Molecular Design Of Lanthanide Binding Tag Peptides For Nanoparticle Assisted Interfacial Separation Of Rare Earth Elements
Rare earth elements (REEs), comprising yttrium, scandium, and the lanthanides, are indispensable to modern technologies owing to their unique luminescent, chemical, and magnetic properties. They play a central role in sustainable technologies such as batteries, wind turbines, and electric vehicles, while their chemical versatility underpins advanced display systems, high-performance magnets, and catalytic processes. Despite their importance, REEs are notoriously difficult to separate because of their highly similar physicochemical properties. Conventional solvent extraction remains the dominant method, though it requires large volumes of hazardous organophosphate surfactants, consumes significant energy, and often demands ∼100 stages for separation at high purities. To address these challenges, biomolecular strategies have recently emerged as promising alternatives, exploiting or engineering the specificity of metalloproteins and their binding loops to achieve selective REE separation. Lanthanide binding tags (LBTs) are short peptides engineered from calcium-binding motifs of metalloproteins such as calmodulin and adapted through directed screening to achieve selective coordination of lanthanide cations (Ln3+). These peptides exhibit high binding affinity and can differentiate among lanthanides across the series. Recently, these molecules have been repurposed for the selective separation of REEs. Understanding how size selectivity arises at the molecular level, however, remains critical. In this work, we investigate how LBTs achieve size selectivity through structural modulation in response to Ln3+ ionic radius, using a combination of solution-state multi-dimensional NMR spectroscopy and molecular dynamics simulations. We show that while overall binding affinity is primarily governed by strongly coordinating charged residues, selectivity is conferred by the structural rearrangement of weaker binding sites. Notably, the N5 asparagine residue functions as a conformational gate: for smaller cations such as Lu3+, it remains locked via an intramolecular hydrogen bond, whereas for larger cations like La3+, the gate opens, allowing water molecules to penetrate the binding pocket. These insights provide crucial guidance for the rational design of LBTs for targeted lanthanide capture and isolation. The subsequent part of this work focuses on developing interfacial separation platforms using LBTs. We show that Ln3+-bound LBTs, which exhibit slight surface activity, can be recovered via foam fractionation. However, this approach is limited by poor foam stability and the formation of wet foam which negatively impacts selectivity. To address these challenges, we introduce positively aminated silica nanoparticles (NPs), which electrostatically bind LBT1:Ln3+ complexes. This interaction facilitates the transport of the NP:LBT complex to the air–water interface, leading to a synergistic reduction in surface tension. Furthermore, the presence of nanoparticles increases interfacial elasticity, which enhances foam stability and improves foam drying. We further characterize this system using Grazing Incidence Small Angle X-ray Scattering (GISAXS), revealing an ordered arrangement of nanoparticles at the interface and well-defined separation distances, which underpins the improved mechanical stability of the foam. The addition of the peptide to the nanoparticles also lowers the zeta potential of the system, reducing colloidal stability; however, this effect can be exploited for recovery via sedimentation. This provides an additional route for Ln3+ separation, complementing the interfacial foam-based strategy. Overall, this work integrates a molecular-level understanding of selective lanthanide complexation, derived from NMR and MD simulations, with the development of LBT-nanoparticle systems for scalable interfacial rare earth element separation.
rare earth elements, bioseparation, peptides, lanthanide binding tags, selectivity, nanoparticles, foam fractionation, Analytical Chemistry, Biochemical and Biomolecular Engineering, Chemical Engineering, Chemistry, Computational Chemistry, Inorganic Chemistry, Other Chemical Engineering, Structural Biology
City University of New York - City College
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