Uranyl–Glutardiamidoxime Binding from First-Principles Molecular Dynamics, Classical Molecular Dynamics, and Free-Energy Simulations

Exploring the structural
interplay of ligands with uranyl can provide
important knowledge for technology advances in uranium extraction
from seawater. However, obtaining such chemical information is not
an easy endeavor experimentally. From a plethora of computational
methods, this work provides both microscopic insights and free-energy
profiles of the binding between uranyl and deprotonated glutardiamidoxime
(H2B) for which experimental structural information is
not available, despite H2B being an important model ligand
with an open-chain conformation for understanding aqueous uranium
extraction chemistry. In our molecular dynamics (MD) simulations,
we explicitly accounted for the water solvent as well as the Na+ and Cl– ions. We found that hydrogen bonding
plays a critical role in dictating the binding configurations of B2– and HB– with uranyl. Simulated
free energies of sequential ligand binding to form UO2B,
[UO2B2]2–, and [UO2(HB)­B]− show very good agreement with the experimental
values, lending support to our structural insights. The potential
of mean force simulations showed the common feature of an important
intermediate state where one end of the ligand binds to uranyl while
the other end is solvated in water. Bringing the loose end of the
ligand to bind with uranyl has a free-energy barrier of 15–25
kJ/mol. The present work shows that the combined simulation approach
can reveal key structural and thermodynamic insights toward a better
understanding of aqueous complexation chemistry for uranium extraction
from the sea.