Thermodynamics of binding of cadmium to bovine serum albumin.

The binding isotherm of Cd2+ ion to bovine serum albumin (BSA) has been investigated by microcalorimetry at 310.15 K and pH 7.0. The thermodynamic parameters of the binding reaction have been determined, and the stoichiometry of the complex is 2:1, indicating that there exist two identical binding sites of BSA with Cd2+ ion. The value of deltarHthetam is -28.4+/-1.7 kJ mol(-1), the free energy of binding deltarGthetam is -25.2 kJ mol(-1), and the entropy of binding deltarSthetam is -10.3 J mol(-1) K(-1). The negative deltarHthetam and deltarSthetam values are observed for the binding reaction of Cd2+ ion and BSA, suggesting that the binding reaction is mainly enthalpy-driven and the entropy is unfavorable for it.