Structure and reactivity of the cysteine methyl ester radical cation

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.