Uranium decontamination gains a great importance with the spread of nuclear waste in both soil and water systems across the planet. All known remediation methods of uranium can be exclusively based either on synthetic materials with high adsorbent power and known physical chemistry or life organisms by which the uranium eventually accumulated inside their tissues. In the present thesis, it was attempted to design a rational approach for uranyl removal primarily from waters using the reducing potential of quercetin, which is a plant-derived small organic molecules, along with its photochemical activities. Such approach, which is neither a fully synthetic nor an organism-based approach, was chosen here to avoid disadvantages with both traditional strategies. Here, complexation experiments were designed to assess the use of uranyl-quercetin complexes for the photoreduction of water-soluble U(VI) to insoluble U(IV) by comparing absorption properties of uranyl-quercetin complexes in acetone, water, and hydrophobic bilayer lipid vesicles.
The UV-vis data show that uranyl quercetin complex can form in both hydrophobic and hydrophilic environments. In both cases the B-ring band in quercetin structure becomes reduced, red shifted and a pronounced absorption arises in the 400-500 nm range. Such data suggests that U(VI) binds at the 3-OH and 4-carbonyl of ring C of quercetin.
Interestingly, the results of UV-Vis spectroscopy part hint at a crucial role of a stable or transiently ionized hydroxyl for the efficient uranyl-dependent photodegradation of quercetin. FTIR spectroscopy absorption changes further demonstrates that the UV-vis-spectroscopic changes are indeed accompanied by changes in the chemical structure of the complex as expected for a uranyl-dependent photodegradation. IR data thus suggest that U(VI) becomes reduced by the photoreaction, rather than merely changing its coordination shell. The frequency shifts in the C=C and C=O absorption range on the other hand are consistent with changes in force constants rather than bond breakage. Upon illumination condition, uranyl quercetin complex in water forms a dark precipitate. Uranyl precipitation and the disappearance of U(VI) IR absorption bands upon illumination further demonstrate that uranyl acts as a redox partner rather than a catalyst in the photoreaction of quercetin.
The formation of uranyl-quercetin complexes in the presence of lipidic phases has been addressed experimentally. The complex is partitioned into the hydrophilic/hydrophobic interface of liposomes. Its electronic absorption properties are influenced by the degree of hydrophobicity provided by the adjacent lipid headgroups. The preference of quercetin to associate with hydrophobic microenvironments can thus be exploited to transfer uranyl to the lipid water biomolecular interface. Illumination of the uranyl-quercetin complex in the presence of different liposomes has been performed in this study for the first time, to the best of my knowledge. The data provide evidence that again uranyl is a redox partner for the photodegradation of quercetin also in this microenvironment. Uranyl in an oxidation state smaller than VI is unsoluble in water.
Therefore, its quercetin-mediated photoreduaction of uranium provides a method to transfer soluble uranium to the liposome and stabilize the reduced photoproduct. Thereby, uranyl could be removed from solution in an insoluble form using cheap natural compounds.
The binding site assignment of uranyl-quercetin complex in acetone have been verified here using NMR spectra and DFT theory. NMR Spectra showed that the observations of broadened and narrow bands in the NMR spectra of quercetin, upon complexation with uranyl, support an intramolecular exchange or site exchange within the quercetin molecule. Moreover, the complexation takes place around the carbonyl group with U(VI) exhibiting two possibly coordination modes, involving the carbonyl and the adjacent O(H) groups. This has been also confirmed from the DFT calculations.
Finally, interaction experiments of uranyl-quercetin complex with DNA have been performed to assess an alternative uranyl-trapping and photoreduction system. The data show that consecutive addition of quercetin and uranyl destabilizes DNA. However, a preformed uranyl quercetin complex has very little effect on DNA structure. On the other hand, quercetin and uranyl appear to bind to DNA as a preformed complex in the loop portion of hairpin DNA. Therefore, also HP DNA is expected to be a suitable but less effective trapping system for the uranyl quercetin complex and its potential photoproducts.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:28224 |
| Date | 21 July 2014 |
| Creators | Attia, Enas |
| Contributors | Fahmy, Karim, Guck, Jochen, Gutzeit, Herwig, 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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