31P NMR and isothermal titration calorimetry studies on polyoxomolybdates-catalyzed hydrolysis of ATP

ATP hydrolysis in the presence of polyoxomolybdates at pH levels of 6, 4, and 2 has been investigated with a help of high pressure liquid chromatography (HPLC) analyses, 31P- and 1H NMR measurements, and isothermal titration calorimetry (ITC). The polyoxomolybdates-induced ATP-hydrolysis proceeded satisfactorily in pH < 6 media at 20 °C with an optimum pH level of 4, while it was significantly depressed at low temperature of ⩽5 °C. At pH levels of 6 and 4, ADP was a main product, and the involvement of [(PO4)2Mo5O15]6−-like ATP–molybdate complex as an intermediate was implied. At pH 2 ATP was decomposed to AMP with small generation of ADP through the formation of the ATP–molybdate complex isostructural with [(O3POPO3)Mo6O18(H2O)4]4− as an intermediate. The ITC result at pH 4 showed an occurrence of two types of the exothermic binding reactions between molybdate and ATP with binding constants (K) of 6.61 × 104 and 9.40 × 103 M−1 and molar enthalpy values (ΔH) of −6.32 × 104 and −4.73 × 103 J mol−1, respectively. Together with the results of 1H NMR measurements, it is deduced that the molybdates interact with not only phosphate sites in the ATP side-chain, but also adenine-ring with an accompanying aggregation of molybdates at pH 4.