Understanding the Binding of a Bifunctional Amidoximate–Carboxylate Ligand with Uranyl in Seawater

Extracting
uranium from seawater remains a formidable challenge
because of its extremely low concentration of 3.3 ppb. State-of-the-art
polymeric sorbents employ both amidoximate and carboxylate groups
on the side chains to achieve optimal U uptake and selectivity, but
little is known about the synergistic effect between the two functional
groups in binding with uranyl. Herein, we simulated the binding of
a model amidoximate–carboxylate bifunctional ligand with uranyl
using a combination of theoretical methods. Gas-phase quantum-mechanical
calculations showed a chelate binding of a η2 amidoximate
and a monodentate carboxylate to uranyl. Ab initio molecular dynamics
(MD) simulations in an explicit water solvation model confirmed the
stability of the chelate mode. Classical MD and free-energy simulations
in 0.5 M NaCl showed that the carboxylate group binds first to uranyl,
leading to a loose intermediate state, and then, the amidoximate group
binds, resulting in a more stable and tight chelate state. Binding
of the second bifunctional ligand follows a similar process, and the
two ligands prefer a trans configuration around the uranyl group.
The simulated free energies indicate that the two bifunctional ligands
bind with uranyl 55 kJ/mol stronger than the two ligands with only
amidoximate groups. This work suggests an important synergy between
amidoximate and carboxylate groups in binding uranyl.