# Rare Earth Element-Induced Condensation of the Block V of the Repeats-in-Toxin Domain from CyaA from Bordetella pertussis for Separations

## TL;DR

Scientists used a protein from Bordetella pertussis to create structures that bind rare earth elements, offering a new way to separate these metals without harmful solvents.

## Contribution

A novel protein-based method for rare earth element separation using Ln-induced phase separation of RTX domain peptides.

## Key findings

- Coral-like cylindrical structures formed with Ln3+-RTX complexes containing ~8 trivalent metals per peptide.
- Structural organization of RTX domains coordinating REEs was revealed using imaging and X-ray scattering.
- Nanosized, metal-rich structures naturally condense, demonstrating potential for bioseparations.

## Abstract

Rare earth elements (REEs) are critical for the development
of
a range of new technologies. However, the current industrial separation
processes of these metals from natural sources, recycled materials,
and industrial effluents involve the large consumption of organic
solvents, resulting in a sizable environmental footprint. We aim to
exploit the high affinity of the block V peptide of the repeats-in-toxin
(RTX) domain of the adenylate cyclase protein from
Bordetella pertussis
for the separation
of REEs. This peptide selectively binds with lanthanide (Ln) cations
and can undergo Ln-induced phase separation, which can be used in
bioseparation processes. Here, we evaluated the self-assembling structures
of complexes of the RTX domain peptide folded in the presence of Ln3+ cations. Size distribution and surface potential measurements
of complexes were taken to understand the Ln-induced changes in the
complexed peptide. Transmission electron microscopy imaging was used
to explore the structures of complexes, while anomalous small-angle
X-ray scattering measurements were used to determine the distribution
of Ln3+ ions within the protein-based macrostructures.
In the presence of excess Ln3+, we observed the formation
of coral-like cylindrical structures comprised of Ln3+-RTX
complexes, with approximately eight trivalent metals per peptide within
the nanosized assemblies. These findings provide new insights into
the structural organization of assembled RTX domains and their ability
to coordinate with REEs, forming nanosized, metal-rich structures
that naturally condense, providing a proof-of-concept for protein-based
separation processes of these critical materials.