Structure and Reactivity of the Cysteine Methyl Ester Radical Cation

Authors

  • Sandra Osburn,

    1. Department of Chemistry and Biochemistry, Northern Illinois University, Dekalb, Illinois (USA) and Center for Biochemical and Biophysical Studies, Northern Illinois University, Dekalb, Illinois (USA), Fax: (+01) 815-753-4802
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  • Dr. Jeffrey D. Steill,

    1. FOM Institute for Plasma Physics, Nieuwegein (The Netherlands) and University of Amsterdam, Amsterdam (The Netherlands)
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  • Prof. Jos Oomens,

    1. FOM Institute for Plasma Physics, Nieuwegein (The Netherlands) and University of Amsterdam, Amsterdam (The Netherlands)
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  • Prof. Richard A. J. O'Hair,

    Corresponding author
    1. School of Chemistry, The University of Melbourne, Mebourne, Victoria 3010 (Australia) and Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Melbourne, Victoria 3010, (Australia) and ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Melbourne, Victoria 3010 (Australia), Fax: (+61) 393-475-180
    • School of Chemistry, The University of Melbourne, Mebourne, Victoria 3010 (Australia) and Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Melbourne, Victoria 3010, (Australia) and ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Melbourne, Victoria 3010 (Australia), Fax: (+61) 393-475-180
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  • Prof. Michael van Stipdonk,

    1. Department of Chemistry, Wichita State University, Wichita, Kansas (USA)
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  • Prof. Victor Ryzhov

    Corresponding author
    1. Department of Chemistry and Biochemistry, Northern Illinois University, Dekalb, Illinois (USA) and Center for Biochemical and Biophysical Studies, Northern Illinois University, Dekalb, Illinois (USA), Fax: (+01) 815-753-4802
    • Department of Chemistry and Biochemistry, Northern Illinois University, Dekalb, Illinois (USA) and Center for Biochemical and Biophysical Studies, Northern Illinois University, Dekalb, Illinois (USA), Fax: (+01) 815-753-4802
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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.

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