The Binding of Sodium Dodecyl Sulfate to Lysozyme in Aqueous Solutions. II. The Effect of Added NaCl

The isotherms for the binding of sodium dodecyl sulfate (SDS) to lysozyme have been investigated as a function of the concentration of added NaCl at pH 5.8 and 25 °C by a potentiometric method utilizing a surfactant-selective electrode. The equilibrium surfactant concentration at which the binding begins to occur increases with increasing NaCl concentration. The equilibrium surfactant concentration giving the same binding number above 10, on the contrary, decreases with increasing NaCl concentration. Thermodynamic analysis shows that there takes place not only the exchange between chloride ion (Cl− ion) and dodecyl sulfate ion (DS− ion) but also the binding of DS− ion to the positively charged site unoccupied by Cl− ion. The addition of NaCl does not change the equilibrium SDS concentration at which the binding number begins to exceed 10. This equilibrium concentration is regarded as a characteristic one at which the affinity between lysozyme and SDS can be evaluated. The value of standard free energy change upon DS− ion binding and the number of Na+ ions bound to a DS− ion which forms a cluster on lysozyme were estimated from a thermodynamic equation based on the phase separation model.