Uranium(VI) Binding Forms in Selected Human Body Fluids: Thermodynamic Calculations versus Spectroscopic Measurements

Human exposure to uranium increasingly becomes a subject of interest in many scientific disciplines such as environmental medicine, toxicology, and radiation protection. Knowledge about uranium chemical binding forms (speciation) in human body fluids can be of great importance to understand not only its biokinetics but also its relevance in risk assessment and in designing decorporation therapy in the case of accidental overexposure. In this study, thermodynamic calculations of uranium speciation in relevant simulated and original body fluids were compared with spectroscopic data after ex-situ uranium addition. For the first time, experimental data on U­(VI) speciation in body fluids (saliva, sweat, urine) was obtained by means of cryogenic time-resolved laser-induced fluorescence spectroscopy (cryo-TRLFS) at 153 K. By using the time dependency of fluorescence decay and the band positions of the emission spectra, various uranyl complexes were demonstrated in the studied samples. The variations of the body fluids in terms of chemical composition, pH, and ionic strength resulted in different binding forms of U­(VI). The speciation of U­(VI) in saliva and in urine was affected by the presence of bioorganic ligands, whereas in sweat, the distribution depends mainly on inorganic ligands. We also elucidated the role of biological buffers, i.e., phosphate (H₂PO₄^–/HPO₄^2–) on U­(VI) distribution, and the system Ca²⁺/UO₂²⁺/PO₄^3– was discussed in detail in both saliva and urine. The theoretical speciation calculations of the main U­(VI) species in the investigated body fluids were significantly consistent with the spectroscopic data. Laser fluorescence spectroscopy showed success and reliability for direct determination of U­(VI) in such biological matrices with the possibility for further improvement

Structural parameters of the uranium complexes formed by the cells of <i>Paenibacillus</i> sp. JG-TB8.

<p>Standard deviations as estimated by EXAFSPAK are given in parenthesis.</p>a<p>Errors in coordination numbers are ±25%.</p>b<p>Errors in distance are ±0.02 Å.</p>c<p>Debye-Waller factor.</p>d<p>Parameter fixed for calculation, Debye-Waller factor of the U-P, U-C and U-Cdis path were fixed acccording to the Debye-Waller factors calculated for the corresponding model compounds (see references 42 and 43).</p>e<p>Coordination number linked twice and Debye-Waller factor once to the N and σ² of the U-P path.</p>f<p>Coordination number (N) linked to N of U-C₁ path.</p><p>Structural parameters of the uranium complexes formed by the cells of <i>Paenibacillus</i> sp. JG-TB8.</p

U <i>L</i>_III-edge <i>k</i>³-weighted EXAFS spectra (left) and the corresponding Fourier Transforms (right) (3.1 Å⁻¹−1) of the uranium complexes formed by <i>Paenibacillus</i> sp. JG-TB8 at pH 2, pH 3, pH 4.5, and pH 6 at different pH and aeration conditions within 48 hours.

<p>For comparison, the spectra of three model compounds, namely uranyl succinate, uranyl-fructose(6)phosphate, meta-autunite are illustrated as well.</p

U(VI) binding capacity calculated for the cells of JG-TB8 in dependency of pH conditions, presence of oxygen and incubation time.

a<p>Experiments were performed in triplicate. The mean and the standard deviation are presented.</p>b<p>At pH 6, the binding capacity is limited to ∼24 mg/g dry biomass due to the lower uranium concentration used in these samples.</p><p>U(VI) binding capacity calculated for the cells of JG-TB8 in dependency of pH conditions, presence of oxygen and incubation time.</p

Representative microscopic pictures of <i>Paenibacillus</i> sp. JG-TB8, stained with the Live/Dead Kit after the treatment with uranium under aerobic (left) and anaerobic (right) conditions at pH 3 (A, B), pH 4.5 (C, D), and pH 6 (E, F) for 48 hours.

<p>Pictures were taken in fluorescence mode using a fluorescence mirror unit (U-MSWB; Olympus Europa Holding GmbH, Hamburg, Germany) with excitation wavelengths between 420 and 460 nm.</p

Normalized U(VI) luminescence spectra recorded from the uranium complexes formed under different pH and aeration conditions within 48 hours by the cells of <i>Paenibacillus</i> sp. JG-TB8.

<p>For comparison, dotted vertical lines indicate the main luminescence emission maxima recorded for the sample incubated at pH 2. The dashed vertical line marks the position of the luminescence emission peak which was assigned to uranyl carboxylate complexes.</p

Transmission electron micrograph (left) of uranium precipitates deposited by the cells of <i>Paenibacillus</i> sp. JG-TB8 at pH 4.5 under oxic conditions.

<p>Energy-dispersive X-ray spectra (right) of the sample points (S1–S6) are marked with arrowheads.</p

Structural models used for the fitting procedure of the EXAFS spectra obtained from the uranium complexes build by the cells of <i>Paenibacillus</i> sp. JG-TB8.

<p>The left model created from the crystall structure of meta-autunite was used for the fitting procedure of the sample incubated at pH 6 under oxic conditions. The right model contains fragments of meta-autunite (monodentate coordination at phosphate groups) as well as uranyl triacetate (bidentate coordination to carboxylic groups) and was used for the fitting procedures of all other samples.</p

Calculated luminescence lifetimes of the U(VI) complexes formed by the cells of <i>Paenibacillus</i> sp. JG-TB8.

<p>τ₁, τ₂, τ₃: organic uranyl phosphate complexes.</p><p>τ₄: mixture of organic and inorganic uranyl phosphate complexes.</p><p>τ₅: inorganic uranyl phosphate complexes.</p><p>τ₆: organic uranyl carboxylate complexes.</p><p>Calculated luminescence lifetimes of the U(VI) complexes formed by the cells of <i>Paenibacillus</i> sp. JG-TB8.</p