Formation of [b 3 − 1 + cat] + ions from metal‐cationized tetrapeptides containing β‐alanine, γ‐aminobutyric acid or ε‐aminocaproic acid residues

The presence and position of a single β‐alanine (βA), γ‐aminobutyric acid (γABu) or ε‐aminocaproic acid (Cap) residue has been shown to have a significant influence on the formation of b n + and y n + product ions from a series of model, protonated peptides. In this study, we examined the effect of the same residues on the formation of analogous [b 3 − 1 + cat] + products from metal(Li + , Na + and Ag + )‐cationized peptides. The larger amino acids suppress formation of b 3 + from protonated peptides with general sequence AAXG (where X = β‐alanine, γ‐aminobutyric acid or ε‐aminocaproic acid), presumably because of the prohibitive effect of larger cyclic intermediates in the ‘oxazolone’ pathway. However, abundant [b 3 − 1 + cat] + products are generated from metal‐cationized versions of AAXG. Using a group of deuterium‐labeled and exchanged peptides, we found that formation of [b 3 − 1 + cat] + involves transfer of either amide or α‐carbon position H atoms, and the tendency to transfer the atom from the α‐carbon position increases with the size of the amino acid in position X. To account for the transfer of the H atom, a mechanism involving formation of a ketene product as [b 3 − 1 + cat] + is proposed. Copyright © 2008 John Wiley & Sons, Ltd.