title: Structure and reactivity of the cysteine methyl ester radical cation.
abstract: The structure and reactivity of the cysteine methyl ester radical cation, CysOMe(.+) , have been examined in the gas phase using a combination of experiment and density functional theory (DFT) calculations. CysOMe(.+) undergoes rapid ion-molecule reactions with dimethyl disulfide, allyl bromide, and allyl iodide, but is unreactive towards allyl chloride. These reactions proceed by radical atom or group transfer and are consistent with CysOMe(.+) possessing structure 1, in which the radical site is located on the sulfur atom and the amino group is protonated. This contrasts with DFT calculations that predict a captodative structure 2, in which the radical site is positioned on the α carbon and the carbonyl group is protonated, and that is more stable than 1 by 13.0 kJ mol(-1) . To resolve this apparent discrepancy the gas-phase IR spectrum of CysOMe(.+) was experimentally determined and compared with the theoretically predicted IR spectra of a range of isomers. An excellent match was obtained for 1. DFT calculations highlight that although 1 is thermodynamically less stable than 2, it is kinetically stable with respect to rearrangement.
authors.affiliation: Department of Chemistry and Biochemistry, Northern Illinois University, Dekalb, USA.
authors.forename: Sandra
authors.initials: S
authors.lastname: Osburn
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Steill
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
authors.forename: Richard A J
authors.initials: RA
authors.lastname: O'Hair
authors.forename: Michael
authors.initials: M
authors.lastname: van Stipdonk
authors.forename: Victor
authors.initials: V
authors.lastname: Ryzhov
mesh_terms: D002412:Cations / Q000737:chemistry*; D003198:Computer Simulation; D003545:Cysteine / Q000031:analogs & derivatives* / Q000737:chemistry; D005740:Gases / Q000737:chemistry; D007536:Isomerism; D007700:Kinetics; D015394:Molecular Structure; D013055:Spectrophotometry, Infrared; D053719:Tandem Mass Spectrometry / Q000379:methods; D013816:Thermodynamics
chemical_list: D002412:Cations; D005740:Gases; D003545:Cysteine; C017507:mecysteine