(735bx) Protein-Based Isocratic Separation of Rees Using Newly Identified Calcium-Binding Proteins.

Rare earth elements comprising the 15 lanthanides, yttrium, and scandium are critical for a green economy. Conventional chemical separation of these vital elements is complex, and the process is often compared to fine chemical processing, not commodity metals. Biological systems have evolved to uptake and utilize calcium, which shares similar physical and chemical properties with lanthanides, such as ionic radius and coordination geometry. We identified possible selective lanthanide-binding proteins via a bioinformatic search of calcium-binding proteins and modeling. The top 10 most promising candidates were expressed as a fusion of fluorescence resonance energy transfer (FRET) pair cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP). The selectivity of the proteins to the ions within the lanthanide series were evaluated using FRET. The results improve our understanding of the effect of the size and chemistry of the binding pocket and the secondary structure of the protein as a whole on the ion selectivity. The repeat in toxin (RTX) and HEW5 domains had the highest selectivity, with the opposite preference for heavy and light REEs. The selected domains have high capacity with 8-10 binding lanthanide sites per protein (15 kDa) and are pH- and temperature-tolerant. The selected proteins were further characterized and used to separate rare earth elements (REEs) from one another and from non-REEs present in both primary and secondary sources with high yield and purity under isocratic chromatography conditions without the use of chelators.

This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions and/or findings expressed are those of the author and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government. Approved for Public Release, Distribution Unlimited.