title: Nucleophilic substitution by amide nitrogen in the aromatic rings of [zn - H]˙⁺ ions; the structures of the [b₂ - H - 17]˙⁺ and [c1 - 17]⁺ ions.
abstract: Peptide radical cations that contain an aromatic amino acid residue cleave to give [zn - H]˙⁺ ions with [b2 - H - 17]˙⁺ and [c1 - 17](+) ions, the dominant products in the dissociation of [zn - H]˙⁺, also present in lower abundance in the CID spectra. Isotopic labeling in the aromatic ring of [Yπ˙GG](+) establishes that in the formation of [b2 - H - 17]˙⁺ ions a hydrogen from the δ-position of the Y residue is lost, indicating that nucleophilic substitution on the aromatic ring has occurred. A preliminary DFT investigation of nine plausible structures for the [c1 - 17](+) ion derived from [Y(π)˙GG](+) shows that two structures resulting from attack on the aromatic ring by oxygen and nitrogen atoms from the peptide backbone have significantly better energies than other isomers. A detailed study of [Y(π)˙GG](+) using two density functionals, B3LYP and M06-2X, with a 6-31++G(d,p) basis set gives a higher barrier for attack on the aromatic ring of the [zn - H]˙⁺ ion by nitrogen than by the carbonyl oxygen. However, subsequent rearrangements involving proton transfers are much higher in energy for the oxygen-substituted isomer leading to the conclusion that the [c1 - 17](+) ions are the products of nucleophilic attack by nitrogen, protonated 2,7-dihydroxyquinoline ions. The [b2 - H - 17]˙⁺ ions are formed by loss of glycine from the same intermediates involved in the formation of the [c1 - 17](+) ions.
authors.affiliation: Department of Chemistry, The University of Hong Kong, Hong Kong, China. ivankchu@hku.hk.
authors.forename: Xiaoyan
authors.initials: X
authors.lastname: Mu
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto ON, M3J 1P3, Canada. ach@yorku.ca and Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada.
authors.forename: Justin Kai-Chi
authors.initials: JK
authors.lastname: Lau
authors.affiliation: Department of Chemistry, The University of Hong Kong, Hong Kong, China. ivankchu@hku.hk.
authors.forename: Cheuk-Kuen
authors.initials: CK
authors.lastname: Lai
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto ON, M3J 1P3, Canada. ach@yorku.ca and Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada.
authors.forename: K W Michael
authors.initials: KW
authors.lastname: Siu
authors.affiliation: Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto ON, M3J 1P3, Canada. ach@yorku.ca.
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
authors.affiliation: Department of Chemistry, The University of Hong Kong, Hong Kong, China. ivankchu@hku.hk.
authors.forename: Ivan K
authors.initials: IK
authors.lastname: Chu
mesh_terms: D000577:Amides / Q000737:chemistry*; D007477:Ions; D013058:Mass Spectrometry; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry*
chemical_list: D000577:Amides; D007477:Ions; D009584:Nitrogen