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

Abstract

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.