Initially, the more reactive library, containing para-substituted aryl sulfonyl fluoride 2, was screened against the three proteins (24 h incubation, 4 or 20 °C) and directly analysed by intact protein LC-MS (0.5 or 1 µM protein, 20 or 50 µM SF) (Fig. ). Across all screens, the majority of wells contained unmodified protein, indicating that the SF reactive moieties were not yielding non-specific covalent modifications. However, for some of the wells, the resultant mass spectra displayed additional peaks with mass shifts consistent with the covalent modification of the protein by the SF fragment, accompanied by the loss of HF as expected for the reaction between the nucleophilic amino acid residue with the SF group: [protein+SF-HF]. A range of modification yields were observed, and the hit threshold for each screen was defined as the mean percentage modification + 2 standard deviations (SDs). Hits were also prioritised based on the overall extent of modification, with some showing modification greater than 50% (e.g. 2b-e with CAII), while others showed modification less than 50% (e.g. 2f-i with BCL6) (Fig. ). All hits gave a single modification event on the protein, consistent with recognition-driven modification. Hits containing reactive moiety 2 from the screens against CAII and BCL6 were resynthesised and purified for use in further investigations. Disappointingly, no hits were observed for KRAS G12D , consistent with the fact that this is considered to be a poorly tractable target. The library containing the less reactive sulfamoyl fluoride 3 was screened against CAII but afforded no hits, suggesting an insufficient intrinsic reactivity of the electrophile.