Nucleophilic substitution by amide nitrogen in the aromatic rings of [zn - H]Ë⺠ions; the structures of the [b2 - H - 17]Ë⺠and [c1 - 17]⺠ions.
Phys Chem Chem Phys
; 18(16): 11168-75, 2016 Apr 28.
Article
em En
| MEDLINE
| ID: mdl-27048940
ABSTRACT
Peptide radical cations that contain an aromatic amino acid residue cleave to give [zn - H]Ë⺠ions with [b2 - H - 17]Ë⺠and [c1 - 17](+) ions, the dominant products in the dissociation of [zn - H]Ëâº, also present in lower abundance in the CID spectra. Isotopic labeling in the aromatic ring of [YπËGG](+) establishes that in the formation of [b2 - H - 17]Ë⺠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)ËGG](+) 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)ËGG](+) 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]Ë⺠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]Ë⺠ions are formed by loss of glycine from the same intermediates involved in the formation of the [c1 - 17](+) ions.
Texto completo:
1
Coleções:
01-internacional
Base de dados:
MEDLINE
Assunto principal:
Amidas
/
Nitrogênio
Idioma:
En
Ano de publicação:
2016
Tipo de documento:
Article