SEQUENCE-ION STUDIES IN PEPTIDES - THE GENERATION OF C'' IONS .2.

The mechanism for the formation of C(x)''-type ions in which x less-th an-or-equal-to n - 2, from protonated peptides containing n amino acid s, produced under conditions of fast atom bombardment and collisional activation, was investigated. Comparison of the tandem mass spectra of the [M + H]+ ions of two model peptides (VGSE and AAA) with those of their corresponding [M(d) + D]+ ions in which all exchangeable hydroge ns are replaced with deuterium, revealed that in the case of VGSE the hydrogen atom from the hydroxyl group in the serine residue and/or fro m the amide group of the adjacent amino acid must be involved in the m igration process which leads to the formation of C2''-type ions. The s econd peptide (AAA) shows the exclusive transfer of the amide hydrogen atom in the formation of the C1'' ion. The mechanism differs from an earlier reported reaction channel which appeared to be applicable to t he formation of C(n-1)''-type ions. The most plausible neutral species which is eliminated in the formation process of the C(x)'' ion from [ M + H]+ is proposed on the basis of relative stability calculations.