Competition of Li(I) and Mg(II) for ATP binding: a 31P NMR study

We have examined the changes in the 31P NMR chemical shifts of ATP in aqueous solution, upon addition of NaCl and LiCl at ambient temperature. NaCl and LiCl cause qualitatively similar downfield changes, although the effect of LiCl is somewhat larger. For a 2:1 mixture of Na-ATP or tris-ATP and MgCl2 at 0 degrees C, separate beta-P peaks are observed for uncomplexed ATP and ATP complexed with Mg(II) at 121.6 MHz. Addition of LiCl slightly shifts the uncomplexed Na-ATP or tris-ATP peak downfield but does not measurably shift the Mg-ATP peak or change relative intensities. Thus, Li behaves more like Na than Mg in its ATP complexation behavior, making this system a poor model for competition between Li and Mg for macromolecular binding.