Uran(VI) complexes with sugar phosphates in aqueous solution

The complex formation in the aqueous systems of uran(VI) with glucose 6-phosphate (G6P) and fructose 6-phosphate (F6P), respectively, were studied by means of potentiometric titration, TRLFS, and EXAFS. In each case two complexes with a metal-to-ligand ratio of 1 : 1 and 1 : 2 could be observed. Complex stability constants were determined by potentiometric titration for both complexes to be log beta₁₁(G6P) = 5.89 ± 0.40, log beta₁₂(G6P) = 9.45 ± 0.08, log beta₁₁(F6P) = 5.72 ± 0.21, and log beta₁₂(F6P) = 9.54 ± 0.09. By TRLFS stability constants could be calculated under the specific experimental conditions only for the 1 : 1 complexes (log beta₁₁(G6P) = 6.35 ± 0.28, and log beta₁₁(F6P) = 5.66 ± 0.17).
The TRLFS measurements resulted in that the UO₂G6P complex shows no fluorescence properties. For this system only a decrease of the fluorescence intensity with increasing ligand concentration could be observed. For the uranyl fructose 6-phosphate 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 different pH values yielded a shortened U - O_eq bond distance (2.31 ± 0.02 to 2.37 ± 0.02 Angström) compared to the UO₂ ²⁺(H₂O)₅ ion (2.40 ± 0.02 Angström) due to a monodentate coordination via the oxygen atoms of the phosphate group.