Studies on the Binding of Bis-Quaternary Ammonium Surfactants to Bovine Serum Albumins by Microcalorimetry and Circular Dichroism

The interactions of bovine serum albumin (BSA) with two cationic gemini surfactants, (CnN)2Cl2 (n=12, 14), in buffer solutions (pH=7.0) were investigated by isothermal titration calorimetry (ITC) and circular dichroism (CD). CD spectra showed that the two surfactants change the secondary structure of BSA. The thermodynamic results suggest that there exist two binding types (high affinity/low affinity) in the interacting process of (CnN)2Cl2 micelles with BSA. The high affinity binding is an endothermic process driven by entropy, in which the synergistic effect among weak interactions plays an important role. The low affinity binding is an exothermic process accompanied by positive entropy effect, in which hydrophobic interaction is dominant in all driving forces. Furthermore, corresponding binding site number of BSA for (C14N)2Cl2 is much smaller than that for (C12N)2Cl2, indicating that the hydrophobic chain length of surfactant plays a key role in low affinity binding process.