Interaction between penicillins and human serum albumin: A thermodynamic study of micellar-like clusters on a protein

The thermodynamic parameters for the interaction of a range of penicillins, nafcillin, cloxacillin, dicloxacillin, and flucloxacillin, with human seru m albumin (HSA) in aqueous solution, pH 7.4, 25 degrees C, have been determ ined using a combination of equilibrium dialysis and microcalorimetric tech niques. The drugs bind largely nonspecifically to HSA to various extents ra nging fr om similar to 1200 (dicloxacillin) to similar to 3000 (nafcillin) drug molecules per HSA molecule as the free drug concentration approaches t he critical concentration (cc) for aggregation of the free drug. Maxima in the binding isotherms are found for nafcillin and cloxacillin, which possib ly relate to maxima in monomeric drug activity in the vicinity of the criti cal concentrations (ccs). In the case of the dicloxacillin-HSA system, the binding isotherm reflects the two ccs observed for dicloxacillin correspond ing to aggregate formation and the sphere-to-rod aggregate transition. The enthalpies of binding are small and exothermic so that the Gibbs energies o f binding are dominated by large increases in entropy consistent with hydro phobic interactions. The magnitudes of the thermodynamic parameters for the interactions are similar to these for the interactions of anionic surfacta nts with globular proteins. The results are consistent with the clustering of drug molecules to the protein to form micellar-like structures.