Binding of the triton X series of nonionic surfactants to bovine serum albumin

The binding of a series of Triton X nonionic surfactants (NIS) tobivine serum albumin (BSA) has been studied by equilibrium dialysis and titration calorimetry. At pH 7.0, Triton X molecules bind to two classes of sites, the first 2 molecules binding with positive cooperativity to high-affinity sites following by the binding of approximately 15 additional molecules to lower affinity, thermodynamically identical, and independent sites. The strength of the binding decreases as the number of oxyethylene units is increased in the surfactants Triton X-114, X-100, X-102, and X-165. Calorimetric measurements show the enthalpy change for the NIS-BSA interaction to be small and endothermic. Increasing the hydrophilic oxyethylene chain length results in a more endothermic enthalpy change and a smaller association constant. Electron spin resonance studies of Triton X binding to BSA, covalently spin-labeled with N-(2,2,6,6-tetramethyl-piperidinyl-1-oxy)maleimide, indicated that the protein conformation in the vicinity of the labeled sulfhydryl was insensitive to NIS binding from dilute monomeric solutions. Calorimetric experiments near the critical micelle concentration indicate, however, that the protein probably undergoes a conformational change associated with the population of the lower affinity NIS binding sites.