Buffer contribution to formation enthalpy of copper(II)–bicine complex determined by isothermal titration calorimetry method

Abstract

The isothermal titration calorimetry (ITC) technique supported by potentiometric titration data was used to study a buffer contribution to the formation enthalpy of copper(II)–bicine complex obtained directly from the ITC experiments. The calorimetric measurements were carried out at 298.15 K in 100 mM buffer solutions with a pH of 6. In experiments two biologically relevant pH buffers were used, namely Mes and Caco. These buffers do not bind copper(II) ions and thus do not affect the binding constant. However, due to their different ionization enthalpy, they affect the conditional (observed) enthalpy of binding of the metal ion to a ligand owing to the proton transfer during the interactions. To calculate the conditional-independent binding enthalpy ∆H, the equation based on the Hess’s law was used. Furthermore, pH-independent and buffer-independent parameters (K, ∆G and ∆S) of the interactions were calculated as well. The relationship between the ionization enthalpy of the buffer components and the thermodynamic parameters has been discussed.