The Solution Structure and Dynamics of MRI Probes Based on Lanthanide(III) DOTA as Investigated by DFT and NMR Spectroscopy

Paramagnetic lanthanide(III) complexes stable in aqueous solutions have gained increasing interest in the recent years due to their importance as contrast agents in magnetic resonance imaging (MRI). Lanthanide(III) complexes with macrocyclic ligands derived from 1,4,7,10-tetraazacyclododecane (cyclen) are widely used for the design of MRI probes because of their high thermodynamic stability and kinetic inertness. The rational design of more efficient contrast agents requires a better understanding of the structure and dynamics of these systems in solution. This contribution reviews the work of the author and his collaborators on the solution structure and dynamics of lanthanide(III) complexes with different cyclen-based ligands and closely related systems. DFT calculations provide molecular geometries and relative energies of the different stereoisomers of these complexes in good agreement with the experimental data. The conformational analysis performed with the aid of density functional theory (DFT) calculations was validated with the investigation of the YbIII-induced 1H NMR paramagnetic shifts, which encode information on the position of the observed NMR nuclei with respect to the LnIII ion. Additionally, DFT calculations provide a better understanding of the dynamic processes responsible for the interconversion between the square-antiprismatic (SAP) and twisted-square-antiprismatic (TSAP) isomers of these complexes in solution, which might proceed through the inversion of the cyclen unit or the rotation of the pendant arms. The activation barriers obtained from theoretical calculations show a good agreement with the experimental values obtained from variable-temperature NMR spectroscopy. The work presented in this paper shows that DFT calculations in combination with NMR spectroscopy provide detailed information on the structure and dynamics of lanthanide(III) complexes at the molecular level and represent a powerful tool for the characterization of lanthanide(III) complexes relevant to the field of MRI contrast agents.