence information as a consequence of macrocycle formation from internal nucleophilic attacks. Here, we show by infrared multiple-photon dissociation spectroscopy and density functional theory that the prototypical a(2) ion generated from protonated triglycine is predominantly a cyclic N-1-protonated 4-imidazolidinone. Cyclization resulting from internal nucleophilic attacks therefore may be a more general phenomenon than anticipated.