Formation of [b₃ - 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 bn ⁺ and yn ⁺ 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 [b₃ - 1 + cat]⁺ products from metal(Li⁺, Na⁺ and Ag⁺)-cationized peptides. The larger amino acids suppress formation of b₃ ⁺ 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 [b₃ - 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 [b₃ - 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 [b₃ - 1 + cat]⁺ is proposed. Copyright © 2008 John Wiley & Sons, Ltd.