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Free cytosolic [Mg2+] can be assessed in vivo by 31P MRS from the chemical shift of β‐ATP which in turn depends on the fraction of total ATP complexed to Mg2+ ions. The reliability of these in vivo measurements depends on the availability of an appropriate in vitro calibration to determine the limits of chemical shifts of unbound ATP and Mg‐ATP complexes, using solutions that mimic the in vivo cytosolic conditions as far as possible. We used an algorithm and software to allow a quantitative definition of the Mg2+‐binding molecules to build a semi‐empirical equation that correlates the chemical shift of the β‐ATP signal to the [Mg2+] taking into account the amount of Mg2+ bound to all other constituents in solution. Our experiments resulted in a simple and reliable equation directly usable to assess in vivo the free cytosolic magnesium concentration of human brain by 31P MRS. Our method is also flexible enough to make it suitable for in vivo measurements of [Mg2+] in other organs and tissues.

Equilibrium data on the interaction of DTMA [(DTMA = DOTA tetrakis(methylammide)] with Gd3+ in aqueous solution, properties of the complexes formed in the pH range 0.6–11.8, water proton relaxation rate enhancement, and the crystal structure analysis of the [Gd(DTMA)H2O]3+ complex are reported. In the crystal structure the metal ion is bound to the nitrogen atoms of the tetraazamacrocyclic moiety, to the amidic oxygen atoms, and to an oxygen atom of a water molecule. The nine donors are located at the vertices of a distorted square antiprism, which is capped by the coordinated water oxygen atom in the axial position. In solution [Gd(DTMA)]3+ is not very stable [logKML = 12.8(1)] and gives rise to the formation of [Gd(DTMA)OH]2+ [pKa = 7.9(1)] and [Gd(HDTMA)]4+ [logK(ML+H) = 3.4(1)]. The proton solvent relaxivity of aqueous complex solutions assumes a constant value in the pH range 3–8, increasing at higher and lower pH. For pH > 3 the data are in good agreement with a previous study on the same compound. For pH < 3 a new interpretation is presented, based on the formation of [Gd(HDTMA)]4+ and the release of Gd3+.