Assessment of magnesium concentrations by 31P NMR in vivo

31P NMR spectra obtained in vivo reveal the presence of a few reasonably well defined chemical species, namely, ATP, orthophosphate (Pi), and, in brain, phosphocreatine. The chemical shifts of these resonances respond to changes in concentrations of ions such as H+ and Mg2+ in a manner that depends on both the chemical shifts intrinsic to individual complexes and the formation or binding constants for the several complexes. Values of the appropriate formation constants are well established in the literature. We have derived estimates of the chemical shifts intrinsic to the individual complexes by analyzing high resolution spectra of solutions whose composition brackets the domain of physiological relevance. This provides information sufficient to estimate intracellular concentrations of H+ and Mg2+ from chemical shifts seen with in vivo spectra. The primary finding is an estimate of 0.3 mM for the concentration of free magnesium in human brain. Differing values are obtained from other tissues.