(113h) 3D Molecular Structures of Lanthanide Binding Peptides for Air-Water Interfacial Separation of Rare Earth Elements

Rare earth elements are of great importance in advanced technologies due to their distinctive magnetic, luminescent, and catalytic properties. To date, rare earth elements are mined and then separated by using liquid extraction processes that are expensive, energy inefficient, and environmentally unfavorable. Given their crucial role in the modern economy, it is important to develop a selective and energy-efficient “green” recovery method for rare earth elements. Bioinspired lanthanide binding tags have received considerable interest due to their high affinity and selectivity for complexing trivalent rare-earth ions. Here, we present the 3D molecular structures of two peptide sequences (LBT1 & LBT1-LLA) and their binding affinity with La³⁺ cations. The addition of the extended hydrophobic amino acid sequence LLA to LBT1 renders the peptide more amphiphilic, assisting the adsorption of the peptide complex to the air-water interface during a subsequent foam fractionation separation and recovery process. In this study, we use a diverse range of experimental and computational techniques to obtain the 3D molecular structures of the peptides and their complexes with La³⁺ cations, as well as understand the interfacial adsorption and stability of the peptide-binding loop at the air-water interface.

High-resolution quantitative ¹H solution-state NMR spectra were acquired for the peptides titrated with different molar ratios of rare earth (e.g., La³⁺) cations. The variation in the ¹H chemical shifts upon titrating La³⁺ ions indicated conformational changes associated with binding. ¹H signal assignments were made using a series of 2D homonuclear and heteronuclear NMR experiments, including 2D ¹H{¹H} TOCSY, ¹H{¹H} NOESY, ¹H{¹⁵N} HSQC, ¹H{¹³C} HSQC spectra. Using through-space distance constraints from ¹H{¹H} NOESY NMR data, the 3D molecular structures of the peptides complexes in the bulk solution were solved (XPLOR NIH). Additionally, the dissociation constants (K_d) for the complexes were estimated from the NMR data. Surface tension values obtained by pendant drop experiments established preferential surface adsorption of the peptide/ peptide ion complex. These results were further corroborated by the higher electron density observed at the air-water interface in X-ray reflectivity studies. Finally, molecular dynamics (MD) simulation methods were used to understand the coordination sites in the peptide sequence and ascertain the binding loop's stability and conformation at the air-water interface. Overall, the results reveal the molecular structures of LBT1, LBT1-LLA, and their complexes with rare earth elements for the first time and lays the scientific groundwork towards realizing an energy-efficient, interface-based bioseparation process for the recovery of rare-earth elements.