Thermodynamics of binding cationic detergents to bovine serum albumin

Molar ratio (Γ_a) for binding cetyl trimethyl ammonium bromide (CTAB), myristyl trimethyl ammonium bromide (MTAB), dodecyl trimethyl ammonium bromide (DTAB) and cetyl pyridinium chloride (CPCL) to globular protein bovine serum albumin have been measured by equilibrium dialysis technique under various conditions of pH, ionic strength and temperatures. In all cases binding isotherms are S-shaped and at high concentrations of an amine, Γ_a appears to reach a constant value Γ^m_a, Γ^m_a may vary from 1 to 836 depending upon temperature, nature of amines and other solution parameters. The standard free energy changes (ΔG°) for BSA-amine binding have been calculated using appropriate reference states. The order of ΔG° for various systems is found to be in agreement with the order of the maximum extent of binding Γ^m_a. The effects of chain length of the surfactants on ΔG° and Γ^m_a are found to be irregular. ΔS° and ΔH° for binding reactions are also found to be significant in magnitude. Binding interactions at various pH and ionic strengths do not exhibit regularities in terms of electrostatic effects. All these observations lead to the conclusion that the binding sites of BSA are highly heterogeneous so that binding interactions become irregular in nature. All types of interactions are involved in the binding process so that the mechanism of binding in an individual system is complex in nature. The thermal stability and intrinsic viscosity of bovine serum albumin in the presence of amines have been examined in the light of the binding process.