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The biological relevance of early lanthanides, such as neodymium, remained undiscovered until methylotrophic bacteria with lanthanide-dependent metabolism were identified. The respective strains incorporate these elements into the active site of their key metabolic enzyme methanol dehydrogenase (MDH). Growth studies with the strictly lanthanide-dependent thermoacidophile Methylacidiphilum fumariolicum SolV and the mesophilic Methylorubrum extorquens AM1 ΔmxaF mutant demonstrate that the trivalent actinides americium and curium support growth in the absence of the essential lanthanides. In fact, the bacteria make no distinction between lanthanide and actinide ions if they have the correct size and oxidation state. Time-resolved laser-induced fluorescence spectroscopy, liquid scintillation counting, and inductively coupled plasma mass spectrometry confirm the bacterial uptake. The interchangeability of f-block elements is supported by very similar enzymatic activities of recombinant methanol dehydrogenase reconstituted with different metal ions. Our combined in vivo and in vitro results firmly establish that actinides support growth of methylotrophic bacteria, suggesting a potential biological role for these radioactive elements. Importantly, bacteria capable of utilizing actinides will also be able to mobilize these elements in the environment. This may lead to applications for bioremediation and recycling/separation of lanthanides and actinides.

Since the discovery of the biological relevance of lanthanides (Lns) for methylotrophic bacteria in the last decade, the field has seen a steady rise in discoveries of bacteria using Lns. The major role of lanthanides here is in the active sites of enzymes: methanol dehydrogenases. Additionally, lanthanide binding proteins have also been identified. One such protein is lanmodulin (LanM) and, with a remarkable selectivity for Lns over Ca(II) and affinities in the picomolar range, it makes an attractive target to address challenges in lanthanide separation. Why LanM has such a high selectivity is currently not entirely understood, both the specific amino acid sequences of the EF-hand loops, together with cooperativity effects have been suggested. Consequently, we decided to remove the effect of cooperativity by focusing on the amino acid level. Thus, we synthesized all four 12-amino acid EF-Hand loop peptides of LanM using solid phase peptide synthesis and investigated their affinity for Lns (Eu(III), Tb(III)), the actinide Cm(III) and Ca(II). Using isothermal titration calorimetry and time resolved laser fluorescence spectroscopy combined with parallel factor analysis, we show that in the absence of cooperativity the short EF-Hand loop peptides have all similar affinities for lanthanides and that these are all in the micromolar range. Furthermore, calcium was shown not to bind to the peptides which was verified with circular dichroism spectroscopy. This technique also revealed that the peptides undergo a change to a more ordered state when lanthanides are added. These experimental observations were further supported by molecular dynamics simulations. Lastly, we put Eu(III) and Cm(III) in direct competition using TRLFS. Remarkably, a slightly higher affinity for the actinide, as was also observed for LanM, was found. Our results demonstrate that the picomolar affinities in LanM are largely an effect of pre-structuring in the full protein and therefore reduction of flexibility in combination with cooperative effects, and that all EF-Hand loops possess similar affinities when detached from the protein backbone, albeit still retaining the high selectivity for lanthanides and actinides over calcium.

The mechanism by which oxygen bound in UO22+ exchanges with that from water under strong alkaline conditions remains a subject of controversy. Two recent NMR studies independently revealed that the key intermediate species is a binuclear uranyl(VI) hydroxide, presumably of the stoichiometry [(UO2(OH)42−)(UO2(OH)53−)]. The presence of UO2(OH)53− in highly alkaline solution was postulated in earlier experimental studies, yet the species has been little characterized. Quantum-chemical calculations (DFT and MP2) show that hydrolysis of UO2(OH)42− yields UO3(OH)33− preferentially over UO2(OH)53−. X-ray absorption spectroscopy was used to study the uranium(VI) speciation in a highly alkaline solution supporting the existence of a species with three U−O bonds, as expected for UO3(OH)33−. Therefore, we explored the oxygen exchange pathway through the binuclear adduct [(UO2(OH)42−)(UO3(OH)33−)] by quantum-chemical calculations. Assuming that the rate-dominating step is proton transfer between the oxygen atoms, the activation Gibbs energy for the intramolecular proton transfer within [(UO2(OH)42−)(UO3(OH)33−)] at the B3LYP level was estimated to be 64.7 kJ mol−1. This value is in good agreement with the activation energy for “yl”−oxygen exchange in [(UO2(OH)42−)(UO2(OH)53−)] obtained from experiment by Szabó and Grenthe (Inorg. Chem. 2010, 49, 4928−4933), which is 60.8 ± 2.4 kJ mol−1. Both the presence of UO3(OH)33− and the scenario of an “yl”−oxygen exchange through a binuclear species in strong alkaline solution are supported by the present study.