Lanthanides as NMR/MRI temperature sensors and probes of moderately fast molecular dynamics in aqueous medium: A dependence of activation energy of racemization in complexes of diethylenetriaminepentaacetat on lanthanide ion

Abstract

In solution, [Ln(H2O)n(dtpa)]2− chelates undergo a dynamic exchange between two enantiomers. The free energy of activation of intramolecular dynamics (for complexes of Pr to Ho) slightly increases with decreasing radius of the lanthanide cation. According to analysis of the paramagnetic lanthanide induced shifts (LIS) in NMR spectra, the structure of [Ho(H2O)n(dtpa)]2− in solution is similar to the related model found in the crystal state. It was found that complexes [Ln(H2O)n(dtpa)]2− may be used (due to the substantial temperature dependence of LIS) as NMR thermometric sensors for controlling the local temperature in aqueous medium. The investigated coordination compounds are suggested to apply as thermo-sensing contrast reagent for MRI diagnostics of cancer and inflammation.