Thermodynamic and structural properties of Eu3+,Gd3+ and Tb3+ complexes with 1,4,7,10-tetra(2-glutaryl)-1,4,7,10-tetraazacyclododecane in solution:: EXAFS, luminescence, potentiometric studies, and quantum calculations

The stability of the various complexes formed by racemic solutions of the title ligand (L) with Gd3+, Eu3+ and Tb3+ was investigated by potentiometry. The reaction of complexation proceeds through the quick formation of metastable species leading, after a slow reorganisation of the macrocycle, to thermodynamically stable complexes. The mean numbers of water molecules coordinated to the lanthanides were determined by luminescence and EXAFS spectroscopy. This last method, applied to solutions of complexes, allowed us to precisely determine the nature of the atoms that surround the metal atom and the distance between the lanthanide ion and the various ligands. These structural data that are in good agreement with the results found using quantum mechanics allow us to propose a reaction mechanism, from the hydrated lanthanide ion to the final stable complexes through intermediate species. The specific stability of these final complexes arises from the formation of transitory bonds between the metal ion and two pendant arms, which bear carboxylate groups. The stability constants of the final complexes have high values, close to those obtained with DOTA [log beta(110)(EuL5-) = 24.01; log beta(110)(GdL5-) = 24.03; log beta(110)(TbL5-) = 23.97]. This induces a notable in vivo dissociation inertness, which is essential for a potential contrast agent in magnetic resonance imaging. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.