title: Competition between Li+ and Mg2+ 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 Mg2+ to the phosphate moieties of ATP and ADP. To examine the binding of Li+ and Mg2+ to the base and ribose moieties, we used 1H and 13C NMR chemical shifts. The 7Li NMR relaxation times of Li+/Mg2+ mixtures of ATP or ADP increased with increasing concentrations of Mg2+, suggesting competition between the two ions for adenine nucleotides. No significant binding of Li+ and Mg2+ 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 Mg2+ and found them to be somewhat greater in the presence of Mg2+. Although competition between Li+ and Mg2+ for ATP and ADP phosphate binding sites in solution is consistent with the 31P chemical shift data, the possibility that the Li+ and Mg2+ form mixed complexes with the phosphate groups of ATP or ADP cannot be ruled out.
authors.affiliation: Department of Chemistry, Loyola University of Chicago, Illinois 60626.
authors.forename: A
authors.initials: A
authors.lastname: Abraha
authors.forename: D E
authors.initials: DE
authors.lastname: de Freitas
authors.forename: M
authors.initials: M
authors.lastname: Margarida
authors.forename: C A
authors.initials: CA
authors.lastname: Castro
authors.forename: C F
authors.initials: CF
authors.lastname: Geraldes
mesh_terms: D000244:Adenosine Diphosphate / Q000737:chemistry*; D000255:Adenosine Triphosphate / Q000737:chemistry*; D001667:Binding, Competitive; D007700:Kinetics; D008094:Lithium / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D012996:Solutions; D014867:Water
chemical_list: D012996:Solutions; D014867:Water; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D008094:Lithium; D008274:Magnesium