How tobacco (Nicotiana tabacum) BY-2 cells cope with Eu(III) - a microspectroscopic study.

The extensive use of lanthanides in science, industry and high-technology products is accompanied by an anthropogenic input of rare earth elements into the environment. Knowledge of a metal's environmental fate is essential for reasonable risk assessment and remediation approaches. In the present study, Eu(III) was representatively used as a luminescent probe to study the chemical environment and to elucidate the molecular interactions of lanthanides with a suspension cell culture of Nicotiana tabacum BY-2. Biochemical methods were combined with luminescence spectroscopy, two-dimensional microspectroscopic mappings, and data deconvolution methods to resolve the bioassociation behavior and spatial distribution of Eu(III) in plant cells. BY-2 cells were found to gradually take up the metal after exposure to 100 µM Eu(III) without significant loss of viability. Time-resolved luminescence measurements were used to specify the occurrence of Eu(III) species as a function of time, revealing the transformation of an initial Eu(III) species into another after 24 h exposure. Chemical microscopy and subsequent iterative factor analysis reveal the presence of four distinct Eu(III) species located at different cellular compartments, e.g., the cell nucleus, nucleolus and cell walls, which could be assigned to intracellular binding motifs. In addition, a special type of bioaccumulation occurs through the formation of a Eu(III)-containing oxalate biomineral, which is already formed within the first 24 hours after metal exposure. Oxalate crystals were also obtained in analogous experiments with Gd and Sm. These results indicate that tobacco BY-2 cells induce the precipitation of metal oxalate biominerals for detoxification of lanthanides, although they also bind to other cellular ligands at the same time.

Equilibrium Thermodynamics of Macropa Complexes with Selected Metal Isotopes of Radiopharmaceutical Interest.

To pursue the design of in vivo stable chelating systems for radiometals, a concise and straightforward method toolbox was developed combining NMR, isothermal titration calorimetry (ITC), and europium time-resolved laser-induced fluorescence spectroscopy (Eu-TRLFS). For this purpose, the macropa chelator was chosen, and Lu3+, La3+, Pb2+, Ra2+, and Ba2+ were chosen as radiopharmaceutically relevant metal ions. They differ in charge (2+ and 3+) and coordination properties (main group vs lanthanides). 1H NMR was used to determine four pKa values (±0.15; carboxylate functions, 2.40 and 3.13; amino functions, 6.80 and 7.73). Eu-TRLFS was used to validate the exclusive existence of the 1:1 Mn+/ligand complex in the chosen pH range at tracer level concentrations. ITC measurements were accomplished to determine the resulting stability constants of the desired complexes, with log K values ranging from 18.5 for the Pb-mcp complex to 7.3 for the Lu-mcp complex. Density-functional-theory-calculated structures nicely mirror the complexes' order of stabilities by bonding features. Radiolabeling with macropa using ligand concentrations from 10-3 to 10-6 M was accomplished by pointing out the complex formation and stability (212Pb > 133La > 131Ba ≈ 224Ra > 177Lu) by means of normal-phase thin-layer chromatography analyses.

Europium(III) as luminescence probe for interactions of a sulfate-reducing microorganism with potentially toxic metals.

Microorganisms show a high affinity for trivalent actinides and lanthanides, which play an important role in the safe disposal of high-level radioactive waste as well as in the mining of various rare earth elements. The interaction of the lanthanide Eu(III) with the sulfate-reducing microorganism Desulfosporosinus hippei DSM 8344T, a representative of the genus Desulfosporosinus that naturally occurs in clay rock and bentonite, was investigated. Eu(III) is often used as a non-radioactive analogue for the trivalent actinides Pu(III), Am(III), and Cm(III), which contribute to a major part of the radiotoxicity of the nuclear waste. D. hippei DSM 8344T showed a weak interaction with Eu(III), most likely due to a complexation with lactate in artificial Opalinus Clay pore water. Hence, a low removal of the lanthanide from the supernatant was observed. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy revealed a bioprecipitation of Eu(III) with phosphates potentially excreted from the cells. This demonstrates that the ongoing interaction mechanisms are more complex than a simple biosorption process. The bioprecipitation was also verified by luminescence spectroscopy, which showed that the formation of the Eu(III) phosphate compounds starts almost immediately after the addition of the cells. Moreover, chemical microscopy provided information on the local distribution of the different Eu(III) species in the formed cell aggregates. These results provide first insights into the interaction mechanisms of Eu(III) with sulfate-reducing bacteria and contribute to a comprehensive safety concept for a high-level radioactive waste repository, as well as to a better understanding of the fate of heavy metals (especially rare earth elements) in the environment.

Localization and chemical speciation of europium(III) in Brassica napus plants.

For the reliable safety assessment of repositories of highly radioactive waste, further development of the modelling of radionuclide migration and transfer in the environment is necessary, which requires a deeper process understanding at the molecular level. Eu(III) is a non-radioactive analogue for trivalent actinides, which contribute heavily to radiotoxicity in a repository. For in-depth study of the interaction of plants with trivalent f elements, we investigated the uptake, speciation, and localization of Eu(III) in Brassica napus plants at two concentrations, 30 and 200 µM, as a function of the incubation time up to 72 h. Eu(III) was used as luminescence probe for combined microscopy and chemical speciation analyses of it in Brassica napus plants. The localization of bioassociated Eu(III) in plant parts was explored by spatially resolved chemical microscopy. Three Eu(III) species were identified in the root tissue. Moreover, different luminescence spectroscopic techniques were applied for an improved Eu(III) species determination in solution. In addition, transmission electron microscopy combined with energy-dispersive X-ray spectroscopy was used to localize Eu(III) in the plant tissue, showing Eu-containing aggregates. By using this multi-method setup, a profound knowledge on the behavior of Eu(III) within plants and changes in its speciation could be obtained, showing that different Eu(III) species occur simultaneously within the root tissue and in solution.

Europium(III) Meets Etidronic Acid (HEDP): A Coordination Study Combining Spectroscopic, Spectrometric, and Quantum Chemical Methods.

Etidronic acid (1-Hydroxyethylidene-1,1-diphosphonic acid, HEDP, H4L) is a proposed decorporation agent for U(VI). This paper studied its complex formation with Eu(III), an inactive analog of trivalent actinides, over a wide pH range, at varying metal-to-ligand ratios (M:L) and total concentrations. Combining spectroscopic, spectrometric, and quantum chemical methods, five distinct Eu(III)-HEDP complexes were found, four of which were characterized. The readily soluble EuH2L+ and Eu(H2L)2- species with log ß values of 23.7 ± 0.1 and 45.1 ± 0.9 are formed at acidic pH. At near-neutral pH, EuHL0s forms with a log ß of ~23.6 and, additionally, a most probably polynuclear complex. The readily dissolved EuL- species with a log ß of ~11.2 is formed at alkaline pH. A six-membered chelate ring is the key motif in all solution structures. The equilibrium between the Eu(III)-HEDP species is influenced by several parameters, i.e., pH, M:L, total Eu(III) and HEDP concentrations, and time. Overall, the present work sheds light on the very complex speciation in the HEDP-Eu(III) system and indicates that, for risk assessment of potential decorporation scenarios, side reactions of HEDP with trivalent actinides and lanthanides should also be taken into account.

Eu(III) and Cm(III) Complexation by the Aminocarboxylates NTA, EDTA, and EGTA Studied with NMR, TRLFS, and ITC-An Improved Approach to More Robust Thermodynamics.

The complex formation of Eu(III) and Cm(III) was studied via tetradentate, hexadentate, and octadentate coordinating ligands of the aminopolycarboxylate family, viz., nitrilotriacetate (NTA3-), ethylenediaminetetraacetate (EDTA4-), and ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetate (EGTA4-), respectively. Based on the complexones' pKa values obtained from 1H nuclear magnetic resonance (NMR) spectroscopic pH titration, complex formation constants were determined by means of the parallel-factor-analysis-assisted evaluation of Eu(III) and Cm(III) time-resolved laser-induced fluorescence spectroscopy (TRLFS). This was complemented by isothermal titration calorimetry (ITC), providing the enthalpy and entropy of the complex formation. This allowed us to obtain genuine species along with their molecular structures and corresponding reliable thermodynamic data. The three investigated complexones formed 1:1 complexes with both Eu(III) and Cm(III). Besides the established Eu(III)-NTA 1:1 and 1:2 complexes, we observed, for the first time, the existence of a Eu(III)-NTA 2:2 complex of millimolar metal and ligand concentrations. Demonstrated for thermodynamic studies on Eu(III) and Cm(III) interaction with complexones, the utilized approach is commonly applicable to many other metal-ligand systems, even to high-affinity ligands.

Minor Actinides Can Replace Essential Lanthanides in Bacterial Life.

Certain f-block elements-the lanthanides-have biological relevance in the context of methylotrophic bacteria. The respective strains incorporate these 4 f elements into the active site of one of their key metabolic enzymes, a lanthanide-dependent methanol dehydrogenase. In this study, we investigated whether actinides, the radioactive 5 f elements, can replace the essential 4 f elements in lanthanide-dependent bacterial metabolism. Growth studies with Methylacidiphilum fumariolicum SolV and the Methylobacterium extorquens AM1 ΔmxaF mutant demonstrate that americium and curium support growth in the absence of lanthanides. Moreover, strain SolV favors these actinides over late lanthanides when presented with a mixture of equal amounts of lanthanides together with americium and curium. Our combined in vivo and in vitro results establish that methylotrophic bacteria can utilize actinides instead of lanthanides to sustain their one-carbon metabolism if they possess the correct size and a +III oxidation state.

Not just a background: pH buffers do interact with lanthanide ions-a Europium(III) case study.

The interaction between Eu(III) ion and different pH buffers, popular in biology and biochemistry, viz. HEPES, PIPES, MES, MOPS, and TRIS, has been studied by solution nuclear magnetic resonance spectroscopy (NMR) and time-resolved laser-induced fluorescence spectroscopy (TRLFS) techniques. The Good's buffers reveal non-negligible interaction with Eu(III) as determined from their complex stability constants, where the sites of interaction are the morpholine and piperazine nitrogen atoms, respectively. In contrast, TRIS buffer shows practically no affinity towards Eu(III). Therefore, when investigating lanthanides, TRIS buffer should be preferred over Good's buffers.

Studies of pyrroloquinoline quinone species in solution and in lanthanide-dependent methanol dehydrogenases.

Pyrroloquinoline quinone (PQQ) is a redox cofactor in calcium- and lanthanide-dependent alcohol dehydrogenases that has been known and studied for over 40 years. Despite its long history, many questions regarding its fluorescence properties, speciation in solution and in the active site of alcohol dehydrogenase remain open. Here we investigate the effects of pH and temperature on the distribution of different PQQ species (H3PQQ to PQQ3- in addition to water adducts and in complex with lanthanides) with NMR and UV-Vis spectroscopy as well as time-resolved laser-induced fluorescence spectroscopy (TRLFS). Using a europium derivative from a new, recently-discovered class of lanthanide-dependent methanol dehydrogenase (MDH) enzymes, we utilized two techniques to monitor Ln binding to the active sites of these enzymes. Employing TRLFS, we were able to follow Eu(III) binding directly to the active site of MDH using its luminescence and could quantify three Eu(III) states: Eu(III) in the active site of MDH, but also in solution as PQQ-bound Eu(III) and in the aquo-ion form. Additionally, we used the antenna effect to study PQQ and simultaneously Eu(III) in the active site.

Spatially resolved Eu(III) environments by chemical microscopy.

Chemical microscopy combines high-resolution emission spectra with Abbe-limited spatial resolution and is used for studies of inhomogeneous samples at the (sub-)micronscale. The spatial distinction of multiple Eu(III) coordination sites allows for a comprehensive understanding of environmental samples and highlights the applicability of Eu(III) as a molecular probe in medicine and biology.

Investigation of the structure and dynamics of gallium binding to high-affinity peptides elucidated by multi-scale simulation, quantum chemistry, NMR and ITC.

Gallium (as Ga3+) is a Group IIIa metal and its recovery from wastewaters has become increasingly important for its reuse. The use of peptides for recycling offers a low-cost and environmentally-friendly option but the structural characteristics of peptides likely to bind Ga3+ are largely unknown. Multiple computational methods, coupled with experimental verification via NMR and Isothermal Calorimetry (ITC), were used to establish that Ga3+ binds with high affinity to peptide sequences and to elucidate the structural characteristics that contributed. It was demonstrated that peptide pre-organisation is key to Ga3+ binding and that a favourable binding position is necessarily governed by the size and shape of the electrostatic environment as much as individual electrostatic interactions with peptide residues themselves. Given favourable conditions, Ga3+ retrieved plausible binding positions involving both charged and uncharged residues that greatly increases the range of bonding possibilities with other peptide sequences and offers insights for binding other metals. The addition of pH buffer substantially improved the affinity of Ga3+ and a structural role for a buffer component was demonstrated.

Quenching Mechanism of Uranyl(VI) by Chloride and Bromide in Aqueous and Non-Aqueous Solutions.

A major hindrance in utilizing uranyl(VI) luminescence as a standard analytical tool, for example, in environmental monitoring or nuclear industries, is quenching by other ions such as halide ions, which are present in many relevant matrices of uranyl(VI) speciation. Here, we demonstrate through a combination of time-resolved laser-induced fluorescence spectroscopy, transient absorption spectroscopy, and quantum chemistry that coordinating solvent molecules play a crucial role in U(VI) halide luminescence quenching. We show that our previously suggested quenching mechanism based on an internal redox reaction of the 1:2-uranyl-halide-complex holds also true for bromide-induced quenching of uranyl(VI). By adopting specific organic solvents, we were able to suppress the separation of the oxidized halide ligand X2·- and the formed uranyl(V) into fully solvated ions, thereby "reigniting" U(VI) luminescence. Time-dependent density functional theory calculations show that quenching occurs through the outer-sphere complex of U(VI) and halide in water, while the ligand-to-metal charge transfer is strongly reduced in acetonitrile.

Uranium(VI) Complexes of Glutathione Disulfide Forming in Aqueous Solution.

The interactions between glutathione disulfide, GSSG, the redox partner and dimer of the intracellular detoxification agent glutathione, GSH, and hexavalent uranium, U(VI), were extensively studied by solution NMR (in D2O), complemented by time-resolved laser-induced fluorescence and IR spectroscopies. As expected for the hard Lewis acid U(VI), coordination facilitates by the ligands' O-donor carboxyl groups. However, owing to the adjacent cationic α-amino group, the glutamyl-COO reveal monodentate binding, while the COO of the glycyl residues show bidentate coordination. The log K value for the reaction UO22+ + H3GSSG- → UO2(H3GSSG)+ (pH 3, 0.1 M NaClO4) was determined for the first time, being 4.81 ± 0.08; extrapolation to infinite dilution gave log K⊖ = 5.24 ± 0.08. U(VI) and GSSG form precipitates in the whole pD range studied (2-8), showing least solubility for 4 < pD < 6.5. Thus, particularly GSSG, hereby representing also other peptides and small proteins, affects the mobility of U(VI), strongly depending on the speciation of either component.

Thermodynamic and Structural Studies on the Ln(III)/An(III) Malate Complexation.

The complexation of the trivalent lanthanides Nd(III) and Eu(III) and of the actinide Am(III) with malate was studied using a multi-method approach. The combination of structural and thermodynamic studies was required for the interpretation of the stoichiometry and thermodynamic data (log ß0, Δr H0m, Δr S0m, Δr G0m) of the lanthanide/actinide malate complexes leading to a profound molecular understanding of the system. The structure-sensitive methods vibrational spectroscopy and extended X-ray absorption fine structure spectroscopy complemented with quantum-mechanical ab initio molecular dynamics calculations revealed a tridentate ring structure of the respective metal complexes. The metal is coordinated by two carboxylate groups and a hydroxyl group. UV-vis, laser fluorescence, and calorimetric studies consistently yielded two complex species having a 1:1 and a 1:2 (metal/malate) stoichiometry. Parallel factor analysis and iterative transformation factor analysis were applied to decompose experimental spectra into their single components and to determine stability constants. The 1:1 and 1:2 Nd(III) malate complexation constants determined by isothermal titration calorimetry were extrapolated to zero ionic strength using the specific ion interaction theory, yielding log ß10 and log ß20 of about 6 and 9, respectively. The respective complexation enthalpies Δr H0m,1 and Δr H0m,2 showed average values of 5 kJ·mol-1 which are typical for small organic molecules. The comparison of Nd(III) and Am(III) malate complexes showed that the malate binding motif, the speciation, and the thermodynamics can be transferred from lanthanides(III) to actinides(III) supporting the 4f/5f element homology.

Cm3+/Eu3+ induced structural, mechanistic and functional implications for calmodulin.

Trivalent actinides and their lanthanide homologues are being scrutinized for their potential health risk when ingested as a result of a range of industrial activities such as mining. Importantly, these ions are known to exhibit high affinity towards calmodulin (CaM). In case of their inadvertent uptake, the holoproteins that are occupied by these cations may block signal transduction pathways or increase the concentration of these ions in intact cells, which could lead to accumulation in human organs. Accordingly, this investigation employed spectroscopy, computational chemistry, calorimetry, and biochemistry to study the results of metal ion substitution on the protein structure, enzymatic activity and chemo- and cytotoxicity of An3+/Ln3+ ions. As will be demonstrated herein, our data confirm the higher affinity of Cm3+ and Eu3+ compared to Ca2+ to all 4 binding sites of CaM, with one site differing from the remaining three. This higher-affinity site will complex Eu3+ in an exothermic fashion; in contrast, ion binding to the three lower-affinity EF-hands was found to be endothermic. The overall endothermic binding process is ascribed to the loss of the hydration shells of the trivalent ions upon protein binding. These findings are supported by extensive quantum chemical calculations of full holo-CaM, which were performed at the MP2 level using the fragment molecular orbital method. The exceptional binding site (EF-hand 3) features fewer negatively charged residues compared to the other EF-hands, thereby allowing Eu3+ and Cm3+ to carry one or two additional waters compared to Ca2+-CaM, while also causing the structure of Cm3+/Eu3+-CaM to become slightly disordered. Moreover, the enzymatic activity decreases somewhat in comparison to Ca2+-CaM. By utilizing a combination of techniques, we were able to generate a comprehensive picture of the CaM-actinide/lanthanide system from the molecular level to its functional impact. Such knowledge could also be applied to other metal-binding proteins.

Impact of Haloarchaea on Speciation of Uranium-A Multispectroscopic Approach.

Haloarchaea represent a predominant part of the microbial community in rock salt, which can serve as host rock for the disposal of high level radioactive waste. However, knowledge is missing about how Haloarchaea interact with radionuclides. Here, we used a combination of spectroscopic and microscopic methods to study the interactions of an extremely halophilic archaeon with uranium, one of the major radionuclides in high level radioactive waste, on a molecular level. The obtained results show that Halobacterium noricense DSM 15987T influences uranium speciation as a function of uranium concentration and incubation time. X-ray absorption spectroscopy reveals the formation of U(VI) phosphate minerals, such as meta-autunite, as the major species at a lower uranium concentration of 30 µM, while U(VI) is mostly associated with carboxylate groups of the cell wall and extracellular polymeric substances at a higher uranium concentration of 85 µM. For the first time, we identified uranium biomineralization in the presence of Halobacterium noricense DSM 15987T cells. These findings highlight the potential importance of Archaea in geochemical cycling of uranium and their role in biomineralization in hypersaline environments, offering new insights into the microbe-actinide interactions in highly saline conditions relevant to the disposal of high-level radioactive waste as well as bioremediation.

Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl].

For the only water coordinated "free" uranyl(VI) aquo ion in perchlorate solution we identified and assigned several different excited states and showed that the 3Δ state is the luminescent triplet state from transient absorption spectroscopy. With additional data from other spectroscopic methods (TRLFS, UV/vis) we generated a detailed Jablonski diagram and determined rate constants for several state transitions, like the inner conversion rate constant from the 3Φ state to the 3Δ state transition to be 0.35 ps-1. In contrast to luminescence measurements, it was possible to observe the highly quenched uranyl(VI) ion in highly concentrated chloride solution by TAS and we were able to propose a dynamic quenching mechanism, where chloride complexation is followed by the charge transfer from the excited state uranyl(VI) to chloride. This proposed quenching route is supported by TD-DFT calculations.

Speciation Studies of Metals in Trace Concentrations: The Mononuclear Uranyl(VI) Hydroxo Complexes.

A direct luminescence spectroscopic experimental setup for the determination of complex stability constants of mononuclear uranyl(VI) hydrolysis species is presented. The occurrence of polynuclear species is prevented by using a low uranyl(VI) concentration of 10­8 M (2.4 ppb). Time-resolved laser-induced fluorescence spectra were recorded in the pH range from 3 to 10.5. Deconvolution with parallel factor analysis (PARAFAC) resulted in three hydrolysis complexes. A tentative assignment was based on thermodynamic calculations: UO22+, UO2(OH)+, UO2(OH)2, UO2(OH)3­. An implementation of a Newton­Raphson algorithm into PARAFAC allowed a direct extraction of complex stability constants during deconvolution yielding log(ß1M,1°C)1:1 = −4.6, log(ß1M,1°C)1:2 = −12.2, log(ß1M,1°C)1:3 = −22.3. Extrapolation to standard conditions gave log(ß0)1:1 = −3.9, log(ß0)1:2 = −10.9, and log(ß0)1:3 = −20.7. Luminescence characteristics (band position, lifetime) of the individual mononuclear hydroxo species were derived to serve as a reference data set for further investigations. A correlation of luminescence spectroscopic features with Raman frequencies was demonstrated for the mononuclear uranyl(VI) hydroxo complexes for the first time. Thereby a signal-to-structure correlation was achieved and the complex assignment validated.

Uranium(VI) chemistry in strong alkaline solution: speciation and oxygen exchange mechanism.

The mechanism by which oxygen bound in UO2(2+) 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)4(2-))(UO2(OH)5(3-))]. The presence of UO2(OH)5(3-) 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)4(2-) yields UO3(OH)3(3-) preferentially over UO2(OH)5(3-). 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)3(3-). Therefore, we explored the oxygen exchange pathway through the binuclear adduct [(UO2(OH)4(2-))(UO3(OH)3(3-))] 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)4(2-))(UO3(OH)3(3-))] 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)4(2-))(UO2(OH)5(3-))] 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)3(3-) and the scenario of an "yl"-oxygen exchange through a binuclear species in strong alkaline solution are supported by the present study.

Effect of Ba(II), Eu(III), and U(VI) on rat NRK-52E and human HEK-293 kidney cells in vitro.

Heavy metals pose a potential health risk to humans when they enter the organism. Renal excretion is one of the elimination pathways and, therefore, investigations with kidney cells are of particular interest. In the present study, the effects of Ba(II), Eu(III), and U(VI) on rat and human renal cells were investigated in vitro. A combination of microscopic, biochemical, analytical, and spectroscopic methods was used to assess cell viability, cell death mechanisms, and intracellular metal uptake of exposed cells as well as metal speciation in cell culture medium and inside cells. For Eu(III) and U(VI), cytotoxicity and intracellular uptake are positively correlated and depend on concentration and exposure time. An enhanced apoptosis occurs upon Eu(III) exposure whereas U(VI) exposure leads to enhanced apoptosis and (secondary) necrosis. In contrast to that, Ba(II) exhibits no cytotoxic effect at all and its intracellular uptake is time-independently very low. In general, both cell lines give similar results with rat cells being more sensitive than human cells. The dominant binding motifs of Eu(III) in cell culture medium as well as cell suspensions are (organo-) phosphate groups. Additionally, a protein complex is formed in medium at low Eu(III) concentration. In contrast, U(VI) forms a carbonate complex in cell culture medium as well as each one phosphate and carbonate complex in cell suspensions. Using chemical microscopy, Eu(III) was localized in granular, vesicular compartments near the nucleus and the intracellular Eu(III) species equals the one in cell suspensions. Overall, this study contributes to a better understanding of the interactions of Ba(II), Eu(III), and U(VI) on a cellular and molecular level. Since Ba(II) and Eu(III) serve as inactive analogs of the radioactive Ra(II) and Am(III)/Cm(III), the results of this study are also of importance for the health risk assessment of these radionuclides.