An integrative approach towards bio-inspired sequestration of rare earth elements

Despite their important role in the development of renewable energy, the process of obtaining lanthanides (Ln) through conventional mining or through recovery processes is problematic due to their negative impact on the environment. Research into new recovery processes therefore will be crucial to enable cost-effective and environmentally friendly enrichment and separation of Ln, particularly from waste streams or mine water. Many proteins contain specific peptide sequences as metal-binding sites. EF-hand loops are a Ca2+ binding motif in a diverse family of proteins, among them calmodulin (CaM). As it is well known that lanthanides can substitute Ca2+ in proteins, peptides based on the proteinic binding motifs are a promising approach to develop materials for Ln binding, separation and recovery. This study characterizes the CaM-EF-hand 4 (EF4) and its binding behaviour for various lanthanides in detail. Isothermal titration calorimetry (ITC), time-resolved laser fluorescence spectroscopy (TRLFS), nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations demonstrate a generally high Ln-binding affinity but indicate slightly different binding constants for different Ln to EF4 (DIDGDGQVNYEE). In addition, thermodynamic, structural and coordination effects upon binding are investigated. The isolated peptide possesses a much higher flexibility than the loop within the protein. Under certain conditions, multiple complex stoichiometries, 1:1 and 2:1 (Ln3+:peptide), are detected. Finally, immobilized EF4 is used to demonstrate the extraction and recovery of Sm3+ from aqueous solution. Overall, the EF4 loop motif of calmodulin turns out to be a promising biological ligand for lanthanide extraction.