title: Elucidation of fragmentation mechanisms of protonated Peptide ions and their products: a case study on glycylglycylglycine using density functional theory and threshold collision-induced dissociation.
abstract: The fragmentation mechanisms of protonated triglycine and its first-generation dissociation products have been investigated using a combination of density functional theory calculations and threshold collision-induced dissociation experiments. The activation barrier measured for the fragmentation of protonated triglycine to the b(2) ion and glycine is in good agreement with a calculated barrier at the B3LYP/6-31++G(d,p) level of theory reported earlier [Rodriquez, C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The b(2) ion fragments to the a(2) ion via a transition state structure that is best described as acylium-like. Contrary to what is commonly assumed, the lowest energy structure of the a(2) ion is not an iminium ion, but a cyclic, protonated 4-imidazolidone. Furthermore, fragmentation of the b(2) to the a(1) ion proceeds not via a mechanism that results in HNCO and H(2)C=C=O as byproducts, as have been postulated, but via a transition state that contains an incipient a(1) ion and an incipient carbene. The fragmentation of a(2) to a(1) proceeds via a transition state structure that contains the a(1) ion, CO and an imine as incipient components.
authors.affiliation: Contribution from the Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3.
authors.forename: Houssain
authors.initials: H
authors.lastname: El Aribi
authors.forename: Christopher F
authors.initials: CF
authors.lastname: Rodriquez
authors.forename: David R P
authors.initials: DR
authors.lastname: Almeida
authors.forename: Yun
authors.initials: Y
authors.lastname: Ling
authors.forename: William W-N
authors.initials: WW
authors.lastname: Mak
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
mesh_terms: D007477:Ions / Q000737:chemistry; D008956:Models, Chemical; D009842:Oligopeptides / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D007477:Ions; D009842:Oligopeptides; C035647:glycyl-glycyl-glycine