Carbazole as Linker for Dinuclear Gadolinium-Based MRI Contrast Agents

Ligands able to complex two gadolinium ions have been synthesized and characterized in view of the ability of the complexes to increase the spin relaxation of water protons. All ligands are based on the heptadentate diethylenetriaminetetraacetic acid (DTTA) chelator and carbazole as a rigid linker. Depending on the derivatization on the nitrogen atom of the five-membered ring, the compounds form small aggregates in aqueous solution, self-assemble to form micelles or bind to human serum albumin. In all cases, this leads to a marked increase in H-1 relaxivity at nuclear Larmor frequencies between 20 and 60 MHz. Water exchange on the gadolinium ions as measured by O-17 NMR relaxation is fast enough not to limit relaxivity. H-1 nuclear magnetic relaxation dispersion profiles were also measured and analyzed using Solomon-Bloembergen-Morgan theory including Lipari-Szabo treatment to include internal motion or anisotropic rotation.