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 ΔrHmΘ is −28.4±1.7 kJ mol−1, the free energy of binding ΔrGmΘ is −25.2 kJ mol−1, and the entropy of binding ΔrSmΘ is −10.3 J mol−1 K−1. The negative ΔrHmΘ and ΔrSmΘ 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.