C ‐terminal amino acid residue loss for deprotonated peptide ions containing glutamic acid, aspartic acid, or serine residues at the C ‐terminus

Deprotonated peptides containing C ‐terminal glutamic acid, aspartic acid, or serine residues were studied by sustained off‐resonance irradiation collision‐induced dissociation (SORI‐CID) in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer with ion production by electrospray ionization (ESI). Additional studies were performed by post source decay (PSD) in a matrix‐assisted laser desorption ionization/time‐of‐flight (MALDI/TOF) mass spectrometer. This work included both model peptides synthesized in our laboratory and bioactive peptides with more complex sequences. During SORI‐CID and PSD, [M − H] − and [M − 2H] 2− underwent an unusual cleavage corresponding to the elimination of the C ‐terminal residue. Two mechanisms are proposed to occur. They involve nucleophilic attack on the carbonyl carbon of the adjacent residue by either the carboxylate group of the C ‐terminus or the side chain carboxylate group of C ‐terminal glutamic acid and aspartic acid residues. To confirm the proposed mechanisms, D was labelled by 18 O specifically on the side chain of the aspartic acid residue. For peptides that contain multiple C ‐terminal glutamic acid residues, each of these residues can be sequentially eliminated from the deprotonated ions; a driving force may be the formation of a very stable pyroglutamatic acid neutral. For peptides with multiple aspartic acid residues at the C ‐terminus, aspartic acid residue loss is not sequential. For peptides with multiple serine residues at the C ‐terminus, C ‐terminal residue loss is sequential; however, abundant loss of other neutral molecules also occurs. In addition, the presence of basic residues (arginine or lysine) in the sequence has no effect on C ‐terminal residue elimination in the negative ion mode. Copyright © 2006 John Wiley & Sons, Ltd.