A nuclear‐magnetic‐resonance study of the binding of novel N ‐hydroxybenzenesulphonamide carbonic anhydrase inhibitors to native and cadmium‐111‐substituted carbonic anhydrase

Various ring‐ and nitrogen‐substituted benzenesulphonamides have been prepared and tested as potential inhibitors of carbonic anhydrase. N ‐Methoxysulphonamides showed no inhibitory activity, as predicted by the classic work of Krebs on N ‐substituted inhibitors. By contrast, N ‐hydroxysulphonamides proved to be very effective inhibitors of carbonic anhydrase. Using 111 Cd‐NMR it has been possible to analyse the molecular interaction of 4‐fluoro‐ N ‐hydroxybenzene‐sulphon[ 15 N]amide, with 111 Cd‐substituted bovine carbonic anhydrase. A large cadmium‐111: nitrogen‐15 spin‐coupling shows that this inhibitor is directly bound to the metal via its nitrogen rather than through an oxygen atom. The mode of this binding is similar to that for the unsubstituted sulphonamide inhibitor, 4‐fluorbenzenesulphon[ 15 N]amide. The 111 Cd‐chemical shift of the signal for the inhibited enzyme shows that the N ‐hydroxysulphonamide is bound as its anion. Form the relative intensities of free and complexed enzyme signals it can be deduced that the cadmium enzyme complex with the N ‐hydroxysulphonamide has a longer life‐time than that formed with the unsubstituted sulphonamide. By contrast, native zinc‐containing bovine carbonic anhydrase shows similar I 50 values with both of these sulphonamides. Attempts to monitor the binding using 15 N‐NMR were unsuccessful, possible due to a very long relaxation time for the nitrogen nucleus in the N ‐hydroxysulphonamide when found to the enzyme leading to loss of the 15 N signal.