Cysteine (Cys) is prone to diverse post-translational modifications in proteins, including oxidation into sulfenic acid (Cys-SOH) by reactive oxygen species generated under oxidative stress. Detection of low concentrated and metastable Cys-SOH within complex biological matrices is challenging due to the dynamic concentration range of proteins in the samples.Herein, visible laser-induced dissociation (LID) implemented in a mass spectrometer was used for streamlining the detection of Cys oxidized proteins thanks to proper derivatization of Cys-SOH with a chromophore tag functionalized with a cyclohexanedione group. Once grafted, peptides undergo a high fragmentation yield under LID, leading concomitantly to informative backbone ions and to a chromophore reporter ion. 79 % of the Cys-containing tryptic peptides deriving from human serum albumin and serotransferrin tracked by Parallel Reaction Monitoring (PRM) were detected as targets subjected to oxidation. These candidates, as well as Cys-containing peptides predicted by in silico trypsin digestion of 5 other human plasma proteins, were then tracked in real plasma samples to pinpoint the endogenous Cys-SOH subpopulation. Most of the targeted peptides were detected in all plasma samples by LID-PRM, with significant differences in their relative amounts. By eliminating the signal of interfering co-eluted compounds, LID-PRM surpasses conventional HCD-PRM in detecting grafted Cys-SOH containing peptides and allows now to foresee clinical applications in large human cohorts.

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