Novel Fragmentation Pathway for CID of (bn − 1 + Cat)+ Ions from Model, Metal Cationized Peptides

We report a new fragmentation pathway for the CID of (b3 − 1 + Cat)+ product ions derived from the model peptide AXAG, where X =β-alanine, γ-aminobutyric acid, ε-amino-n-caproic acid, or 4-aminomethylbenzoic acid. By changing the amino acid to the C-terminal side of the amino acid X, and incorporating 15N and 13C labeled residues at the same position, we conclude that the dissociation pathway most likely leads to a metal cationized nitrile. With respect to the various amino acids at position X, the putative nitrile product becomes more prominent, relative to the conventional (a3 − 1 + Cat)+ species, in the order β-alanine < γ-aminobutyric acid < ε-aminocaproic acid < 4-aminomethylbenzoic acid. The pathway is not observed for peptides with α-amino acids at position X. The product ion is observed most prominently during the CID of Li+ and Na+ cationized peptides, only to a small extent for Ag+ cationized peptides, and not at all from protonated analogues.