Competition between Li + and Mg 2+ for ATP and ADP in aqueous solution: A multinuclear NMR study

Abstract

We used 7Li NMR spin-lattice relaxation times and 31P NMR chemical shifts to study the binding of Li + and Mg 2+ to the phosphate moieties of ATP and ADP. To examine the binding of Li + and Mg 2+ to the base and ribose moieties, we used 1H and 13C NMR chemical shifts. The 7Li NMR relaxation times of Li +/Mg 3+ mixtures of ATP or ADP increased with increasing concentrations of Mg 2+, suggesting competition between the two ions for adenine nucleotides. No significant binding of Li + and Mg 2+ to the base and ribose moieties occurred. At the pH and ionic strength used, 2:1 and 1:1 species of the Li +-ATP and Li +-ADP complexes were present, with the 2:1 species predominating. In contrast, 1:1 species predominated for the Mg 2+-ADP and Mg 2+-ATP complexes. We calculated the Li +-nucleotide binding constants in the presence and absence of Mg 2+ and found them to be somewhat greater in the presence of Mg 2+. Although competition between Li + and Mg 2+ for ATP and ADP phosphate binding sites in solution is consistent with the 31P chemical shift data, the possibility that the Li + and Mg 2+ form mixed complexes with the phosphate groups of ATP or ADP cannot be ruled out.