Capillary electrophoretic study of the complexation of nucleotides with magnesium and calcium ions

The complexation equilibrium between nucleotides (ATP, ADP) and inorganic cations (Mg²⁺, Ca²⁺) has been studied by capillary electrophoresis. The equilibrium constant and the stoichiometry of nucleotide-inorganic cation complexes can be deduced from the dependence of the electrophoretic mobility of each nucleotide on the negative logarithm of the inorganic cation concentration. The experimental values of complexation constants determined by CE compare favorably with those in the literature. As expected, Mg²⁺ forms more stable complexes with ATP (logK=2.30 and 4.10 at pH 5 and 8, respectively) than with ADP (logK=1.92 and 3.15 at pH 5 and 8, respectively). In the pH range 4–8, the stoichiometry of ADP-Mg²⁺ and ADP-Ca²⁺ complexes is always 1∶1 whereas that of the complexes between these cations and ATP depends on pH-hence ATP-Mg²⁺ and ATP-Ca²⁺ complexes have 1∶1 stoichiometry at pH 5 and 1∶2 at pH 8.