Sequence-ion studies in peptides: the generation of C" ions.

The mechanism for the formation of C"-type ions from protonated peptides, produced under conditions of fast-atom bombardment and collisional activation was investigated. Comparison of the tandem mass spectra of the [M + H]+ ions of a model peptide and the corresponding [Md + D]+ ions, in which all exchangeable hydrogens are replaced with deuterium, revealed that neither the carboxylic hydrogen nor a hydrogen from a nitrogen atom is involved in the process of migration of a hydrogen which leads to the formation of C"n-type ions. The most feasible position from which the transferred hydrogen originates is that at the first C-atom in the side-chain of the adjacent amino acid.