Metastable decomposition of peptide [M + H]⁺ ions via rearrangement involving loss of the C-terminal amino acid residue

Abstract

A novel fragmentation of metastable peptide [M + H] + ions is described. Loss of the C-terminal amino acid residue is accomqanied by retention of one of the carboxyl oxygens, as judged by 18 O-labeling. The retained 8 O label is located at the new C-terminus. Sequential mass spectrometric analyses indicate that the structure of the first-generation product ion is indistinguishable from that of the [M + H] + ion of the peptide with one fewer amino acid residues. Thus, for example, the metastable decompositions of ions of m/z 904 are similar whether they correspond to des-Arg 9 -bradykinin [M + H] + ions or to fragments derived from bradykinin [M + H] + ions. No corresponding rearrangements have been observed for peptides with C-terminal amide or ester functions. The mechanism of this fragmentation may be considered to be analogous to that previously suggested for fragmentations of [M + alkali metal cation] + ions. For the examples of bradykinin and related peptides, the rearrangement is strongly promoted when arginine is the amino acid residue lost. The same fragmentation is also favored by the presence of an arginine residue at or near the N-terminus. The strong influence of peptide amino acid composition, including residues remote from the C-terminus, on the prevalence of this fragmentation suggests mechanistic complexities that require further elucidation.