Characterization of Americium and Curium Complexes with the Protein Lanmodulin: A Potential Macromolecular Mechanism for Actinide Mobility in the Environment

Abstract

Anthropogenic radionuclides, including long-lived heavy actinides such as americium and curium, represent the primary long-term challenge for management of nuclear waste. The potential release of these wastes into the environment necessitates understanding their interactions with biogeochemical compounds present in nature. Here, we characterize the interactions between the heavy actinides, Am3+ and Cm3+, and the natural lanthanide-binding protein, lanmodulin (LanM). LanM is produced abundantly by methylotrophic bacteria, including Methylorubrum extorquens, that are widespread in the environment. We determine the first stability constant for an Am3+-protein complex (Am3LanM) and confirm the results with Cm3LanM, indicating a ∼5-fold higher affinity than that for lanthanides with most similar ionic radius, Nd3+ and Sm3+, and making LanM the strongest known heavy actinide-binding protein. The protein’s high selectivity over 243Am’s daughter nuclide 239Np enables lab-scale actinide-actinide separations as well as provides insight into potential protein-driven mobilization for these actinides in the environment. The luminescence properties of the Cm3+-LanM complex, and NMR studies of Gd3+-LanM, reveal that lanmodulin-bound f-elements possess two coordinated solvent molecules across a range of metal ionic radii. Finally, we show under a wide range of environmentally relevant conditions that lanmodulin effectively outcompetes desferrioxamine B, a hydroxamate siderophore previously proposed to be important in trivalent actinide mobility. These results suggest that natural lanthanide-binding proteins such as lanmodulin may play important roles in speciation and mobility of actinides in the environment; it also suggests that protein-based biotechnologies may provide a new frontier in actinide remediation, detection, and separations.

Engineering lanmodulin's selectivity for actinides over lanthanides by controlling solvent coordination and second-sphere interactions

“…WT LanM strongly outcompetes DFOB in this pH range, with 50% of Cm 3 LanM still formed in the presence of ∼1800 equivalents of DFOB. These results are in excellent agreement with independent measurements 55 at different pH values and slightly different Cm 3+ concentrations. The results indicate that the 3D 9 N and 3D 9 H variants have lower affinity for Cm 3+ than WT LanM.…”

“…Thermodynamic data on americium–protein species are scarce, and the five new stability constants for such complexes reported herein more than doubles the total number previously published. Besides these and the WT Am–LanM complex, 55 the only other reported examples are siderocalin–siderophore–americium adducts 45 with K d values of 240–29 000 pM, at pH 7.4, i.e. , orders of magnitude less stable than for LanM and its variants.…”

“…Radiochemistry experiments contrasted with circular dichroism measurements revealed 54 that the actinium complex of LanM is more stable than its lanthanum counterpart with K d values of 0.865 pM and 1.8 pM (at pH 7.0) for Ac 3 LanM and La 3 LanM, respectively. Using independent spectroscopic techniques, we determined 55 the stability constants of the Am 3+ and Cm 3+ complexes of LanM and also found the actinide complexes to be more stable than their lanthanide analogues, with K d values of 1.3 and 1.2 pM (at pH 5.0) for Am 3 LanM and Cm 3 LanM, compared to ∼10–20 pM for Pr 3 LanM, Nd 3 LanM, and Sm 3 LanM. It was also found that NpO 2 + does not interact significantly with LanM, allowing its convenient separation from the LanM-bound Am 3+ by simple filtration.…”

Americium preferred: lanmodulin, a natural lanthanide-binding protein favors an actinide over lanthanides

“…21 Previously, an NMR structure of LanM with Y 3+ has been reported, and during the review process of this manuscript, a study suggesting a similar coordination behavior for An (Am and Cm) was published. 19,22 Here, we evaluate the potential of LanM to bind the An neptunium, plutonium, americium and curium and compare the relative binding affinity ǂ of these An to each other and with the lanthanide series (Figure 1) at pH 6.7 to mimic physiological conditions. The isolation of americium from lanthanides is of particular interest, as they occur commonly together in nuclear waste and are challenging to separate.…”

A Natural Lanthanide-Binding Protein Facilitates Separation and Recovery of Rare Earth Elements

“… 6 The young field of lanthanides in biology has already made an impact on other chemistry-related fields such as Ln-based catalysis 7 as well as actinide chelation ( Figure 2 ). 8 , 9 Open questions remain (i.e., on Ln-uptake by bacteria), and their answers will certainly push and advance the field of biobased REE sequestration and separation and maybe even inspire new medical applications of REE and actinides. For example, small chelators 10 (lanthanophores) that have been proposed to be excreted by Ln-utilizing bacteria could, along with the existing identified proteins and chelators, be used in medicine (e.g., to chelate Gd 3+ or radiometals such as 90 Y).…”