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

The complexation equilibrium between nucleotides (ATP, ADP) and inorganic c ations (Mg2+, Ca2+) has been studied by capillary electrophoresis. The equi librium constant and the stoichiometry of nucleotide-inorganic cation compl exes can be deduced from the dependence of the electrophoretic mobility of each nucleotide on the negative logarithm of the inorganic cation concentra tion. The experimental values of complexation constants determined by CE co mpare favorably with those in the literature. As expected, Mg2+ forms more stable complexes with ATP (log K = 2.30 and 4.10 at pH 5 and 8, respectivel y) than with ADP (log K = 1.92 and 3.15 at pH 5 and 8, respectively). In th e pH range 4-8, the stoichiometry of ADP-Mg2+ and ADP-Ca2+ complexes is alw ays 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.