THERMODYNAMICS OF PROTONATION OF AMP, ADP, AND ATP FROM 50-DEGREES-C TO 125-DEGREES-C

The interaction of adenosine 5'-monophosphate (AMP), adenosine 5'-diph osphate (ADP), and adenosine 5'-triphosphate (ATP) ions with protons i n aqueous solution has been studied calorimetrically from 50 to 125-de grees-C and 1.52 MPa. At each temperature, the reaction of acidic AMP with tetramethylammonium hydroxide was combined with the heat of ioniz ation for water to obtain the enthalpy of protonation of AMP, while th e reactions of HCl with deprotonated tetramethylammonium salts of ADP and ATP were used to obtain the enthalpies of protonation of ADP and A TP. Equilibrium constant K, enthalpy change DELTAH-degrees, entropy ch ange DELTAS-degrees, and heat capacity change DELTAC(p)-degrees values were calculated for the stepwise protonation reactions as a function of temperature. The reactions involving the first protonation of AMP, ADP, and ATP and the third protonation of ADP and ATP were endothermic over the temperature range studied, while that involving the second p rotonation is exothermic for AMP and ADP, but is exothermic below 100- degrees-C and endothermic at 125-degrees-C in the case of ATP. Consequ ently, log K values for the first and third protonation reactions (pho sphate groups) increase while those for the second protonation reactio n (N1-adenine) decrease in the cases of AMP and ADP and go through a m inimum in the case of ATP as temperature increases. The DELTAH-degrees values for all protonation reactions increase with temperature. The m agnitude and the trend for the DELTAH-degree, DELTAS-degree, and DELTA C(p)-degree values with temperature are discussed in terms of solvent- solute interactions. The magnitude of the DELTAC(p)-degree values for the second protonation is consistent with little interaction between t he phosphate ion and the protonated N1 site of the adenine moiety in A MP, but indicates moderate interaction between these groups in ADP, an d strong interaction in ATP.