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Supramolecular assembly of peptides and proteins into amyloid fibrils is of multifold interest, going from materials science to physiopathology. The binding of metal ions to amyloidogenic peptides is associated with several amyloid diseases, and amyloids with incorporated metal ions are of interest in nanotechnology. Understanding the mechanisms of amyloid formation and the role of metal ions can improve strategies toward the prevention of this process and enable potential applications in nanotechnology. Here, studies on ZnII binding to the amyloidogenic peptide Aβ11-28 are reported. ZnII modulates the Aβ11-28 aggregation, in terms of kinetics and fibril structures. Structural studies suggest that Aβ11-28 binds ZnII by amino acid residues Glu11 and His14 and that ZnII is rapidly exchanged between peptides. Structural and aggregation data indicate that ZnII binding induces the formation of the dimeric ZnII1(Aβ11-28)2 species, which is the building block of fibrillar aggregates and explains why ZnII binding accelerates Aβ11-28 aggregation. Moreover, transient ZnII binding, even briefly, was enough to promote fibril formation, but the final structure resembled that of apo-Aβ11-28 amyloids. Also, seeding experiments, i.e., the addition of fibrillar ZnII1(Aβ11-28)2 to the apo-Aβ11-28 peptide, induced aggregation but not propagation of the ZnII1(Aβ11-28)2-type fibrils. This can be explained by the dynamic ZnII binding between soluble and aggregated Aβ11-28. As a consequence, dynamic ZnII binding has a strong impact on the aggregation behavior of the Aβ11-28 peptide and might be a relevant and so far little regarded parameter in other systems of metal ions and amyloidogenic peptides. [ABSTRACT FROM AUTHOR]

We report and discuss here the unambiguous uranium valence state determination on the complex compound [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] by using high-energy-resolution fluorescence detection-X-ray absorption near-edge structure spectroscopy (HERFD-XANES). The spectra at both U L3- and M4-edges confirm that all five nonequivalent U atoms are solely in the hexavalent form in this compound, as previously suggested by bond-valence-sum analysis and X-ray diffraction pattern refinement. Moreover, the presence of the preedge feature, due to the 2p3/2-5f quadrupole transition, has been observed in the U L3-edge HERFD-XANES spectrum, in agreement with theoretical and experimental observations of other uranium-based compounds. Recently, this feature has been proposed as a possible tool to determine the uranium oxidation state in a manner similar to that of 3d and 4d metals. Nevertheless, this feature is also very sensitive to the uranium local environment, as revealed by our theoretical calculations, and consequently could not be used to attribute without ambiguity the uranium valence state. In contrast, U M4-edge HERFD-XANES appears to be the most straightforward and reliable way to assess the uranium valence state in very complex materials such as [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] or a mixture of compounds. [ABSTRACT FROM AUTHOR]

Uranium-plutonium mixed oxides, containing few mol % of Am, are currently studied as fuel for Sodium Fast Reactors. The study of the O/M ratio of these fuels is of main interest, as its variation can induce issues for reactor safety. For this purpose, four U 1- y - z Pu y Am z O 2± x samples (0.235 ≤ y ≤ 0.39 and 0.005 ≤ z ≤ 0.02) were studied using electron probe microanalysis (EPMA), X-ray powder diffraction (XRD), and X-ray absorption spectroscopy (XAS) experiments. EPMA analyses revealed a homogeneous matrix phase with few U- and Pu-rich agglomerates. A matrix phase of fluorite structure (face-centered cubic), as well as a small contribution of a UO 2 phase of the same structure, was evidenced for all of the samples by XRD. The O/M ratios of the (U,Pu)O 2± x matrix were calculated based on the obtained lattice parameters. XANES experiments highlighted the simultaneous presence of U 5+ and Pu 3+ /Am 3+ for all of the samples, indicating a charge compensation mechanism even for stoichiometric samples. Based on the EPMA and XRD results, it was evidenced that this coexistence of U 4+ , U 5+ , Pu 3+ , Pu 4+ , and Am 3+ was not originating from the U- and Pu-rich agglomerates. It was found to be rather a peculiarity of the U 1- y - z Pu y Am z O 2± x matrix phase, discussed with the help of thermodynamic calculations performed on the U-Pu-Am-O system.

In the context of the separation of uranium and plutonium from spent fuel allowed by N,N -dialkylamides, three regioisomers of N,N -di(2-ethylhexyl) butyramide (DEHBA or ββ) and the diastereopure isomers of N -(2-ethylhexyl)- N -(oct-3-yl)butyramide (EHOBA or αβ) were synthesized to assess their extraction performance and to study the mechanisms at the origin of the differences observed between the stereo- and regioisomers. The N,N -dialkylamides showed differences in extraction, with a greater effect of regio- than stereoisomerism. A mechanistic study at both the molecular and supramolecular scales was initially applied to explain these effects. X-ray absorption and UV-vis spectroscopy showed that uranium is extracted by a UO 2 (NO 3 ) 2 L 2 complex, which is not very sensitive to steric hindrance, while plutonium is extracted by two complexes, Pu(NO 3 ) 4 L 2 and Pu(NO 3 ) 6 (HL) 2 , which are differently affected by stereo- and regioisomerism. Investigations at the supramolecular scale also showed that Pu(NO 3 ) 4 L 2 complexes are disadvantaged by the bulkiness of the extractants, while Pu(NO 3 ) 6 (HL) 2 is favored by the preformation of larger supramolecular aggregates.

Plutonium (Pu) is an anthropogenic element involved in the nuclear industry cycle. Located at the bottom of the periodic table within the actinide family, it is a chemical toxic but also a radiological toxic, regardless of isotopy. After nearly 80 years of Pu industrialization, it has become clear that inhalation and wounds represent the two main ways a person may become contaminated after an accident. In order to reduce the deleterious health effects of Pu, it is crucial to limit chronic exposure by removing it or preventing its incorporation into the body. Diethylenetriaminepentaacetic acid (DTPA) has emerged as the gold standard for Pu decorporation, although it suffers from very short retention time in serum. Other molecules like the hydroxypyridonate family with high chemical affinity have also been considered. We have been considering alternative polymeric chelates and, in particular, polyethylenimine (PEI) analogues of DTPA (the carbonate or phosphonate version), which may present a real breakthrough in Pu decorporation not only because of their higher loading capacity but also because of their indirect vectorization properties correlated with a specific biodistribution into the lungs, bone, kidney, or liver. In the first part of this Forum Article, new data on the structural characterization of the complexation of Pu IV with polyethylenimine methylphosphonate (PEI-MP) were obtained using the combination of extended X-ray absorption fine structure spectroscopy and ab initio molecular dynamics (AIMD) calculations. The use of thorium (Th) as a Pu chemical surrogate is also discussed because its unique oxidation state is IV+ in solution. In the second part of the paper, we put this new set of data on PEI-MP-Pu into perspective with use of the PEI platform to complex Th IV and Pu IV . Uptake curves of Th IV witth polyethylenimine methylcarboxylate (PEI-MC) are compared with those of PEI-MP and DTPA, and the AIMD data are discussed.

A comprehensive molecular analysis of a simple aqueous complexing system-U(VI) acetate-selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry., Competing Interests: The authors declare no competing financial interest.

The structures of plutonium(IV) and uranium(VI) ions with a series of N,N-dialkyl amides ligands with linear and branched alkyl chains were elucidated from single-crystal X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and theoretical calculations. In the field of nuclear fuel reprocessing, N,N-dialkyl amides are alternative organic ligands to achieve the separation of uranium(VI) and plutonium(IV) from highly concentrated nitric acid solution. EXAFS analysis combined with XRD shows that the coordination structure of U(VI) is identical in the solution and in the solid state and is independent of the alkyl chain: two amide ligands and four bidentate nitrate ions coordinate the uranyl ion. With linear alkyl chain amides, Pu(IV) also adopt identical structures in the solid state and in solution with two amides and four bidentate nitrate ions. With branched alkyl chain amides, the coordination structure of Pu(IV) was more difficult to establish unambiguously from EXAFS. Density functional theory (DFT) calculations were consequently performed on a series of structures with different coordination modes. Structural parameters and Debye-Waller factors derived from the DFT calculations were used to compute EXAFS spectra without using fitting parameters. By using this methodology, it was possible to show that the branched alkyl chain amides form partly outer-sphere complexes with protonated ligands hydrogen bonded to nitrate ions.