Investigation of interaction of cationic surfactant with HSA in the presence of alcohols using PFG-NMR and potentiometric technique

The interaction between human serum albumin (HSA) and cationic surfactant in mixed solvent (water/ethanol or water/1-propanol) was investigated using potentiometry and pulsed field gradient-NMR spectroscopy. The concentration at which hexadecyltrimethylammoniumbromide (CTAB) commenced binding to HSA was lowered by the presence of alcohol. However, no interaction between CTAB and HSA was detected in solvents containing more than 30% (v/v) ethanol or 20% 1-propanol. The same behavior was observed for HSA/DOTAB (dodecyltrimethylammoniumbromide) systems in water/ethanol mixed solvents. The total diffusion coefficient of cationic surfactants and alcohols were determined in the presence of HSA. The effect of alcohols on the binding of surfactant to protein was analyzed by a thermodynamic treatment of the mixture based on the regular mixing model. The values of C1 (the surfactant concentration at which binding is initiated) and C2 (protein saturated by surfactant) were determined. The degrees of dissociation (α) of the cationic surfactants were determined in the region between C1 and C2. The changes of the free energy of binding were obtained in the presence of alcohols using Wayman’s method.