Study of Al³⁺ interaction with AMP, ADP and ATP in aqueous solution

The interaction of Al³⁺ and nucleotide ligands, namely adenosine-5'-monophosphate, (AMP), adenosine-5'-diphosphate, (ADP), adenosine-5'-triphosphate, (ATP), has been studied in aqueous solution at T = 298.15 K and I = 0.15 mol L^-1 in NaCl (only for Al³⁺-ATP system at I = 0.1 mol L^-1). Formation constants and speciation models for the species formed are discussed on the basis of potentiometric results. The speciation models found for the three systems include ML and ML₂ species in all the cases, and for Al³⁺-ADP and ATP systems, MLH, MLOH and ML₂OH species as well. The formation constant value for ML species shows the trend, AMP < ADP < ATP. ¹H NMR spectroscopy was also employed for the study of Al³⁺-ATP system. The ¹H NMR results are in agreement with the speciation model obtained from analysis of potentiometric titration data, confirming the stabilities of the main species. Enthalpy change values were obtained by titration calorimetry; for the main Al³⁺-ATP species (at T = 298.15 K and I = 0.1 mol L^-1 in NaCl), they resulted always higher than zero, as typical for hard-hard interactions. The dependence of formation constants on ionic strength over the range I = 0.1 to 1 mol L^-1 in NaCl is also reported for Al³⁺-ATP system. The sequestering ability of the nucleotides under study towards Al³⁺ was also evaluated by the empirical parameter pL_0.5.