title: Discovery and mechanistic studies of facile N-terminal Cα-C bond cleavages in the dissociation of tyrosine-containing peptide radical cations.
abstract: Fascinating N-terminal Cα-C bond cleavages in a series of nonbasic tyrosine-containing peptide radical cations have been observed under low-energy collision-induced dissociation (CID), leading to the generation of rarely observed x-type radical fragments, with significant abundances. CID experiments of the radical cations of the alanyltyrosylglycine tripeptide and its analogues suggested that the N-terminal Cα-C bond cleavage, yielding its [x2 + H](•+) radical cation, does not involve an N-terminal α-carbon-centered radical. Theoretical examination of a prototypical radical cation of the alanyltyrosine dipeptide, using density functional theory calculations, suggested that direct N-terminal Cα-C bond cleavage could produce an ion-molecule complex formed between the incipient a1(+) and x1(•) fragments. Subsequent proton transfer from the iminium nitrogen atom in a1(+) to the acyl carbon atom in x1(•) results in the observable [x1 + H](•+). The barriers against this novel Cα-C bond cleavage and the competitive N-Cα bond cleavage, forming the complementary [c1 + 2H](+)/[z1 - H](•+) ion pair, are similar (ca. 16 kcal mol(-1)). Rice-Ramsperger-Kassel-Marcus modeling revealed that [x1 + H](•+) and [c1 + 2H](+) species are formed with comparable rates, in agreement with energy-resolved CID experiments for [AY](•+).
authors.affiliation: Department of Chemistry, The University of Hong Kong , Hong Kong, China.
authors.forename: Xiaoyan
authors.initials: X
authors.lastname: Mu
authors.forename: Tao
authors.initials: T
authors.lastname: Song
authors.forename: Minjie
authors.initials: M
authors.lastname: Xu
authors.forename: Cheuk-Kuen
authors.initials: CK
authors.lastname: Lai
authors.forename: Chi-Kit
authors.initials: CK
authors.lastname: Siu
authors.forename: Julia
authors.initials: J
authors.lastname: Laskin
authors.forename: Ivan K
authors.initials: IK
authors.lastname: Chu
mesh_terms: D002412:Cations / Q000737:chemistry*; D003198:Computer Simulation; D007700:Kinetics; D013058:Mass Spectrometry; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D002412:Cations; D010455:Peptides; D014443:Tyrosine