Selective and Efficient Biomacromolecular Extraction of Rare-Earth Elements using Lanmodulin

Abstract

Lanmodulin (LanM) is a recently discovered protein that undergoes a large conformational change in response to rareearth elements (REEs). Here, we use multiple physicochemical methods to demonstrate that LanM is the most selective macromolecule for REEs characterized to date and even outperforms many synthetic chelators. Moreover, LanM exhibits metal-binding properties and structural stability unseen in most other metalloproteins. LanM retains REE binding down to pH ≈ 2.5, and LanM-REE complexes withstand high temperature (up to 95 °C), repeated acid treatments, and up to molar amounts of competing non-REE metal ions (including Mg, Ca, Zn, and Cu), allowing the protein's use in harsh chemical processes. LanM's unrivaled properties were applied to metal extraction from two distinct REE-containing industrial feedstocks covering a broad range of REE and non-REE concentrations, namely, precombustion coal and electronic waste leachates. After only a single all-aqueous step, quantitative and selective recovery of the REEs from all non-REEs initially present (Li, Na, Mg, Ca, Sr, Al, Si, Mn, Fe, Co, Ni, Cu, Zn, and U) was achieved, demonstrating the universal selectivity of LanM for REEs against non-REEs and its potential application even for industrial low-grade sources, which are currently underutilized. Our work indicates that biosourced macromolecules such as LanM may offer a new paradigm for extractive metallurgy and other applications involving felements.

Protein-Based Platform for Purification, Chelation, and Study of Medical Radiometals: Yttrium and Actinium

“…CD experiments using La 3+ , a non-radioactive Ac 3+ analogue, yielded a Kd,app of 1.8(±0.1) pM for nat La3LanM (Figure S12). The sub-picomolar stability of Ac3LanM extends a trend previously noted for the early lanthanides 17,26 where LanM displays higher affinity for cations with lower Lewis acidity (Figure 4f). This reversed preference is remarkable when compared to small chelators 42 used in medicine since they typically exhibit decreasing affinity 43 as the metal gets larger (Figure S14), leading to weak La 3+ or Ac 3+ complexes.…”

“…By contrast, LanM displays unprecedented selectivity for trivalent lanthanides against most other cations. 17,25,26 Instead of using minor differences in extraction reaction constants (as with current extractants, resulting in arduous metal ion separations 11,12 ), a selective macromolecular extractant such as LanM (~12 kDa, hydrodynamic radius ~4 nm 26 ) would enable a new and more robust mechanism for recovery and purification of radiometals based on size differences. We hypothesized that protein-bound versus unbound metal ions could easily be separated using inexpensive methods like spin filtration or size exclusion chromatography (Figure 1b-c).…”

“…This approach revealed that LanM has very strong affinity for Ac 3+ , with a log β31 of 36.2±0.5 for 228 Ac3LanM at pH 7, equivalent to an average Kd of 865 fM per site (Figure 4c-f). We previously showed 17,26,41 that lanthanide ions form polymetallic complexes with LanM, Ln3LanM, at macroscopic concentrations (micromolar to millimolar range). However, LanM has four metal binding sites 17 and it was unclear whether the fourth site could play a role in the binding of actinium.…”

E3 Ubiquitin Ligase SPL2 Is a Lanthanide-Binding Protein

“…These features make LBTs particularly well-suited for luminescence and imaging and explain why Ln-based probes have gained much interest in recent years. Some of the latest applications involve phasing protein X-ray crystal structures using only the anomalous signal [ 2 ] or in vivo visualization of specific proteins tagged with X-ray-sensitive peptide sequences to improve the image resolution beyond the diffraction limit of visible light [ 3 ].…”

“…Another natural, and probably the best-studied, rare earth element (REE)-binding protein is lanmodulin (LanM). Since some REEs are now widely used in high-technology industries, LanM offers an attractive possibility for extracting rare earth metals, especially since the protein offers unprecedented selectivity against non-REE elements under acidic pH and high temperature while outperforming many chemical processes currently in use [ 3 ]. Consequently, lanthanides are now regarded as essential cofactors in certain enzymes of methylotrophic bacteria.…”