Amide bond dissociation in protonated peptides. Structures of the N-terminal ionic and neutral fragments

Abstract

Three-stage tandem mass spectrometry (MS 3) and neutral fragment reionization (N fR) are utilized to investigate the structures of the N-terminal ionic (b i ) and neutral backbone fragments, respectively, produced from break-up of the amide bond in protonated peptides that have been collisionally activated. The b-type ion from [M+H] + of C 6H 5CO-GF, which is produced by loss of the C-terminal phenylalanine, has the structure of protonated 2-phenyl-5-oxazolone. Conversely, the neutral fragment accompanying the y-type ion (protonated phenylalanine) is 2-phenyl-5-oxazolone. The b 2 ions arising from [M+H] + of underivatized tripeptides are also found to be protonated oxazolones. On the other hand, the neutral fragments released from the N-terminus of the tripeptides upon formation of y 1 are shown to be diketopiperazines and not oxazolones. The combined MS 3 and N fR data help propose dissociation mechanisms that account for the observed structures of ionic and neutral backbone fragments.