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  1. Ligand selectivity to specific lanthanide (Ln) ions is key to the separation of rare earth elements from each other. Ligand selectivity can be quantified with relative stability constants (measured experimentally) or relative binding energies (calculated computationally). The relative stability constants of EDTA (ethylenediaminetetraacetic acid) with La 3+ , Eu 3+ , Gd 3+ , and Lu 3+ were predicted from relative binding energies, which were quantified using electronic structure calculations with relativistic effects and based on the molecular structures of Ln–EDTA complexes in solution from density functional theory molecular dynamics simulations. The protonation state of an EDTA amine group was varied to study pH ∼7 and ∼11 conditions. Further, simulations at 25 °C and 90 °C were performed to elucidate how structures of Ln–EDTA complexes varying with temperature are related to complex stabilities at different pH conditions. Relative stability trends are predicted from computation for varying Ln 3+ ions (La, Eu, Gd, Lu) with a single ligand (EDTA at pH ∼11), as well as for a single Ln 3+ ion (La) with varying ligands (EDTA at pH ∼7 and ∼11). Changing the protonation state of an EDTA amine site significantly changes the solution structure of the Ln–EDTA complex resulting inmore »a reduction of the complex stability. Increased Ln–ligand complex stability is correlated to reduced structural variations in solution upon an increase in temperature.« less
    Free, publicly-accessible full text available May 4, 2023
  2. Free, publicly-accessible full text available March 1, 2023
  3. We report the solution structure of a europium-nicotianamine complex predicted from ab initio molecular dynamics simulations with density functional theory. Emission and excitation spectroscopy show that the Eu 3+ coordination environment changes in the presence of nicotianamine, suggesting complex formation, such as what is seen for the Eu 3+ –nicotianamine complex structure predicted from computation. We modeled Eu 3+ –ligand complexes with explicit water molecules in periodic boxes, effectively simulating the solution phase. Our simulations consider possible chemical events ( e.g. coordination bond formation, protonation state changes, charge transfers), as well as ligand flexibility and solvent rearrangements. Our computational approach correctly predicts the solution structure of a Eu 3+ –ethylenediaminetetraacetic acid complex within 0.05 Å of experimentally measured values, backing the fidelity of the predicted solution structure of the Eu 3+ –nicotianamine complex. Emission and excitation spectroscopy measurements were also performed on the well-known Eu 3+ –ethylenediaminetetraacetic acid complex to validate our experimental methods. The electronic structure of the Eu 3+ –nicotianamine complex is analyzed to describe the complexes in greater detail. Nicotianamine is a metabolic precursor of, and structurally very similar to, phytosiderophores, which are responsible for the uptake of metals in plants. Although knowledge that nicotianamine binds europiummore »does not determine how plants uptake rare earths from the environment, it strongly supports that phytosiderophores bind lanthanides.« less