Uranium(VI) bio-coordination chemistry from biochemical, solution and protein structural data

The biocoordination chemistry of the uranyl cation (uranium(VI), [UO2]2+) is assessed utilizing data from the Protein Databank, solution thermodynamic data and biochemical experiments. Uranyl cation is effectively transported in the bloodstream through interactions with carbonate and protein ligands. Under physiologic conditions, carboxylate donors from biological ligands to uranyl are expected to compete favorably against carbonate in the inner sphere of the tris-carbonato uranyl complex. Carboxylate donors from aspartate, glutamate or the carboxyl terminus in proteins make up the majority of inner-sphere interactions with uranyl in protein structures that include this cation. Other significant interactions include amide oxygen donors in the inner-sphere, and H-bonding to uranyl oxo ligands by amide NH donors. These inner- and outer-sphere interactions are also present in structures of uranyl–amino acid complexes, and in other uranyl complexes with small molecules. In the solution state, uranyl–carboxylate interactions have been measured with thermodynamic values for stepwise conditional formation constants on the order of log K ≈ 2–5, similar to uranyl–acetate interactions. In addition to inner-sphere coordination, H-bonding and charge effects, as well as local chemical environmental conditions, play significant roles in the outer sphere; these interactions are somewhat less characterized in the literature.