Stability assessment of gadolinium complexes by P-31 and H-1 relaxometry

Longitudinal P-31 relaxation rate enhancements of phosphate groups have been measured at pH 7-7.2 and 310 degrees K on aqueous solutions containing adenosine triphosphate (ATP), phosphocreatine (PCr), inorganic phosphate (Pi) and some lanthanide complexes (Gd-DOTA, Gd-HPDO3A, Gd-DO3A, Gd-DTPA, Gd-DTPA-BMA). The macrocyclic complexes induce linear enhancements of the relaxation rates of all phosphorus nuclei. For Gd-DOTA and Gd-HPDO3A, the mechanism of the interaction with the P-31 nuclei seems to be of the outer sphere type and a better efficiency is noted for the "neutral" Gd-HPDO3A. A short-lived ternary complex between Gd-DO3A and the phosphorylated metabolites appears to be formed enabling an inner sphere interaction. In solutions containing the open chain complexes, Gd-DTPA and Gd-DTPA-BMA, P-31 relaxation rates of ATP exhibit significant and nonlinear enhancements that are much larger than those observed for PCr and Pi. A ternary complex involving the lanthanide ion, its original chelator, and the ATP molecule is precluded by various experiments which confirm that the lanthanide ion shifts from the original complexes to the ATP phosphate groups.