Influence of "Alternative" C-terminal amino acids on the formation of [b3 + 17 + Cat]+ products from metal cationized synthetic tetrapeptides

The aim of this study was to investigate the dissociation patterns, and in particular the relative abundance of [b3 + 17 + Cat]+, for peptides with C‐termini designed to allow transfer of the —OH required to generate the product ion, but not necessarily as the most favored pathway. Working with the hypothesis that formation of a five‐membered ring intermediate, including intramolecular nucleophilic attack by a carbonyl oxygen atom, is an important mechanistic step, several model peptides with general sequence AcFGGX were synthesized, metal cationized by electrospray ionization and subjected to collision‐induced dissociation (CID). The amino acid at position X was one that either required a larger ring intermediate (β‐alanine, γ‐aminobutyric acid and ε‐amino‐n‐caproic acid to generate six‐, seven‐ or nine‐ membered rings, respectively) to transfer —OH, lacked a structural element required for nucleophilic attack (aminoethanol) or prohibited cyclization because of the inclusion of a rigid ring (p‐ and m‐aminobenzoic acid). For Ag+, Li+ and Na+ cationized peptides, our results show that amino acids requiring the adoption of larger ring intermediates suppressed the formation of [b3 + 17 + Cat]+, while amino acids that prohibit cyclization eliminated the reaction pathway completely. Formation of [b3 − 1 + Cat]+ from the alkali metal cationized versions was not a favorable process upon suppression or elimination of the [b3 + 17 + Cat]+ pathway: the loss of H2O to form [M − H2O + Cat]+ was instead the dominant dissociation reaction observed. Multiple‐stage dissociation experiments suggest that [M − H2O + Cat]+ is not [b4 − 1 + Cat]+ arising from the loss of H2O from the C‐terminus, but may instead be a species that forms via a mechanism involving the elimination of an oxygen atom from an amide group. Copyright © 2004 John Wiley & Sons, Ltd.