Intra-molecular reactions between cysteine sulfinyl radical and a disulfide bond within peptide ions

• Intra-molecular reactions assisted by collisional activation has proven to be an effective means to probe the reactivity of cysteine sulfinyl radical.
• A new reaction channel, i.e., sulfinyl radical exchange with disulfide bond, was discovered for the first time, which was absent under traditional ion/molecule reaction conditions.
• The reactivity of sulfinyl radical toward disulfide bond resulted in disulfide bond opening and scrambling within peptide systems.

Cysteine sulfinyl radical (SO
• Cys) is a reactive intermediate discovered in the inactivation of enzymes utilizing the glycyl/thiyl radical in their catalytic functions upon exposure to air. SO
• Cys has been recently formed and investigated in the gas phase via mass spectrometry (MS), with the aim being to acquire direct experimental evidence of the radical’s intrinsic chemical reactivity. Ion/molecule reaction studies showed that SO
• Cys was relatively chemically inert toward thiol (SH) and disulfide (SS) functional groups under the explored experimental conditions. Herein, we utilized intra-molecular reactions aided by collision-induced dissociation (CID) to overcome the limitations associated with the traditional bimolecular reactions and explore the reactivity of SO
• Cys. Our results revealed a new reaction pathway in which the sulfinyl radical exchanged with an intrachain or interchain disulfide bond within a peptide ion, leading to the formation of a new disulfide bond and a sulfinyl radical. As a consequence, CID of peptide disulfide regio-isomers consisting of SO
• Cys led to enhanced sequence information, however the disulfide bond linkage patterns could not be accurately assigned. This reaction pathway also has implications on disulfide bond scrambling in proteins initiated by a radical intermediate.

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