Positive and negative ion FAB mass spectrometry were found to be useful for the structural analysis of phosphorylated peptides containing multiple O-phosphoseryl residues. The positive ion FAB mass spectra obtained for Ac-Ser(P)-Ser(P)-NHMe and Ac-Ser(P)-Ser(P)-Ser(P)-NHMe showed that β-eliminative loss of H3PO4 from the Ser(P)-residue was a major event in the fragmentation of the two phosphopeptides and that successive losses of H3PO4 from the [M+H]+ ion occurred when the Ser(P)-cluster was located at the N-terminus. In contrast, the FAB mass spectrum of Ac-Glu-Ser(P)-Leu-Ser(P)-Ser(P)-Ser(P)-Glu-Glu-NHMe showed only a single loss of H3PO4 from the [M+H]+ ion, with further losses of H3PO4 from internal Ser(P)-residues only occurring when fragmentation of the parent phosphopeptide generated daughter fragments that contained (part of) an N-terminal Ser(P)-residue. Negative ion FAB mass spectrometry also proved useful for the structural analysis of the three Ser(P)-peptides and showed high-intensity [M-H]- ions along with minor [M-H-80]- fragment ions.
- FAB mass spectrometry
- Ser(P)-containing peptides