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

Three-stage tandem mass spectrometry (MS3) and neutral fragment reionization (NfR) 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 C6H5CO-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 MS3 and NfR data help propose dissociation mechanisms that account for the observed structures of ionic and neutral backbone fragments. (C) 1997 Elsevier Science B.V.