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1. Modulating metal-centered dimerization of a lanthanide chaperone protein for separation of light lanthanides

   https://doi.org/10.1073/pnas.2410926121  

   Larrinaga, Wyatt B; Jung, Jonathan J; Lin, Chi-Yun; Boal, Amie K; Cotruvo_Jr, Joseph A (October 2024, Proceedings of the National Academy of Sciences)

   Elucidating details of biology’s selective uptake and trafficking of rare earth elements, particularly the lanthanides, has the potential to inspire sustainable biomolecular separations of these essential metals for myriad modern technologies. Here, we biochemically and structurally characterizeMethylobacterium(Methylorubrum)extorquensLanD, a periplasmic protein from a bacterial gene cluster for lanthanide uptake. This protein provides only four ligands at its surface-exposed lanthanide-binding site, allowing for metal-centered protein dimerization that favors the largest lanthanide, La^III. However, the monomer prefers Nd^IIIand Sm^III, which are disfavored lanthanides for cellular utilization. Structure-guided mutagenesis of a metal-ligand and an outer-sphere residue weakens metal binding to the LanD monomer and enhances dimerization for Pr^IIIand Nd^IIIby 100-fold. Selective dimerization enriches high-value Pr^IIIand Nd^IIIrelative to low-value La^IIIand Ce^IIIin an all-aqueous process, achieving higher separation factors than lanmodulins and comparable or better separation factors than common industrial extractants. Finally, we show that LanD interacts with lanmodulin (LanM), a previously characterized periplasmic protein that shares LanD’s preference for Nd^IIIand Sm^III. Our results suggest that LanD’s unusual metal-binding site transfers less-desirable lanthanides to LanM to siphon them away from the pathway for cytosolic import. The properties of LanD show how relatively weak chelators can achieve high selectivity, and they form the basis for the design of protein dimers for separation of adjacent lanthanide pairs and other metal ions. 

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2. X-ray absorption spectroscopy of lanmodulin-derived peptides bound to rare earth elements

   https://doi.org/10.1107/S1600577525007726  

   Smerigan, Adam; Hoffman, Adam S; Perez-Aguilar, Jorge; Shi, Rui; Bare, Simon R (November 2025, Journal of Synchrotron Radiation)

   A sustainable and robust supply chain of rare earth elements (REEs) is necessary to meet our consumer, national security and clean energy goals. However, current intra-REE separation technologies (e.g.solvent extraction) are costly and carry a heavy environmental burden. Therefore, the development of new aqueous based ligands that are selective for individual REEs will be integral in future REE production systems. To develop these ligands, an understanding of how ligand coordination structure relates to selectivity is imperative. We used X-ray absorption spectroscopy (XAS) to observe the local structure around four lanthanide (Ln) ions (La, Ce, Pr and Nd) complexed by water and several relevant chelating ligands [lanmodulin EF-hand 1 peptides (LanM1), ethylenediaminetetraacetic acid (EDTA), aminotris(methylenephosphonic acid) (ATMP) and citric acid]. To collect these liquid-phase XAS spectra, we developed a new flow cell that prevents bubble interference and beam damage to the samples. In the X-ray absorption near-edge structure (XANES), we observed energy shifts in the white line, white line broadening and differences in the white line intensity of different Ln–ligand complexes between ligands. In the extended X-ray absorption fine structure (EXAFS), we distinguished differences in peak intensity and distance between coordinating ligands. Differences in the local coordination structure between Ln–LanM1 peptide complexes were more subtle compared with the other ligands (La–water, La–EDTA, La–ATMP and La–citric acid complexes). Further XANES and EXAFS studies, in combination with modelling and other techniques, could greatly improve our structural knowledge of how these aqueous ligands bind Ln ions and how they can be used to design more selective ligands for more efficient and sustainable REE separations. 

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3. Selective Crystallization of Rare‐Earth Ions into Cationic Metal‐Organic Frameworks for Rare‐Earth Separation

   https://doi.org/10.1002/anie.202017042  

   Yang, Huajun; Peng, Fang; Schier, Danielle E; Markotic, Stipe A; Zhao, Xiang; Hong, Anh N; Wang, Yanxiang; Feng, Pingyun; Bu, Xianhui (May 2021, Angewandte Chemie International Edition)

   Abstract For rare‐earth separation, selective crystallization into metal‐organic frameworks (MOFs) offers new opportunities. Especially important is the development of MOF platforms with high selectivity toward target ions. Here we report a MOF platform (CPM‐66) with low‐coordination‐number environment for rare‐earth ions. This platform is highly responsive to the size variation of rare‐earth ions and shows exceptional ion‐size selectivity during crystallization. CPM‐66 family are based on M₃O trimers (M=6‐coordinated Sc, In, Er‐Lu) that are rare for lanthanides. We show that the size matching between urea‐type solvents and metal ions is crucial for their successful synthesis. We further show that CPM‐66 enables a dramatic multi‐fold increase in separation efficiency over CPM‐29 with 7‐coordinated ions. This work provides some insights into methods to prepare low‐coordinate MOFs from large ions and such MOFs could serve as high‐efficiency platforms for lanthanide separation, as well as other applications. 

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4. Electron Paramagnetic Resonance, Electronic Ground State, and Electron Spin Relaxation of Seven Lanthanide Ions Bound to Lanmodulin and the Bioinspired Chelator, 3,4,3‐LI(1,2‐HOPO)

   https://doi.org/10.1002/chem.202303215  

   Larrinaga, Wyatt B.; Cotruvo, Joseph A.; Worrell, Brady T.; Eaton, Sandra S.; Eaton, Gareth R. (December 2023, Chemistry – A European Journal)

   Abstract The electron paramagnetic resonance (EPR) spectra of lanthanide(III) ions besides Gd³⁺, bound to small‐molecule and protein chelators, are uncharacterized. Here, the EPR properties of 7 lanthanide(III) ions bound to the natural lanthanide‐binding protein, lanmodulin (LanM), and the synthetic small‐molecule chelator, 3,4,3‐LI(1,2‐HOPO) (“HOPO”), were systematically investigated. Echo‐detected pulsed EPR spectra reveal intense signals from ions for which the normal continuous‐wave first‐derivative spectra are negligibly different from zero. Spectra of Kramers lanthanide ions Ce³⁺, Nd³⁺, Sm³⁺, Er³⁺, and Yb³⁺, and non‐Kramers Tb³⁺and Tm³⁺, bound to LanM are more similar to the ions in dilute aqueous:ethanol solution than to those coordinated with HOPO. Lanmodulins from two bacteria, with distinct metal‐binding sites, had similar spectra for Tb³⁺but different spectra for Nd³⁺. Spin echo dephasing rates (1/T_m) are faster for lanthanides than for most transition metals and limited detection of echoes to temperatures below ~6 to 12 K. Dephasing rates were environment dependent and decreased in the order water:ethanol>LanM>HOPO, which is attributed to decreasing librational motion. These results demonstrate that the EPR spectra and relaxation times of lanthanide(III) ions are sensitive to coordination environment, motivating wider application of these methods for characterization of both small‐molecule and biomolecule interactions with lanthanides. 

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5. Lanthanide extraction selectivity of a tripodal carbamoylmethylphosphine oxide ligand system

   https://doi.org/10.1039/c8dt02239a  

   Patterson, Michael G.; Mulville, Andrew K.; Connor, Emily K.; Henry, Alyssa T.; Hudson, Michael L.; Tissue, Kirsten; Biros, Shannon M.; Werner, Eric J. (October 2018, Dalton Transactions)

   Four tripodal carbamoylmethylphosphine oxide (CMPO)-based ligands are reported here and assessed with regard to lanthanide (Ln) coordination chemistry and selective extraction of lanthanide ions from aqueous solution. Inspired by previous liquid–liquid extraction studies that suggested a preference for terbium( iii ), the current work further probes the extraction behavior of a tris-(2-aminoethyl)amine (TREN) capped, ethoxy substituted CMPO ligand with respect to the entire series of lanthanides. Upon confirmation of Tb 3+ extraction selectivity versus the whole series, experiments were conducted to assess the effect of increasing the alkyl chain length within the ligand TREN cap, as well as changing the CMPO substituents by replacing the ethoxy groups with more hydrophobic phenyl groups to promote solubility in the organic extraction solvent. Extraction efficiencies remained low for most lanthanides upon increasing the cap size, with % E values consistently around 5%, and a complete loss of Tb 3+ preference was noted with a decrease in % E from 18% to 3.5%. For the agent employing the original, smaller TREN cap but with phenyl substituents on the CMPO units, an increase in extraction toward the middle of the row was again observed, albeit modest, with relatively high % E values for both Gd 3+ and Tb 3+ versus the other lanthanides (13 and 11%, respectively). A more dramatic extraction selectivity for the phenyl substituted ligand was achieved upon modification of the ligand to metal ratio, with a 100 : 1 ratio resulting in a near linear decrease in % E from 41% for La 3+ to 3.7% for Lu 3+ . Finally, modification of the TREN capping scaffold by adding an oxygen atom to the central nitrogen led to consistently low % E values, revealing the effect of TREN cap oxidation on Ln extraction for this tripodal CMPO ligand system. 

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