Gas-Phase Unimolecular Dissociation Reveals Dominant Base Property of Protonated Homocysteine Sulfinyl Radical Ions.

Homocysteine sulfinyl radical ((SO⋅) Hcy) is a reactive intermediate involved during oxidative damage of DNA in the presence of high concentrations of homocysteine (Hcy). The short lifetime of (SO⋅) Hcy makes its preparation, isolation, and characterization challenging using traditional chemical measurement tools. Herein, we demonstrate the first study on mass-selected protonated (SO⋅) Hcy ions in the gas phase by investigating its unimolecular dissociation pathways from low energy collision-induced dissociation (CID). Tandem mass spectrometry (MS/MS), stable-isotope labeling, and theoretical calculations were employed to rationalize the observed fragmentation pathways. The dominant dissociation channel of protonated (SO⋅) Hcy was a charge-directed H2 O loss from the protonated sulfinyl radical (-SO⋅) moiety, forming a thiyl radical (-S⋅), which further triggered sequential radical-directed ⋅SH loss through multiple pathways. Compared to cysteine sulfinyl radical ((SO⋅) Cys), the small structural change induced by one additional methylene group in the side chain of (SO⋅) Hcy significantly promotes its base property while reducing the radical reactivity of sulfinyl radical. This observation provides new insight into studying reactions of (SO⋅) Hcy with biomolecules, which are critical in understanding toxicity induced by high levels of Hcy in biological conditions.