title: S-to-αC radical migration in the radical cations of Gly-Cys and Cys-Gly.
abstract: The radical cations of Cys-Gly and Gly-Cys were studied using ion-molecule reactions (IMR), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. Homolytic cleavage of the S-NO bond of nitrosylated precursors generated radical cations with the radical site initially located on the sulfur atom. Time-resolved ion-molecule reactions showed that radical site migration via hydrogen atom transfer (HAT) occurred much more quickly in Gly-Cys(•+) than in Cys-Gly(•+). IRMPD and DFT calculations indicated that for Gly-Cys, the radical migrated from the sulfur atom to the α-carbon of glycine, which is lower in energy than the sulfur radical (-53.5 kJ/mol). This migration does not occur for Cys-Gly because the glycine α-carbon is higher in energy than the sulfur radical (10.3 kJ/mol). DFT calculations showed that the highest energy barriers for rearrangement are 68.2 kJ/mol for Gly-Cys and 133.8 kJ/mol for Cys-Gly, which is in agreement with both the IMR and IRMPD data and explains the HAT in Gly-Cys.
authors.affiliation: Department of Chemistry and Biochemistry, and Center for Biochemical and Biophysical Studies, Northern Illinois University, DeKalb, IL, USA.
authors.forename: Sandra
authors.initials: S
authors.lastname: Osburn
authors.forename: Giel
authors.initials: G
authors.lastname: Berden
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: Richard A J
authors.initials: RA
authors.lastname: O'Hair
authors.forename: Victor
authors.initials: V
authors.lastname: Ryzhov
mesh_terms: D002244:Carbon / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D004151:Dipeptides / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D008968:Molecular Conformation; D013055:Spectrophotometry, Infrared; D013455:Sulfur / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D002412:Cations; D004151:Dipeptides; D005609:Free Radicals; C028505:cysteinylglycine; D013455:Sulfur; D002244:Carbon