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NMR spectroscopy of selected aqueous systems investigated at HZDR–IRE

Kretzschmar, J.; Stumpf, T.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful method for both structure elucidation of molecules and their metal complexes, and also for studying their behavior in terms of thermodynamics and kinetics, as well as reactions occurring in situ in dependence on a variety of physico-chemical parameters. In this context, NMR spectroscopy of aqueous (D₂O) solutions features some peculiarities. That is, first and foremost, the pH (pD) of the solution affects the speciation of both the molecule (ligand) and the metal ion (actinide or lanthanide) under study. Furthermore, the ligand can be subject to deuteration, and either component can undergo redox reactions.

The intracellularly occurring tripeptide glutathione (GSH) constitutes a redox equilibrium with its oxidized (dimeric) form glutathione disulfide (GSSG). Hexavalent uranium, U(VI), forms complexes with the latter over a wide pH range, while GSH reduces U(VI) to U(IV). However, the redox reaction occurs only between pH 6 and 10, i.e. close to the thiol group’s pKa, presumably due to homolytic cleavage of the S–H group in GSH’s cysteine residue. The redox reaction appears to take place intermolecularly without the need for U(VI) complexation by the reductant [1, 2].
Uranyl(VI) citrate dimeric and trimeric complexes exhibit interesting structural and (¹⁷O) NMR spectroscopic features such as superstructure formation upon varying pH or concentration, and polarization of uranyl units acting as Lewis base in metal ion coordination (O=U=O⟶Mn+) [3, 4]. Irradation of uranyl(VI) citrate by visible light yields complexes of lower valent uranium. The reaction again occurs intermolecularly, whereby in situ oxidation of excessive ligand through several intermediates can be comprehended by NMR spectroscopy.
In studies investigating the interaction of radionuclides (RNs) with a solid phase in equilibrium with an aqueous phase, the influence of organics on RN retention can be complemented by qualitative and quantitative solution NMR methods by determining their speciation (free, metal ion-bound, oxidized) and concentration in the supernatant.
NMR spectroscopy can also be utilized as a robust and elegant method for determining ligand’s pKa along with the originating site of the abstracted proton as shown for GSH/GSSG, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), as well as nitrilotriacetate (NTA) and ethylene glycol-bis(aminoethyl ether)-N,N,N’,N’-tetraacetic acid (EGTA) [5]. The latter two, being representatives of the so-called complexones, show Lewis acid-catalyzed in situ deuteration of the N acetyl methylene groups in NaOD media, which is applied for speciation analyses in artificial body fluids by means of ²H NMR spectroscopy as the only deuterated component.

This research received funding by the German Federal Ministry for Economic Affairs and Energy (BMWi) with-in the GRaZ II projects, nos. 02E11860B and 02E11860G, the German Federal Ministry of Education and Research (BMBF) within the RADEKOR project, no. 02NUK057A, as well as by the European Union’s Horizon 2020 research and innovation programme’s CORI project, no. 847593.

[1] Kretzschmar, J.; Haubitz, T.; et al. Chem. Commun., 2018, 54, 8697.
[2] Kretzschmar, J.; Strobel, A.; et al. Inorg. Chem., 2020, 59, 4244.
[3] Kretzschmar, J.; Tsushima, S.; et al. Chem. Commun., 2020, 56, 13133.
[4] Kretzschmar, J.; Tsushima, S.; et al. Inorg. Chem., 2021, 60, 7998.
[5] Kretzschmar, J.; Wollenberg, A.; et al. Molecules, 2022, 27, 4067.

Keywords: NMR spectroscopy; Uranium; Radioecology; Aqueous system; Thermodynamics; Kinetics; Redox reaction; Speciation

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