The mechanism for the formation of C,"-type ions in which x < n -2, from protonated peptides containing n amino acids, produced under conditions of fast atom bombardment and collisional activation, was invmtigated.
Comparison of the tandem mass spectra of the [ M + H I + ions of two model peptides (VGSE and AAA) with those of their corresponding [ M, + D]
- ions, in which all exchangeable hydrogens are replaced with deuterium, revealed that in the case of VGSE the hydrogen atom from the hydroxyl group in the serine residue and/or from the amide group of the adjacent amino acid must be involved in the migration process which leads to the formation of C,"-type ions. The second peptide (AAA) shows the exclusive transfer of the amide hydrogen atom in the formation of the C," ion. The mechanism differs from an earlier reported reaction channel which appeared to be applicable to the formation of Ci-I-type ions. The most plausible neutral species which is eliminated in the formation process of the C," ion from 1 M + H ] + is proposed on the basis of relative stability calculations.