Gallium-71 and Phosphorus-31 Nuclear Magnetic Resonance Studies of the Interactions of Gallium with Phosphoric Acid in Aqueous Solution

Because inorganic phosphate has been shown to inhibit gallium uptake by L1210 leukemic cells, the interactions of gallium with phosphoric acid was investigated using both 71Ga and 31P FT-NMR spectroscopy. The 7lGa NMR results are consistent with the formation of several types of complexes with different stoichiometries. The31P resonances of the phosphate species present were used to characterize the complexes more fully. At 5-7 °C, the 31P NMR spectrum of various gallium phosphate solutions can contain as many as five distinct peaks at 0, ~2.5, 4.0, 5.1, 8.5, and 11 ppm from external phosphoric acid reference. The peak at 0 ppm can be assigned to the uncomplexed phosphate species which are in rapid chemical exchange with one another on the 31P chemical shift time scale. The peaks at 4.0 and 5.1 ppm have been identified as the GaH3PO43+ and GaH2PO42+ complexes, respectively. We suggest that the broad peak observed at 2.5 ppm arises from the formation of gallium phosphate polymers and that the remaining resonance at 11 ppm can be assigned to a gallium complex of the dimeric ion of phosphoric acid, H5P2O8-.