Amide bond cleavage in deprotonated tripeptides: a newly discovered pathway to ″b 2 ions

The fragmentation reactions of the [MH] − 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 ([MHCO 2 ] − ) ion rather than being formed directly by fragmentation of the [MH] − 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. Copyright © 2003 John Wiley & Sons, Ltd.