Thermodynamic parameters for the interaction of adenosine 5'-diphosphate, and adenosine 5'-triphosphate with Mg2+ from 323.15 to 398.15 K

The interaction of adenosine 5'-diphosphate (ADP) and adenosine 5'-triphosphate (ATP) with Mg2+ in water has been studied calorimetrically at 323.15, 348.15, 373.15 and 398.15 K for ATP and at 348.15 and 373.15 K for ADP. The enthalpies of reaction of Mg2+ with ADP and ATP were obtained from the heats of mixing of aqueous solutions of tetramethylammonium salts of ADP and ATP with MgCl2 solutions in an isothermal flow calorimeter Equilibrium constant (K), enthalpy change (Delta H degrees), entropy change (Delta S degrees), and heat capacity change (Delta Cp degrees) values were calculated for the interaction: Mg2+ + L(n-) = MgL(2-n) and Mg2+ + MgL(2-n) = Mg(2)L(4-n), where n = 4 for L = ATP and n = 3 for L = ADP. The results are consistent with those at lower temperatures. For the two nucleotides studied, the above two reactions are endothermic and entropy-driven in the temperature range studied. Large Delta Cp degrees values for the interaction of Mg2+ with ADP with ATP indicate the involvement of phosphate groups of nucleotides in the coordination of Mg2+. The coordination of the first and second Mg2+ ions involves the phosphate chain in both ADP and ATP. No evidence was found for the involvement of the adenine ring or the ribose moiety in the coordination of Mg2+ with these nucleotides. Approximate values of log K, Delta H degrees, and Delta S degrees and Delta Cp degrees for the self-association of ADP and ATP in the presence of Mg2+ are also given.