The interaction of europium(III) ion with nucleotides

Apparent composite stability constants have been determined at pH 7.0 and 8.0 for the interaction of Eu(III) with ADP and ATP in 0.1 M N-ethylmorpholine buffer at 20°C. The values were obtained using a competitive spectrophotometric technique with 8-hydroxyquinoline as the competing ligand and experiments were performed in the presence of relatively low concentration of europium so as to avoid precipitation of hydrolysed species of the metal ion. The data have been used, together with an assumed hydrolysis constant for europium of 10^?8 M, to calculate that the stability constant for the Eu-ADP⁰ complex is about 10⁶ M^?1. The results were not sufficiently accurate to determine the stability constant of the Eu(OH)-ADP^? complex.Values are also reported for the stability constants and molar extinction coefficients of the complexes formed by the reaction of europium and magnesium with 8-hydroxyquinoline.