Solution Structure of a Europium-Nicotianamine Complex Supports That Phytosiderophores Bind Lanthanides

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 measurements show that the Eu³⁺coordination environment changes in the presence of nicotianamine, suggesting complex formation, and strongly supporting the predicted Eu³⁺-nicotianamine complex structure from computation. We used our recently optimized pseudopotentials and basis sets for lanthanides to model Eu³⁺-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 Eu³⁺-ethylenediaminetetraacetic acid complex within 0.05 Å of experimentally measured values, backing the fidelity of the predicted solution structure of the Eu³⁺-nicotianamine complex. Emission and excitation spectroscopy measurements were also performed on the well-known Eu³⁺-ethylenediaminetetraacetic acid complex to validate our experimental methods. The electronic structure of the Eu³⁺-nicotianamine complex is analyzed to describe electron densities and coordination bonds 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 europium does not determine how plants uptake rare earths from the environment, it strongly supports that phytosiderophores bind lanthanides.