title: Stable gas-phase radical cations of dimeric tryptophan and tyrosine derivatives.
abstract: Stable radical cations of dimeric amino acid derivatives of tryptophan and tyrosine were generated by collision-induced dissociation of [Cu(II)(diethylenetriamine)(amino acid derivative)2]*2+. The yields of the dimer radical cations were dependent on both the auxiliary ligand and the tryptophan or tyrosine derivatives used. Amino acid derivatives with an unmodified carboxylic acid group did not generate dimer radical cations. For the amino acid derivatives Ac-Trp-OMe and Ac-Trp-NH2 (Ac is N-acetyl; OMe and NH2 are the methyl ester and amide modifications of the C-terminal carboxylic group), no auxiliary ligand was required for generating the dimer radical cations. Collision-induced dissociation of the [Cu(II)(amino acid derivative)4]*2+ precursor generated the dimer radical cation [(amino acid derivative)2]*+. Stabilizing interactions, most likely involving hydrogen bonding, between the two amino acid derivatives are proposed to account for observation of the dimer radical cations. Dissociation of these ions yields protonated or radical cationic amino acid derivatives; these observations are consistent with the expectation of proton competition between monomeric units, whose proton affinities were calculated using density functional theory.
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada.
authors.forename: Yuyong
authors.initials: Y
authors.lastname: Ke
authors.forename: Udo H
authors.initials: UH
authors.lastname: Verkerk
authors.forename: P Y Iris
authors.initials: PY
authors.lastname: Shek
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
mesh_terms: D000107:Acetylation; D002412:Cations; D006860:Hydrogen Bonding; D008024:Ligands; D013058:Mass Spectrometry; D014364:Tryptophan / Q000031:analogs & derivatives* / Q000737:chemistry*; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000737:chemistry*
chemical_list: D002412:Cations; D008024:Ligands; D014443:Tyrosine; D014364:Tryptophan