Lanthanide and alkaline-earth complexes of EDTA in water: a molecular dynamics study of structures and binding selectivities

Abstract

We report a molecular dynamics (MD) study on M³⁺ lanthanide and M²⁺ alkaline-earth complexes of ethylenediaminetetraacetate tetraanion (EDTA⁴⁻) in aqueous solution. First, a consistent set of Lennard-Jones parameters for La³⁺, Eu³⁺ and Lu³⁺ cations has been derived from MD and free energy calculations. It reproduces the experimental differences in their hydration free energies, lanthanide–water distances and coordination numbers in water. Next, the uncomplexed EDTA⁴⁻ ligand has been simulated in the presence of Na⁺, Ca²⁺ or Eu³⁺ neutralizing counterions, leading to the spontaneous complexation of Na⁺, but not of Ca²⁺ or Eu³⁺ whose complexes are more stable. The endo 1∶1 complexes of M²⁺ and M³⁺ cations, simulated for up to 1 ns, remain of inclusive type during the whole simulation. The calculated binding selectivities nicely reproduce experimental trends in relative stabilities with EDTA⁴⁻ in each cation series: Ca²⁺ > Sr²⁺ > Ba²⁺ and Lu³⁺ > Eu³⁺ > La³⁺. Thus, despite the simplicity of the cation models (1–6–12 pairwise additive interactions without explicit polarization or charge transfer effects), structural and energy features of lanthanide and alkaline-earth complexation by polyaminocarboxylate ligands are, at least, qualitatively, accounted for.