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 [b(3) + 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 (beta-alanine, gamma-aminobutyric acid and epsilon-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) 14 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 [b(4) - 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 (C) 2004 John Wiley Sons, Ltd.