The Interactions of a Homologous Series of Cationic Surfactants with Bovine Serum Albumin (BSA) Studied Using Surfactant Membrane Selective Electrodes

The binding of a homologous series of n-alkyltrimethylammonium bromides to bovine serum albumin were measured in 2.7 mM phosphate buffer, pH 7.0, and at various temperatures using surfactant membrane selective electrodes as a fast and accurate method. The obtained binding isotherms have been analyzed and interpreted using the binding capacity concept, Hill equation and Wyman binding potential. The system behaved as a system with two sets of binding sites in all studied situations. The results represent the essential role of hydrophobic interactions in the first binding set. At high cationic surfactant concentrations, the occupation of the second binding set occurred, with an accompanying unfolding and exposure of numerous hydrophobic binding sites. However, this unfolding process occurred for the cationic surfactant at much higher concentrations than was required for sodium dodecyl sulfate. The Gibbs free energy change calculated on the basis of the Wyman binding potential concept decreases in the initial stages of binding and passes through a minimum, followed by occupation of the second binding set. The affinity of binding increased with increasing temperature, indicating an endothermic and essentially entropy driven process.