title: Influence of "alternative" C-terminal amino acids on the formation of [b3 + 17 + Cat]+ products from metal cationized synthetic tetrapeptides.
abstract: 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) 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 [b(3) + 17 + Cat](+), while amino acids that prohibit cyclization eliminated the reaction pathway completely. Formation of [b(3) - 1 + Cat](+) from the alkali metal cationized versions was not a favorable process upon suppression or elimination of the [b(3) + 17 + Cat](+) pathway: the loss of H(2)O to form [M - H(2)O + Cat](+) was instead the dominant dissociation reaction observed. Multiple-stage dissociation experiments suggest that [M - H(2)O + Cat](+) is not [b(4) - 1 + Cat](+) arising from the loss of H(2)O 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.
authors.affiliation: Department of Chemistry, Wichita State University, Wichita, Kansas 67260-0051, USA.
authors.forename: V
authors.initials: V
authors.lastname: Anbalagan
authors.forename: A T M
authors.initials: AT
authors.lastname: Silva
authors.forename: S
authors.initials: S
authors.lastname: Rajagopalachary
authors.forename: K
authors.initials: K
authors.lastname: Bulleigh
authors.forename: E R
authors.initials: ER
authors.lastname: Talaty
authors.forename: M J
authors.initials: MJ
authors.lastname: Van Stipdonk
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D013058:Mass Spectrometry; D008670:Metals / Q000737:chemistry*; D015394:Molecular Structure; D009842:Oligopeptides / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D000596:Amino Acids; D002412:Cations; D008670:Metals; D009842:Oligopeptides