Electrochemical study of thermodynamics of interaction of lysozyme with sodium dodecyl sulfate in binary ethanol–water mixtures

Dodecyl sulfate doped polypyrrole (PPy-DS) was prepared electrochemically by anodic polymerization of pyrrole in the presense of dodecyl sulfate (DS−) ions in aqueous solution and used as a DS− ion selective electrode. This surfactant selective electrode was used to study the effect of DS− ion on the conformation changes of lysozyme in binary ethanol–water mixtures. The binding of DS− ion to lysozyme have been studied by electrochemical measurements at 293 and 310 K. DS− ion interact cooperatively with lysozyme in different composition of ethanol. The binding isotherms are discussed in terms of the binding potential concept of Wyman and Gibbs free energy, enthalpy and entropy of binding were calculated. The thermodynamic parameters (ΔHν̄ and ΔSν̄) were evaluated from the temperature dependence of equilibrium binding constants. The binding process in 0, 5 and 10% ethanol are entropy controlled and with increasing the percent of ethanol (15%), both enthalpy and entropy are favored. The values of ΔSν̄ being more positive with decease in the content of the mixture. The resulting T ΔSν̄ against ΔHν̄ plot showed a fairly good linear correlation, indicating the existence of an enthalpy–entropy compensation in the binding process.