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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, LaIII. However, the monomer prefers NdIIIand SmIII, 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 PrIIIand NdIIIby 100-fold. Selective dimerization enriches high-value PrIIIand NdIIIrelative to low-value LaIIIand CeIIIin 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 NdIIIand SmIII. 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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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)
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/Tm) 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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- Award ID(s):
- 1945015
- PAR ID:
- 10473082
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 29
- Issue:
- 72
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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