Gas-phase reactivity of sulfur-based radical ions of cysteine derivatives and small peptides

The gas-phase reactivity of sulfur-based radical ions of cysteine derivatives and Cys-containing small peptides was studied via the use of ion-molecule reactions inside a quadrupole ion trap mass spectrometer. Both the radical cations M+ and the anions (M−2H)− were generated via the gas-phase homolysis of the SNO bond of the S-nitrosylated precursors of the following: Cys, N-Ac-Cys, CysOMe, Gly-Cys, Cys-Gly, γ-Glu-Cys, γ-Glu-Cys-Gly, and Gly-Cys-Arg. The radical ions were allowed to react with the following volatile neutral reagents: allyl iodide, allyl bromide, nitric oxide, 1-propane thiol, 3-mercaptopropionic acid, dimethyl disulfide and dimethyl sulfide. The charge has little effect on the types of products formed, with typical S-based radical reactivity being observed. In several systems (N-Ac-Cys, Cys-Gly, Gly-Cys, γ-Glu-Cys-Gly, and Gly-Cys-Arg) the radical was found to lose its reactivity very rapidly, which is consistent with S-to-α-carbon radical rearrangement via hydrogen atom transfer. The observed radical reactivity serves as a model for studying the chemistry of cysteine-based thiyl radicals in protein systems.

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► Cysteine-based thiyl radical ions display typical sulfur radical reactivity in the gas phase.
► Reactivity is radical-based and is not affected by charge of the radical ion (positive or negative).
► Kinetic plots of ion-molecule reactions can be used to detect radical rearrangement.