Adenosine 5'-triphosphate, ATP, is used as the molecular currency of energy in living systems, where divalent ions are bond to ATP strongly and ATP interacts with the protein through the cation in many cases. Therefore, the thermodynamic evaluation of the interaction between ATP and divalent cations is important to understand the biological function of ATP molecules. In this study, the interaction of ATP and Mg ion at pH 8.5 with 300 mM KCl was evaluated by isothermal titration calorimetry. The temperature and ATP concentration dependence of the binding heat were analyzed by the global fitting with a new multi-binding model including ATP, MgATP, Mg(2)ATP, and Mg(ATP)(2) state for ATP molecules. The binding constant and the binding enthalpy of ATP to MgATP at 25 degrees C were determined to be 15 M-1 and 10 kJ mol(-1), respectively. (C) 2013 Elsevier B.V. All rights reserved.