Comparison of fluorescence, ³¹P NMR, and ⁷Li NMR spectroscopic methods for investigating Li⁺/Mg²⁺ competition for biomolecules

The biochemical action of lithium in the treatment of manic-depressive illness is still unknown. One hypothesis is that Li⁺ competes for Mg²⁺- binding sites in biomolecules. We report here our studies on metal ion competition by three distinct methods: fluorescence, ³¹P NMR, and ⁷Li NMR spectroscopy, using ATP as a model ligand. By fluorescence spectroscopy, we used the dye, furaptra, by measuring the increases in Mg²⁺ levels in an ATP solution as Li⁺ levels were increased in the solution. This increase in Mg²⁺ levels was indicated by increases in the fluorescence intensity ratio (335/370) of furaptra. By ³¹P NMR spectroscopy, this competition was demonstrated by changes in the ³¹P NMR spectrum of ATP. The Li⁺/Mg²⁺ competition was indicated by predictable changes in the separation between the α and β resonances of the phosphates of ATP. For ⁷Li NMR spectroscopy, spin-lattice relaxation measurements were used, which provided free Li⁺ concentrations that could be used for determining the free Mg²⁺ values in ATP solutions. The values of the free Mg²⁺ concentrations obtained by all three methods were in good agreement. The fluorescence and ⁷Li NMR methods, however, proved to be more sensitive to low concentrations of Li⁺ than the ³¹P NMR method.