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Updated: Mar 7, 2026

Preparation, Purification, and Characterization of Lanthanide Complexes for Use as Contrast Agents for Magnetic Resonance Imaging
Published on: July 21, 2011
Eu(III) Complex with DO3A-amino-phosphonate Ligand as a Concentration-Independent pH-Responsive Contrast Agent for
Tereza Krchová1, Vít Herynek2, Andrea Gálisová2
1Department of Inorganic Chemistry, Faculty of Science, Charles University , Hlavova 2030, Prague 2 128 43, Czech Republic.
Abstract:
A new DOTA-like ligand H5do3aNP with a 2-[amino(methylphosphonic acid)]ethyl-coordinating pendant arm was prepared, and its coordinating properties were studied by NMR spectroscopy and potentiometry. The study revealed a rare slow exchange (on the 1H and 31P NMR time scale) between protonated and unprotonated complex species with a corresponding acidity constant pKA ∼ 8.0. This unusually slow time scale associated with protonation is caused by a significant geometric change from square-antiprismatic (SA) arrangement observed for protonated complex SA-[Eu(Hdo3aNP)]- to twisted-square-antiprismatic (TSA) arrangement found for deprotonated complex TSA-[Eu(do3aNP)]2-. This behavior results in simultaneous occurrence of the signals of both species in the 31P NMR spectra at approximately -118 and +70 ppm, respectively. Such an unprecedented difference in the chemical shifts between species differing by a proton is caused by a significant movement of the principal magnetic axis and by a change of phosphorus atom position in the coordination sphere of the central Eu(III) ion (i.e., by relative movement of the phosphorus atom with respect to the principal magnetic axis). It changes the sign of the paramagnetic contribution to the 31P NMR chemical shift. The properties discovered can be employed in the measurement of pH by MRS techniques as presented by proof-of-principle experiments on phantoms.
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