Adsorption of a cationic amphiphilic drug on human serum albumin: characterization of the complex

The complex formed by the interaction of the amphiphilic drug verapamil hydrochloride and human serum albumin (HSA) in water at 25°C was investigated using a range of physico-chemical techniques. The colloidal dispersion was considered as a binary system in which water and verapamil molecules are regarded as the solvent for the HSA–verapamil complex. Measurements of the solution conductivity and the electrophoretic mobility of the complexes showed an ionic adsorption of the drug on the protein surface leading to surface saturation at a verapamil concentration between 10 and 15 mmol kg−1. Measurements of the size of the complex and the thickness of the adsorbed layer by dynamic light scattering showed a gradual change in hydrodynamic radius of the complex with increasing drug concentration typical of a saturation rather than a denaturation process, the magnitude of the change being insufficient to account for any appreciable extension or unfolding of the HSA molecule. The interaction potential between the HSA–verapamil complexes and their stability were determined from the dependence of the diffusion coefficients on protein concentration by application of the DLVO colloidal stability theory. The results indicate decreasing stability of the colloidal dispersion of the drug–protein complexes with increase in the concentration of added drug.