The site of binding was investigated by displacement studies. For this, we used the CAII inhibitor ethoxzolamide (4) (Ki = 8 nM), which is known to bind within the Zn 2+ pocket. The resynthesised and purified SFs (100 µM) were co-incubated with either ethoxzolamide (50 µM) or DMSO as a control, with CAII (0.5 µM). Inspection of the resultant mass spectra after a 24 h incubation revealed that the presence of ethoxzolamide had abolished covalent modification for all hits 2b-e, indicating binding to the same site (Fig. ).