Paramagnetic lanthanides as magnetic resonance thermo-sensors and probes of molecular dynamics: Holmium-DOTA complex

Investigation of lanthanide (Ln) сhelates and their paramagnetic properties with NMR spectroscopy is an established area of research. However, respective data for coordination compounds of holmium (Ho) are scarce. To fill this gap, the present work focuses on detailed determination of intramolecular dynamics and paramagnetic properties for holmium complexes. 1H NMR measurements are reported for D2O solutions of paramagnetic [Ho(H2O)(DOTA)]− (I) complex in the 273–348 K temperature range. Diamagnetic complex [Lu(H2O)(DOTA)]− (II) was used as a reference compound. The spectra obtained have been analyzed using band-shape analysis technique in the framework of dynamic NMR (D NMR). Temperature dependences of lanthanide-induced shifts (LIS) were taken into consideration, too. Сonformational dynamic process has been found. The dynamics is caused by an interconversion of square-anti-prismatic (SAP) and twisted-square-anti-prismatic (TSAP) conformers (the estimated activation free energy ΔG‡(298 K) is 65 ± 3 kJ mol−1). Thermodynamics of equilibrium between SAP and TSAP conformers of I was investigated too. The results obtained are found to be consistent with those collected for other Ln-DOTA complexes. In accordance with literature reviewed, the fulfilled experimental study is the first example of intramolecular dynamics determination for holmium complexes. Taking Ho3+ as an example, the methodology of paramagnetic 4f-element probe applications for the study of free-energy changes in chemical exchange processes is discussed. The advantages of this method, compared to D NMR studies of diamagnetic substances are illustrated. Among them is an extension of the range of NMR-accessible rate constants for paramagnetic 4f-element complexes, compared to diamagnetic ones. And last, usage of coordination compounds investigated as a new type of thermometric NMR sensors and lanthanide paramagnetic probes for in situ temperature control in solutions is demonstrated. The investigated coordination compounds are suggested to apply as thermo-sensing contrast reagent for MRI diagnostics of cancer and inflammation.

Conformational dynamics processes in the holmium-DOTA complex was identified by 1H NMR. It is caused by an interconversion of SAP and TSAP conformers in the complex (ΔG‡(298 K) = 65 ± 3 kJ mol−1). Coordination compound investigated in the paper represents new type of thermometric NMR sensors.Figure optionsDownload as PowerPoint slide