Chelation Behaviors of 3,4,3-LI(1,2-HOPO) with Lanthanides and Actinides Implicated by Molecular Dynamics Simulations.

The hydroxypyridinone ligand 3,4,3-LI(1,2-HOPO) (denoted as -HOPO) is a potential chelator agent for decorporation of actinides (An), while its coordination modes with actinides and the dynamics of the complexes (An(-HOPO)) in aqueous phase remain unclear. Here, we report molecular dynamics simulations of the complexes with key actinides (Am, Cm, Th, U, Np, Pu) to study their coordination and dynamic behaviors. For comparison, the complexation of the ligand with a ferric ion and key lanthanides (Sm, Eu, Gd) was also studied. The simulations show that the nature of metal ions determines the properties of the complexes. The -HOPO in the Fe(-HOPO) complex ion formed a compact and rigid cage to encapsulate the ferric ion, which was hexa-coordinated. Ln/An cations were ennea-coordinated with eight ligating oxygen atoms from -HOPO and one from an aqua ligand, and An cations were deca-coordinated with a second aqua ligand. The -HOPO shows strong affinity for metal ions (stronger for An than Ln/An) benefited from its high denticity and its flexible backbone. Meanwhile, the complexes displayed different dynamic flexibilities, with the An(-HOPO) complexes more significant than the others, and in the An(-HOPO) complexes, the fluctuation of the -HOPO ligand was highly correlated with that of the eight ligating O atoms. This is attributed to the more compact conformation of the ligand, which raises backbone tension, and the competition of the aqua ligand against the -HOPO ligand in coordinating with the tetravalent actinides. This work enriches our understanding on the structures and conformational dynamics of the complexes of actinides with -HOPO and is expected to benefit the design of HOPO analogues for actinide sequestering.