LACK OF EFFECT OF THE LENGTH OF OLIGOGLYCINE-DERIVED AND OLIGO(ETHYLENE-GLYCOL)-DERIVED PARA-SUBSTITUENTS ON THE AFFINITY OF BENZENESULFONAMIDES FOR CARBONIC-ANHYDRASE-II IN SOLUTION

Using H-1 NMR spectroscopy, values of T-2 have been determined for the methylene protons of the oligoglycine moieties of para-substituted be nzenesulfonamides having structures H2NO2SC6H4CO(Gly)(n)OH (n = 1-6) b ound at the active site of bovine carbonic anhydrase II (CA, EC 4.2.1. 1). These values have been correlated with measurements of dissociatio n constants of these complexes, in order to infer motion of these liga nds when bound to the enzyme. Motion of glycines 1-3 (those closest to the aryl ring) is hindered by their proximity to the protein; motion of glycines 4-6 is relatively unhindered. Despite the restriction to m otion inferred for glycines 1-3, the values of K-d for the six compoun ds (n = 1-6, 1-6) are indistinguishable within experimental uncertaint y (+/-20%): K-d in mu M (n) 0.30 (1); 0.26 (2); 0.33 (3); 0.37 (4); 0. 37 (5); 0.34 (6). There is, therefore, an unexpected compensation of t he loss in conformational entropy on binding by another contributor to the free energy.