Investigation of Ln 3+ complexation by a DOTA derivative substituted by an imidazothiadiazole: synthesis, solution structure, luminescence and relaxation properties
Résumé
We investigated the coordination properties of original macrocyclic Ln(3+) complexes comprising an imidazothiadiazole heterocycle. The thermodynamic stability of the Gd3+ complex was determined by a combination of potentiometric and photophysical measurements. The kinetic inertness was assessed in highly acidic media. The solution structure of the Ln(3+) complex was unambiguously determined by a set of photophysical measurements and H-1, C-13, Y-89 NMR data in combination with DFT calculations, which proved coordination of the heterocycle to Ln(3+). The ability of the imidazothiadiazole moiety to sensitize Tb3+ luminescence was investigated. Finally, the relaxation properties were investigated by recording H-1 nuclear magnetic relaxation dispersion (NMRD) profiles and O-17 measurements. The water exchange rate is similar to that of GdDOTA as the less negative charge of the ligand is compensated for by the presence of a bulky heterocycle. Relaxivity is constant over a large range of pH values, demonstrating the favorable properties of the complex for imaging purposes.