Amide bond cleavage in deprotonated tripeptides: a newly discovered pathway to "b2 ions

The fragmentation reactions of the M-H](-) ions of the tripeptides H-Gly-Leu-Sar-OH, H-Leu-Gly-Pro-OH and H-Gly-Leu-Gly-OH have been investigated in detail using energy-resolved mass spectrometry, isotopic labelling and MS(3) experiments. It is shown that the major route to the "b(2) ions involves loss of a neutral amine from the a(3) (M-H-CO(2)](-)) ion rather than being formed directly by fragmentation of the M-H](-) ion. When there is no C-terminal amidic hydrogen (Sar, Pro), loss of a neutral amine is the dominant primary fragmentation reaction of the a(3) ion. However, when there is a C-terminal amidic hydrogen (Gly), elimination of the N-terminal amino acid residue is the major fragmentation reaction of the a(3) ion and formation of the "b(2) ion is greatly reduced in importance. It is proposed that the "b(2) ions are deprotonated oxazolones.