Synthesis and Characterization of 1,4,7-Triazacyclononane Derivatives with Methylphosphinate and Acetate Side Chains for Monitoring Free MgII by 31P and 1H NMR Spectroscopy

A series of 1,4,7-triazacyclononane-based ligands containing one, two, or three methylphosphinate (MP) side chains has been prepared. The macrocycle with three methylphosphinates, NOTMP, appeared to be a very promising ligand for monitoring MgII in biological samples by 31P NMR spectroscopy. The 31P resonances of the free ligand and the MgII complex were well resolved, in slow exchange, and well downfield of typical tissue phosphate and phosphate ester metabolite resonances. The Kd value of the Mg(NOTMP) complex was 0.35 mM at physiological pH and temperature, a value which is optimal for accurate assessment of free MgII in cells and blood plasma. The latter was confirmed experimentally. Furthermore, the selectivity of NOTMP for MgII over CaII exceeded 2 orders of magnitude. The Kd values of the MgII complexes were sensitive to pH and temperature. The pH effects could be easily predicted from potentiometric pH titration data. Examination of the MgII ligand equilibria by 31P NMR over a wide temperature range provided complexation enthalpies and entropies. It was shown that the smaller Kd values at higher temperatures were largely due to a temperature effect on the highest ligand protonation constant. Only a small endothermic complexation enthalpy contributed to this phenomenon.