Physicochemical study of the complexation of nortriptyline and human serum albumin

The complexes formed by the interaction of the cationic amphiphilic drug nortriptyline hydrochloride and human serum albumin (HSA) were investigated at 25 °C in aqueous solution at pH 3.2, where interaction is by hydrophobic binding, and at the isoelectric point of the protein (pH 4.9), where binding is both hydrophobic and electrostatic. A variety of techniques have been used to determine changes in the physicochemical properties of the drug/protein systems as a function of drug concentration, including conductivity, ζ-potential, static and dynamic light scattering methods, and UV-vis spectroscopy. Measurements of the solution conductivity of the complexes have shown increasing adsorption of the amphiphilic drug onto the protein surface with an increase of drug concentration, leading to surface saturation and the formation of micelles in solution at a drug concentration of approximately 0.035 mol kg-1. The number of adsorption sites was determined from the observed increases of the ζ-potential as a function of drug concentration. The Gibbs energies of adsorption of the drug onto the protein showed an exponential increase with an increase of drug concentration as a consequence of the hydrophobic interaction. Measurements of the molar mass and hydrodynamic radius of the HSA/nortriptyline complexes as a function of drug concentration by static and dynamic light scattering techniques have shown a gradual increase of size typical of a saturation rather than a denaturation process. Plots of hydrodynamic radius against nortriptyline concentration and changes in the UV-vis spectra have been interpreted by comparison with typical binding isotherms.