Tuning the lanthanide binding tags for preferential actinide chelation: an all atom molecular dynamics study

The present study focuses on designing mutant peptides derived from the lanthanide binding tag (LBT) to enhance selectivity for trivalent actinide (An³⁺) ions over lanthanide (Ln³⁺) metal ions (M). The LBT is a short peptide consisting of only 17 amino acids, and is known for its high affinity towards Ln³⁺. LBT was modified by substituting hard-donor ligands like asparagine (ASN or N) and aspartic acid (ASP or D) with softer ligand cysteine (CYS or C) to create four mutant peptides: M-LBT (wild-type), M-N103C, M-D105C, and M-N103C-D105C. All atom molecular dynamics (MD) simulations were employed to analyze the binding dynamics and affinities of these mutants with Eu³⁺ and Am³⁺ as representatives for trivalent Ln and An ions, respectively. Hydrogen bond dynamics and short-range Coulomb interactions are evaluated from the equilibrium run for all the systems. The study utilized an enhanced sampling method, namely, well-tempered meta-dynamics (WT-MtD), to overcome sampling challenges and obtain converged free energy profiles for the metal-binding interactions. Our simulations studies indicate that both single and double mutations alter the coordination environment within the peptide's binding pocket, potentially increasing Am³⁺ selectivity over the Eu³⁺ ion. The binding of Eu³⁺ and Am³⁺ to LBT systems was analyzed, showing an unbinding energy barrier of ∼60 kJ mol^-1 for the wild-type. The N103C variant increases the binding strength with a barrier over 100 kJ mol^-1, while D105C shows a preference for Am³⁺ with a barrier around 70 kJ mol^-1. The doubly mutated N103C-D105C variant favors Am³⁺ by more than 20 kJ mol^-1. The findings suggest N103C for general chelation and N103C-D105C for preferential trivalent Ln/An separation. These insights contribute to the development of more effective and selective chelating agents for preferential actinide binding.