Nucleophilic substitution by amide nitrogen in the aromatic rings of [zn − H]˙+ ions; the structures of the [b2 − H − 17]˙+ and [c1 − 17]+ ions

Xiaoyan Mu; Justin Kai-Chi Lau; Cheuk-Kuen Lai; K. W. Michael Siu; Alan C. Hopkinson; Ivan K. Chu

doi:10.1039/C6CP00405A

Nucleophilic substitution by amide nitrogen in the aromatic rings of [z_n − H]˙⁺ ions; the structures of the [b₂ − H − 17]˙⁺ and [c₁ − 17]⁺ ions†

Xiaoyan Mu,^a Justin Kai-Chi Lau,^bc Cheuk-Kuen Lai,^a K. W. Michael Siu,^bc Alan C. Hopkinson*^b and Ivan K. Chu*^a

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* Corresponding authors

^a Department of Chemistry, The University of Hong Kong, Hong Kong, China
E-mail: ivankchu@hku.hk

^b Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto ON, Canada
E-mail: ach@yorku.ca

^c Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, ON, Canada

Abstract

Peptide radical cations that contain an aromatic amino acid residue cleave to give [z_n − H]˙⁺ ions with [b₂ − H − 17]˙⁺ and [c₁ − 17]⁺ ions, the dominant products in the dissociation of [z_n − H]˙⁺, also present in lower abundance in the CID spectra. Isotopic labeling in the aromatic ring of [Y_π˙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 [z_n − 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.

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DOI: https://doi.org/10.1039/C6CP00405A
Article type: Paper
Submitted: 19 Jan 2016
Accepted: 04 Mar 2016
First published: 08 Mar 2016

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Phys. Chem. Chem. Phys., 2016,18, 11168-11175

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Nucleophilic substitution by amide nitrogen in the aromatic rings of [z_n − H]˙⁺ ions; the structures of the [b₂ − H − 17]˙⁺ and [c₁ − 17]⁺ ions

X. Mu, J. K. Lau, C. Lai, K. W. M. Siu, A. C. Hopkinson and I. K. Chu, Phys. Chem. Chem. Phys., 2016, 18, 11168 DOI: 10.1039/C6CP00405A

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Physical Chemistry Chemical Physics

Nucleophilic substitution by amide nitrogen in the aromatic rings of [z_n − H]˙⁺ ions; the structures of the [b₂ − H − 17]˙⁺ and [c₁ − 17]⁺ ions†

Abstract

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Nucleophilic substitution by amide nitrogen in the aromatic rings of [z_n − H]˙⁺ ions; the structures of the [b₂ − H − 17]˙⁺ and [c₁ − 17]⁺ ions

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Physical Chemistry Chemical Physics