title: Formation, isomerization, and dissociation of ε- and α-carbon-centered tyrosylglycylglycine radical cations.
abstract: The fragmentation products of the ε-carbon-centered radical cations [Y(ε)˙LG](+) and [Y(ε)˙GL](+), made by 266 nm laser photolysis of protonated 3-iodotyrosine-containing peptides, are substantially different from those of their π-centered isomers [Y(π)˙LG](+) and [Y(π)˙GL](+), made by dissociative electron transfer from ternary metal-ligand-peptide complexes. For leucine-containing peptides the major pathway for the ε-carbon-centered radical cations is loss of the side chain of the leucine residue forming [YG(α)˙G](+) and [YGG(α)˙](+), whereas for the π-radicals it is the side chain of the tyrosine residue that is lost, giving [G(α)˙LG](+) and [G(α)˙GL](+). The fragmentations of the product ions [YG(α)˙G](+) and [YGG(α)˙](+) are compared with those of the isomeric [Y(ε)˙GG](+) and [Y(π)˙GG](+) ions. The collision-induced spectra of ions [Y(ε)˙GG](+) and [YGG(α)˙](+) are identical, showing that interconversion occurs prior to dissociation. For ions [Y(ε)˙GG](+), [Y(π)˙GG](+) and [YG(α)˙G](+) the dissociation products are all distinctly different, indicating that dissociation occurs more readily than isomerization. Density functional theory calculations at B3LYP/6-31++G(d,p) gave the relative enthalpies (in kcal mol(-1) at 0 K) of the five isomers to be [Y(ε)˙GG](+) 0, [Y(π)˙GG](+) -23.7, [YGG(α)˙](+) -28.7, [YG(α)˙G](+) -31.0 and [Y(α)˙GG](+) -38.5. Migration of an α-C-H atom from the terminal glycine residue to the ε-carbon-centered radical in the tyrosine residue, a 1-11 hydrogen atom shift, has a low barrier, 15.5 kcal mol(-1) above [Y(ε)˙GG](+). By comparison, isomerization of [Y(ε)˙GG](+) to [YG(α)˙G](+) by a 1-8 hydrogen atom migration from the α-C-H atom of the central glycine residue has a much higher barrier (50.6 kcal mol(-1)); similarly conversion of [Y(ε)˙GG](+) into [Y(π)˙GG](+) has a higher energy (24.4 kcal mol(-1)).
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.forename: Xiaoyan
authors.initials: X
authors.lastname: Mu
authors.forename: Qiang
authors.initials: Q
authors.lastname: Hao
authors.forename: Alan C
authors.initials: AC
authors.lastname: Hopkinson
authors.forename: Ivan K
authors.initials: IK
authors.lastname: Chu
mesh_terms: D005609:Free Radicals / Q000737:chemistry*; D007536:Isomerism; D056004:Molecular Dynamics Simulation; D015394:Molecular Structure; D009842:Oligopeptides / Q000737:chemistry*; D010446:Peptide Fragments / Q000737:chemistry*; D010782:Photolysis; D021241:Spectrometry, Mass, Electrospray Ionization; D013816:Thermodynamics
chemical_list: D005609:Free Radicals; D009842:Oligopeptides; D010446:Peptide Fragments; C044757:tyrosyl-glycyl-glycine