Thermodynamics of the interaction of human serum albumin (HSA) with bis-quaternary ammonium surfactants, (C_nN)₂Cl₂ (n＝12 and 14), in buffer solution (pH＝7.0) has been investigated by isothermal titration calorimetry at 298.15 K. The results show that there are two types of binding sites on HSA for the two surfactants. One is endothermic high-affinity binding, and the other is exothermic low-affinity binding. For the two surfactants, the first type of binding, high-affinity binding is entropy driven process, and the differences of binding site number, binding constants and thermodynamic parameters are small. While as for the second binding sites, low-affinity binding sites, only portion of hydrophobic chains of (C₁₄N)₂Cl₂ can penetrate into the hydrophobic cavity because its hydrophobic chains are too long, which leads to the reduction of binding site number and evolved heat as well as the increasing of entropy. The low-affinity binding of (C_nN)₂Cl₂ to HSA is driven by a favorable entropy increas-ing with a less favorable enthalpy decrease. Circular dichroism (CD) spectra show that the two surfactants can change the secondary structure of HSA. These results indicate that the interaction of (C_nN)₂Cl₂ with HSA includes contributions of the binding and the partial change of structure of the protein induced by the two surfactants.

Key words: isothermal titration calorimetry, circular dichroism, bis-quaternary ammonium surfactant, human serum albumin