The ultimate goal of this work is providing insights into fundamental (physico-) chemical (redox) behavior of hexavalent uranium (U(VI)), trivalent europium (Eu(III)) and selenium (Se), and upon their interaction with ubiquitous small biomolecules (in case of U(VI) and Eu(III)) or alkaline earth metal ions (in case of Se(IV) and Se(VI)) by application of Nuclear Magnetic Resonance (NMR) spectroscopy. NMR spectroscopy is a powerful method proving its usefulness also to environmental and nuclear waste related studies in aqueous solutions by determination of (potential) binding sites, molecular structures (even conformation and configuration) as well as intra- and intermolecular dynamics, (redox) reaction pathways and mechanisms.
The present work comprises extensive NMR spectroscopic investigations in aqueous (D2O) solutions on (i) glutathione (GSH) and glutathione disulfide (GSSG) interactions with trivalent lanthanides (Ln(III), particularly Eu(III)) and U(VI), (ii) molecular structures of citrate (Cit) complexes of U(VI), and their reactions upon light-irradiation, as well as (iii) pH- and temperature-dependent speciation of selenium oxyanions, i.e., Se(VI) (selenate) and Se(IV) (selenite and, notably, hydrogen selenite) as well as Se(VI) and Se(IV) interaction with alkaline earth metal ions. These investigations are supported by time-resolved laser-induced fluorescence spectroscopy (TRLFS), ultraviolet-visible-near infrared (UV-Vis-NIR), IR/Raman, and extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy (TEM), as well as quantum chemical calculations on density functional theory (DFT) level.
For NMR spectroscopic data on GSH/GSSG complexation towards both Eu(III) and U(VI) are lacking, the herein presented results are new, and nicely complement other spectroscopic studies. Ln(III) complexes of GSH are characterized by their high solubility at least up to 300 mM and pD 5. However, the formation constant of the Eu(III)–GSH 1:1 complex is quite low with log K = 1.71 ± 0.01 as determined by Eu(III)-TRLFS. The diamagnetic La(III) and Lu(III) showed only little effect on the NMR spectra (< 2 ppm) while analogous Eu(III) solutions revealed hyperfine shifts up to 40 ppm. Eu(III)-induced 1H chemical shift changes are solely upfield and attributed to be predominantly due to pseudocontact contribution caused by dipolar interaction. In contrast, Eu(III)-induced 13C chemical shift changes of adjacent atoms – at least for the carboxyl and α-carbons – show alternating signs, indicating spin polarization effects owing to contact contribution. As expected for hard LEWIS acids and shown by other spectroscopies, complexation facilitates by the carboxyl groups. Qualitative differences between the glutamyl and glycyl carboxylate in metal ion complexation are ascribed to COULOMB repulsion due to the positively charged NH3+ in direct vicinity.
Investigations of the U(VI)–GSH system covered experiments under both oxidizing and reducing conditions, performed with GSH’s oxidized form, GSSG, at ambient conditions, while samples with reduced GSH were handled under N2 atmosphere. For either condition, U(VI) showed interaction in aqueous (D2O) solution with both GSH and GSSG as determined by U(VI)-induced 1H and 13C chemical shift changes and U(VI) TRLFS, the latter comprising measurements at 25 °C and –120 °C. In principle, the interactions are stronger as compared to the Ln(III) system, and the speciation in both solution and solid is more complex owing to the aqueous chemistry of uranium. Observed binary GSH complexes are [UO2(H2GSH)]2+ for pD values up to ≈ 2.3, and [UO2(HGSH)]+ predominating for pD > 2.3. Complementary to the Eu(III) results, whenever net neutral binary GSH/GSSG or ternary hydroxo GSH/GSSG U(VI) complexes form in solution, both these U(VI) systems revealed extensive precipitation because of the low solubility of these complexes. Binary U(VI) GSSG and ternary U(VI) hydroxo GSSG complexes yield solid phases from pD 2 through 8, even in carbonatic media. The largest quantities of aqueous GSSG–U(VI) complexes are observed for pD ≈ 3.5, with the association constant for pH 3 determined by TRLFS as log K = 4.81 ± 0.08 for a 1:1 complex. GSH cannot compete with hydroxo ligands for complexation as of pD 6, whereas GSSG can at least partially compete with hydroxo and carbonate ligands upon formation of both quaternary U(VI) hydroxo carbonate GSSG, and ternary U(VI) carbonate GSSG (poly-)anionic species of high solubility.
Under reducing and near-neutral conditions (pD 6 – 9) GSH immediately reduced U(VI) with subsequent formation of nanocrystalline UO2+x. After centrifugation of the starting material and allowing the decanted supernatant to age, the dissolved nanocrystals assemble network-like as disclosed by TEM, and further analysed by selected-area electron diffraction (SAED), energy-dispersive X-ray (EDX) and UV-Vis spectroscopy, revealing hyper-stoichiometric UO2+x phases. Such network-like assembled actinide containing nanocrystals, with the arrangement most likely provoked by the presence of GSSG, have never been shown before. Complementary, the precipitate that has also been allowed to age as a wet paste, showed color changes from yellow via olive to black, indicating a reaction to proceed. The repeatedly probed and dissolved material exhibited GSSG in NMR spectra, and UV-Vis-NIR absorption bands attributed to U(IV) and, notably, U(V), the latter implying a one-electron transfer with subsequent disproportionation of U(V) to U(IV) and U(VI). Therefore, obtained results advance the understanding of both fundamental redox behavior of uranium and the role of GSH (and related molecules) in U(VI) detoxification processes in vivo.
Although investigated for over 70 years, there are still controversial discussions on both speciation and structures of U(VI)–Cit complexes. By means of NMR’s strength in both structure determination and sensitivity to dynamic processes, studies regarding the U(VI)–Cit system allowed further fundamental insights into the structures of the formed complexes on a molecular level. Upon complexation a chiral center is induced in Cit’s central carbon, resulting in the formation of two diastereomeric pairs of enantiomers, whereupon the dimeric complexes exhibit syn and anti configured isomers. In fact, the combination of 17O NMR (note: at natural abundance) and quantum chemical calculations allowed an unambiguous decision on complex geometry and overall configurations. It is evidenced for the first time that the syn isomer is favored in aqueous solution in contrast to the preferably crystallizing anti isomer. Both isomers coexist and interconvert among one another, with a rate estimated to be in the order of 102 s–1 at 25 °C in acidic media, and a corresponding activation energy of approximately 60 kJ mol–1. Moreover, clear indications for uranium chirality is observed for U4+, with the 1:1 U(IV)–Cit complexes also forming two diastereomeric pairs of enantiomers. Comprehensive spectroscopic experiments combined with quantum chemical calculations improved basic understanding of the photo-reaction mechanism in the U(VI)–Cit system. Regardless of sample conditions, Cit is degraded to β-ketoglutarate, acetoacetate, and acetone, while U(VI) was reduced to U(IV) at pD 2 and U(V) at pD 5, suggesting a two- and a one-electron transfer, respectively. NMR signals observed for pD 5 samples at remarkable 1H chemical shift values between 25 and 53 ppm, in combination with UV-Vis-NIR absorptions at about 750 and 930 nm, are assigned to U(V) complexes of citrate. With regard to reported pH dependence on reaction rate and yield in the literature combined with observations in this work, H+/D+ are considered mechanistically crucial constituents. Furthermore, the photoreaction proceeds intermolecularly, requiring for free Cit to be present in solution.
In consideration of both the U(VI)–Cit photoreaction and the U(VI)–GSH chemical redox reaction, regardless of the particular mechanism, in both cases the process is intermolecular. This is not only a highly interesting, but the more a very important result, rendering the reductants not required to be bound to U(VI) in order to reduce it.
Owing to the suitability of 77Se as NMR-active but non-radioactive Se isotope, this spectroscopy was also applied to study chemical behavior of the nuclear waste related long-lived 79Se. For the first time spectroscopic evidence is given for hydrogen selenite dimerization in aqueous solution upon formation of homo-dimers by hydrogen bonding that are stable up to 60 °C and so are other selenium oxyanionic species. Additionally, a remarkably higher 77Se chemical shift temperature coefficient of the dimer – as compared to corresponding selenite and selenous acid – was found. These findings are attributed to a significant deshielding upon heating due to remarkably different rovibrational modes upon stretching the dimer as a whole instead of its dissociation into monomers owing to the rather strong hydrogen bonds. Interaction of selenium oxyanions with ubiquitous alkaline earth metals, i.e., Ca2+ and Mg2+, showed formation of weak aqueous complexes of both selenite and hydrogen selenite dimer for excessive selenium, however, at high ionic strength (5.6 M) for equimolar Ca2+ and Se(IV) even at pHc 5 crystalline calcium selenite is formed.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:34071 |
| Date | 27 May 2019 |
| Creators | Kretzschmar, Jerome |
| Contributors | Stumpf, Thorsten, Brunner, Eike, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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