Sequence‐ion studies in peptides: The generation of C″ ions—part 2

Abstract

AbstractThe mechanism for the formation of Cx″‐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 investigated. Comparison of the tandem mass spectra of the [M + H]+ ions of two model peptides (VGSE and AAA) with those of their corresponding [Md + 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 C2″‐type ions. The second 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 the formation of C‐type ions. The most plausible neutral species which is eliminated in the formation process of the Cx″ ion from [M + H]+ is proposed on the basis of relative stability calculations.