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 t-HOPO) is a potential chelator agent for decorporation of in vivo actinides (An), while its coordination modes with actinides and the dynamics of the complexes (An(t-HOPO)) in aqueous phase remain unclear. Here, we report molecular dynamics simulations of the complexes with key actinides (Am3+, Cm3+, Th4+, U4+, Np4+, Pu4+) to study their coordination and dynamic behaviors. For comparison, the complexation of the ligand with a ferric ion and key lanthanides (Sm3+, Eu3+, Gd3+) was also studied. The simulations show that the nature of metal ions determines the properties of the complexes. The t-HOPO in the FeIII(t-HOPO)1- complex ion formed a compact and rigid cage to encapsulate the ferric ion, which was hexa-coordinated. Ln3+/An3+ cations were ennea-coordinated with eight ligating oxygen atoms from t-HOPO and one from an aqua ligand, and An4+ cations were deca-coordinated with a second aqua ligand. The t-HOPO shows strong affinity for metal ions (stronger for An4+ than Ln3+/An3+) benefited from its high denticity and its flexible backbone. Meanwhile, the complexes displayed different dynamic flexibilities, with the AnIV(t-HOPO) complexes more significant than the others, and in the AnIV(t-HOPO) complexes, the fluctuation of the t-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 t-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 t-HOPO and is expected to benefit the design of HOPO analogues for actinide sequestering.