ReviewCoordination chemistry of f-block metal ions with ligands bearing bio-relevant functional groups
Graphical abstract
Introduction
With the discovery and utilisation of nuclear fission, actinides (An) have become of high socio-political relevance. This applies especially to the elements Th, U, Np, Pu, Am, and Cm, of which only Th and U can be found in significant amounts in nature. The remaining elements are products of nuclear fission applications. Due to malfunction or damage occurring in nuclear plants, or leakage from disposal containers, these elements may contaminate the environment [1]. The risk associated with these elements is two-fold. On the one hand it is due to their nuclear radiation and, on the other hand, a consequence of their heavy metal toxicity. An appreciation of the means for preventing the environmental dispersion of radionuclides clearly depends on an understanding of their environmental pathways and ultimate fate [2]. Consequently, an in-depth understanding of the chemistry of the An elements, especially focused on their coordination chemistry is necessary for this to be achieved.
The effect of radioactive compounds on the environment as well as on the food chain has been an interdisciplinary area of investigation for decades. Many natural and anthropogenic processes causing environmental contamination have been identified. The latter arise from nuclear tests, the use of depleted uranium in munitions and accidental releases from nuclear power plants as well as from storage facilities. Other contributions have involved the recycling of used nuclear fuel rods and the disposal of radioactive waste. Pollution can also result from mining; for example, of uranium and the rare earth elements (REE), or from the use of natural resources contaminated with U (i.e. fossil fuels, construction materials, fertilisers) [3], [4], [5], [6], [7]. Metal complexation is often involved in such processes as well as in the corresponding remediation procedures [8], [9]. The overall radiobiological impact of the various radionuclides depends on various factors such as the nature of their interaction with potential complexing agents, the effectiveness of their uptake into organisms [10], [11], their surface interactions with solids, and the transport and targeting that occurs within particular organisms [12]. For the An elements, many of these complicated relationships have yet to be investigated or have only been studied to a limited extent. For example, detailed information concerning metal-ion speciation for different oxidation states, ligand exchange with bio-relevant ligands, and details of transport behaviour are frequently not available. Further, crucial experimental data such as concentration, pH, temperature, redox potential, carbon dioxide level, and ionic strength are also often missing [3], [4], [5], [6].
While data, especially thermodynamic, speciation, and structural data, for An complex formation with small inorganic ligands is relatively abundant [13], [14], [15], in contrast this is generally less so for organic ligand systems [16], [17] and especially for bio-relevant organic ligand systems [18], [19], [20]. Notably, there is a lack of detailed investigations of complexes of multifunctional ligand systems possessing typical ‘natural’ functional groups such as carboxylate, phosphate, amine, imine, amide, ether, hydroxyl, phenol, and heterocyclic groups. For that reason, in 2016 the joint research project FENABIUM, which included several working groups from Dresden and Leipzig, was established with its focus on the interaction of f-elements with organic ligands carrying binding sites related to those of natural products. This consortium aims to not only explore the interaction of f-block elements with organic ligands but also to ensure that the knowledge so obtained is made available to future workers in the area.
An aspect of the coordination chemistry of 5f-elements is their often similarity to that of the 4f-analogues, the lanthanides (Ln). Although this similarity may not be observed for the early Ans, there remains an opportunity for using the Ln ions as non-radiative model species. Nevertheless, the An ions exhibit a stronger tendency towards covalent bonding that leads to somewhat enhanced interaction with softer nitrogen, phosphorous, and/or sulfur donor atoms in contrast to the Ln, which are distinguished by a distinct preference for hard oxygen donors [21], [22]. Thus, various heteroaromatic N-donor ligands have been reported for the separation of trivalent Ans and Lns [23], [24], [25], [26].
The investigation of f-block metal ion binding to ligand systems carrying phenolic groups has attracted wide interest over many years. The relatively hard aryl –OH or –O− group in fact has a substantial affinity for both Ln and An ions [27]. This group is common to a range of biomolecules, such as particular peptides and proteins (e.g. in the form of tyrosine) [28], siderophores [29], [30], fulvic acids (FA), and humic acids (HA) [31], [32], [33], [34]. Various open-chain and macrocyclic polyphenol ligands have been investigated with respect to f-metal ion complexation. The motivations for undertaking these studies are diverse and include the investigation of biomolecule f-element interactions [35], f-element separation by liquid–liquid extraction, nuclear fuel reprocessing [36], [37], [38], [39], the design and synthesis of biomimetic radionuclide chelators [40], radionuclide decorporation [29], the creation of sorbents for the removal of contrast agents [41] and the development of the next generation of radiopharmaceuticals [42]. A further impetus for studying f-metal phenolato systems is due to recent advances in the development of new Ln probes [43], [44] for use in molecular sensing and optical imaging [45], [46], [47], [48], [49], [50], [51]. Finally, it is noted that complexation with polyphenols can drastically increase the solubility of particular metal ions, which clearly has important implications for their mobilisation in the environment [52].
To deliver a short insight into the wide field of Ln and An complexation, this review focusses on a few examples of naturally occurring ligands (such as Maillard reaction products, siderophors, porphyrins, cellulose, chitin and chitosan, peptides, and proteins) and their synthetic derivatives. Because of structural analogy (in terms of the presence of phenolic groups) a brief survey on the current state of research of the f-block interaction with calix[4]arenes is also included.
Section snippets
Methods
Several experimental methods (such as UV-Vis and EXAFS spectroscopy, ESI- or ICP-MS) allow the direct study of actinide-(bio)ligand interactions. Especially NMR, IR and TRLF spectroscopy are often used and these will be discussed briefly.
Maillard reaction products
The Maillard reaction, named after the French chemist Louis Camille Maillard, has become one of the most important chemical reactions in food chemistry [154]. Generally referred to as non-enzymatic browning or “glycation”, it describes various stages of the reaction of amino compounds such as amino acids, peptides or proteins, respectively, with reducing sugars. This complex reaction is responsible for the formation of colour, aroma and flavour during the preparation of many foods. The
Cellulose and its derivatives for f-element adsorption
Cellulose, a polymer consisting of β-(1,4)-d-glucopyranose monomer units (see Fig. 30, top), is the most abundant biopolymer on earth. Naturally synthesized as a structural component in plants, fungi, bacteria, and algae, it has long been a subject of scientific interest. Combining mechanical durability, biodegradability, sustainability, and hydrophilicity, cellulose continues to be a promising material for applications that include filtration, membrane separation, pharmaceutical preparations,
Conclusion
An aim of the above discussion was to present an overview of a selection of the very large number of diverse, but often related, studies that involve the interaction of Ln and An ions with a wide range of both naturally-occurring and synthetic ligands (the latter bearing bio-relevant functional groups) and materials. Often these investigations have been paralleled by model laboratory and/or computation studies that, in combination, have enabled considerable insight into both the transport and
Acknowledgements
The authors thank the German Federal Ministry of Education and Research (BMBF – project 02NUK046A, 02NUK046B and 02NUK046C) for financial support and Dr. Kathleen Schnaars for the design of the FENABIUM logo. We also like to acknowledge financial support of Fondazione Cariplo – Regione Lombardia grant (Sottomisura A - 2015).
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