Citrate complexes are the dominant binding form of trivalent actinides and lanthanides in human urine at pH < 6. Hence, an accurate prediction of the speciation of these elements in the presence of citrate is crucial for the understanding of their impact on the metabolism of the human organism and the corresponding health risks. We studied the complexation of Cm(III) and Eu(III), as representatives of trivalent actinides and lanthanides, respectively, in aqueous citrate solution over a wide pH range using time-resolved laser-induced fluorescence spectroscopy. Four distinct citrate complexes were identified and their stability constants were determined, which are MHCit⁰, M(HCitH)HCit²⁻, M(HCit)₂ ³⁻, and M(Cit)₂ ⁵⁻ (M = Cm, Eu). Additionally, there were also indications for the formation of MCit⁻ complexes. Structural details on the EuHCit⁰ and EuCit⁻ complexes were obtained with FT-IR spectroscopy in combination with density functional theory calculations. IR spectroscopic evidence for the deprotonation of the hydroxyl group of the citrate ion in the EuCit⁻ complex is presented, which also revealed that the complexation of the Eu³⁺ ion takes place not only through the carboxylate groups, like in EuHCit⁰, but additionally via the hydroxylate group. In both EuHCit⁰ and EuCit⁻ the carboxylate binding mode is mono-dentate. Under a very low metal : citrate ratio that is typical for human body fluids, the Cm(III) and Eu(III) speciation was found to be strongly pH-dependent. The Cm(III) and Eu(III) citrate complexes dominant in human urine at pH < 6 were identified to be Cm(HCitH)HCit²⁻ and a mixture of Eu(HCitH)HCit²⁻ and EuHCit⁰. The results specify our previous in vitro study using natural human urine samples (Heller et al., Chem. Res. Toxicol., 2011, 24, 193–203).

The complexation of uranyl ions with lipopolysaccharide (LPS), the main component of the cell wall of Gram-negative bacteria, was investigated on a molecular level with U L_III-edge extended X-ray absorption fine structure (EXAFS) and attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy over a wide pH range (2.6 to 7.0). For the first time, structural determinations of uranyl complexes with cell wall compounds were extended from acidic up to neutral pH. The main functionalities responsible for uranyl binding are phosphoryl and carboxyl groups. At an excess of LPS, related to environmental conditions, the uranyl ion is mainly complexed by phosphoryl groups four-fold monodentately coordinated in the equatorial plane of the uranyl dioxo cation UO₂ ²⁺ showing great homologies to the uranyl mineral phase meta-autunite in the EXAFS spectra. At equimolar ratios of uranyl and functional groups of LPS, according to a slight deficit of phosphoryl groups, additional carboxyl coordination in a bidentate manner becomes important as it is shown by IR spectroscopy. From the vibrational spectra, a mixed coordination of UO₂ ²⁺ with both phosphoryl and carboxyl groups is derived. The coordination of uranyl ions to the LPS molecule is obviously mainly controlled by the U/LPS concentration ratio, and the influence of pH is only of minor significance at the investigated range.

In the presence of polyborates (detected by ¹¹B-NMR) the formation of a weak Eu(III) borate complex (lg β₁₁ ∼ 2, estimated) was observed by time-resolved laser-induced fluorescence spectroscopy (TRLFS). This complex is a precursor for the formation of a solid Eu(III) borate species. The formation of this solid in solution was investigated by TRLFS as a function of the total boron concentration: the lower the total boron concentration, the slower is the solid formation. The solid Eu(III) borate was characterized by IR spectroscopy, powder XRD and solid-state TRLFS. The determination of the europium to boron ratio portends the existence of pentaborate units in the amorphous solid.

Bacteria have a great influence on the migration behaviour of heavy metals in the environment. Lipopolysaccharides form the main part of the outer membrane of Gram-negative bacteria. We investigated the interaction of the uranyl cation (UO₂ ²⁺) with lipopolysaccharide (LPS) from Pseudomonas aeruginosa by using potentiometric titration and time-resolved laser-induced fluorescence spectroscopy (TRLFS) over a wide pH and concentration range. Generally, LPS consists of a high density of different functionalities for metal binding such as carboxyl, phosphoryl, amino and hydroxyl groups. The dissociation constants and corresponding site densities of these functional groups were determined using potentiometric titration. The combination of both methods, potentiometry and TRLFS, show that at an excess of LPS uranyl phosphoryl coordination dominates, whereas at a slight deficit on LPS compared to uranyl, carboxyl groups also become important for uranyl coordination. The stability constants of one uranyl carboxyl complex and three different uranyl phosphoryl complexes and the luminescence properties of the phosphoryl complexes are reported.

Summary: The complex formation in the aqueous systems of uranium(VI) with glucose 6-phosphate (G6P) and fructose 6-phosphate (F6P) were studied using potentiometric titration, TRLFS, and EXAFS. Two complexes with a uranyl-to-ligand ratio of 1:1 and 1:2 for both sugars were observed. Complex stability constants were determined by potentiometric titration for both complexes to be log β₁₁(G6P)=5.89±0.40, log β₁₂(G6P)=9.45±0.08, log β₁₁(F6P)=5.72±0.21, and log β₁₂(F6P)=9.54±0.09. Stability constants could be determined with TRLFS under the specific experimental conditions only for the 1:1 complexes: log β₁₁(G6P)=6.35±0.28, and log β₁₁(F6P)=5.66±0.17. The TRLFS measurements results show the UO₂G6P complex to exhibit no fluorescence properties. For this system, only a decrease of the fluorescence intensity with increasing ligand concentration was observed. For the UO₂F6P system, a red shift of the fluorescence emission bands of about 8 to 9 nm compared to the free uranyl ion was observed. The fluorescence emission wavelengths of the UO₂F6P complex were determined to be 483, 496, 518, 542, and 567 nm, and the lifetime of this complex is 0.13±0.05 μs. Uranium L _III-edge EXAFS measurements at pH 3.5, 4.0, and 5.5 yielded a shortened U–O_eq bond distance (2.30±0.02 to 2.37±0.02 Å), compared to the UO₂ ²⁺(H₂O)₅ ion (2.40±0.02 Å), due to a monodentate coordination via the oxygen atoms of the phosphate group.