Molecular dynamics and paramagnetic Nuclear Magnetic Resonance coupling for the coordination mode determination of An VI complexes in solution
Résumé
Paramagnetic Nuclear Magnetic Resonance (pNMR) plays an important role in structural determination of complexes in solution, particularly in biological systems where pNMR chemical shifts are induced by lanthanide(III) cations[1] (as probe in metalloproteins). The use of actinides (An) for complex structure determination has never really been employed, and can be useful in nuclear fuel research and environmental science since their toxicology can be investigated through the cation - peptide coordination chemistry owing to structural information studies[2]. For AnO2 2+ complexes (An = Np, Pu), chemical shifts in 1H pNMR are mainly induced by the pseudocontact contribution, corresponding to the magnetic interaction between the 5f unpaired electron of metallic cation and the nuclear spin of ligand protons[3]. This pseudocontact contribution depends on both the magnetic susceptibility anisotropy of the metal ion and a geometrical factor related to the protons position in the complex (Fig. 1.a). The aim of this work is to verify the relationship between the coordinates of protons around the cation, which are determined by classical molecular dynamics (MD) simulations, and their 1H chemical shifts. To this end, MD simulations of complexes composed of AnO2 2+ cation and one or two diglycolamides (TEDGA) have been performed (Fig. 1.b) to confirm the 1H pNMR chemical shifts observed experimentally (Fig. 1.a). The relative flexibility of the TEDGA molecule gives, in the case of the 1:2 complex, two types of coordination mode, i.e., either bidentate or tridentate, that corresponds either to 5 or 6 TEDGA oxygens in the AnO2 2+ first coordination sphere, which is experimentally confirmed by EXAFS experiments. This study provides important structural information on the complex, such as the different coordination modes and allows the determination of magnetic susceptibility anisotropy parameters of the paramagnetic cation.
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