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Coordination Properties of GdDO3A-Based Model Compounds of Bioresponsive MRI Contrast Agents.

We report a detailed characterization of the thermodynamic stability and dissociation kinetics of Gd3+ complexes with DO3A derivatives containing a (methylethylcarbamoylmethylamino)acetic acid (L1), (methylpropylcarbamoylmethylamino)acetic acid (L2), 2-dimethylamino- N-ethylacetamide (L3), or 2-dimethylamino- N-propylacetamide (L4) group attached to the fourth nitrogen atom of the macrocyclic unit. These ligands are model systems of Ca2+- and Zn2+-responsive contrast agents (CA) for application in magnetic resonance imaging (MRI). The results of the potentiometric studies ( I = 0.15 M NaCl) provide stability constants with log KGdL values in the range 13.9-14.8. The complex speciation in solution was found to be quite complicated due to the formation of protonated species at low pH, hydroxido complexes at high pH, and stable dinuclear complexes in the case of L1,2. At neutral pH significant fractions of the complexes are protonated at the amine group of the amide side chain (log KGdL×H = 7.2-8.1). These ligands form rather weak complexes with Mg2+ and Ca2+ but very stable complexes with Cu2+ (log KCuL = 20.4-22.3) and Zn2+ (log KZnL = 15.5-17.6). Structural studies using a combination of 1H NMR and luminescence spectroscopy show that the amide group of the ligand is coordinated to the metal ion at pH ∼8.5, while protonation of the amine group provokes the decoordination of the amide O atom and a concomitant increase in the hydration number and proton relaxivity. The dissociation of the complexes occurs mainly through a rather efficient proton-assisted pathway, which results in kinetic inertness comparable to that of nonmacrocyclic ligands such as DTPA rather than DOTA-like complexes.