Search for: All records

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. 

Abstract Technologically critical rare-earth elements are notoriously difficult to separate, owing to their subtle differences in ionic radius and coordination number^1–3. The natural lanthanide-binding protein lanmodulin (LanM)^4,5is a sustainable alternative to conventional solvent-extraction-based separation⁶. Here we characterize a new LanM, fromHansschlegelia quercus(Hans-LanM), with an oligomeric state sensitive to rare-earth ionic radius, the lanthanum(III)-induced dimer being >100-fold tighter than the dysprosium(III)-induced dimer. X-ray crystal structures illustrate how picometre-scale differences in radius between lanthanum(III) and dysprosium(III) are propagated toHans-LanM’s quaternary structure through a carboxylate shift that rearranges a second-sphere hydrogen-bonding network. Comparison to the prototypal LanM fromMethylorubrum extorquensreveals distinct metal coordination strategies, rationalizingHans-LanM’s greater selectivity within the rare-earth elements. Finally, structure-guided mutagenesis of a key residue at theHans-LanM dimer interface modulates dimerization in solution and enables single-stage, column-based separation of a neodymium(III)/dysprosium(III) mixture to >98% individual element purities. This work showcases the natural diversity of selective lanthanide recognition motifs, and it reveals rare-earth-sensitive dimerization as a biological principle by which to tune the performance of biomolecule-based separation processes. 

The biological importance of lanthanides, and the early lanthanides (La 3+ –Nd 3+ ) in particular, has only recently been recognized, and the structural principles underlying selective binding of lanthanide ions in biology are not yet well established. Lanmodulin (LanM) is a novel protein that displays unprecedented affinity and selectivity for lanthanides over most other metal ions, with an uncommon preference for the early lanthanides. Its utilization of EF-hand motifs to bind lanthanides, rather than the Ca 2+ typically recognized by these motifs in other proteins, has led it to be used as a model system to understand selective lanthanide recognition. Two-dimensional infrared (2D IR) spectroscopy combined with molecular dynamics simulations were used to investigate LanM's selectivity mechanisms by characterizing local binding site geometries upon coordination of early and late lanthanides as well as calcium. These studies focused on the protein's uniquely conserved proline residues in the second position of each EF-hand binding loop. We found that these prolines constrain the EF-hands for strong coordination of early lanthanides. Substitution of this proline results in a more flexible binding site to accommodate a larger range of ions but also results in less compact coordination geometries and greater disorder within the binding site. Finally, we identify the conserved glycine in the sixth position of each EF-hand as a mediator of local binding site conformation and global secondary structure. Uncovering fundamental structure–function relationships in LanM informs the development of synthetic biology technologies targeting lanthanides in industrial applications. 

Actinium-based therapies could revolutionize cancer medicine but remain tantalizing due to the difficulties in studying and limited knowledge of Ac chemistry. Current efforts focus on small synthetic chelators, limiting radioisotope complexation and purification efficiencies. Here, we demonstrate a straightforward strategy to purify medically relevant radiometals, actinium(III) and yttrium(III), and probe their chemistry, using the recently discovered protein, lanmodulin. The stoichiometry, solution behavior, and formation constant of the 228 Ac 3+ -lanmodulin complex and its 90 Y 3+ / nat Y 3+ / nat La 3+ analogs were experimentally determined, representing the first actinium-protein and strongest actinide(III)-protein complex (sub-picomolar K d ) to be characterized. Lanmodulin’s unparalleled properties enable the facile purification recovery of radiometals, even in the presence of >10 +10 equivalents of competing ions and at ultratrace levels: down to 2 femtograms 90 Y 3+ and 40 attograms 228 Ac 3+ . The lanmodulin-based approach charts a new course to study elusive isotopes and develop versatile chelating platforms for medical radiometals, both for high-value separations and potential in vivo applications.