Interaction of U(VI) with Some Bioligands or the Influence of Different Functional Groups on Complex Formation

U(VI) released anthropogenically, e.g. through mining activities, can be accumulated for instance in plants and consequently can enter further parts of the food chain. Hence it is of crucial importance to study the interaction of U(VI) with cellular ligands, like glutathione, uric acid and benzoic acid. Glutathione, the most abundant thiol compound of a cell, has a well known affinity for heavy metal ions. It is a precursor for phytochelatin synthesis and thus also a model substance for the study on U(VI) phytochelatin interaction. Uric acid (UA) was investigated as a further ligand being potentially capable of chelate formation with U(VI). It is present e.g. in bio-fluids such as urine and sweat. Benzoic acid occurs naturally free and bound as benzoic acid esters in many plant and animal species. Furthermore it is a model ligand to study the interaction with bacterial siderophores and humic acids.
UV-vis and time-resolved laser-induced fluorescence spectroscopy (TRLFS) were applied to investigate complex formation between U(VI) and glutathione, uric acid and benzoic acid. Additionally one of the four major potential binding sites of glutathione, the thiol group, was blocked by derivatization to assess the coordination chemistry more detailed.
In the U(VI)-glutathione system a 1:1 complex with a large stability constant of 39.07 ± 0.15 (at zero ionic strength) could be identified. On the contrary, in the U(VI)-urate and -benzoic acid systems a comparably weak complexation was found.
What are the structure-related reasons for such a considerable discrepancy in complex stability? Investigating the interaction of U(VI) with glutathione-S-conjugates, no decrease in complex stability in comparison to U(VI) complexation by glutathione was found. Thus here a significant involvement of the thiol group in coordination can be excluded. Since benzoic acid offers one carboxyl functionality but exhibits a very weak U(VI) coordination, for U(VI) complexation by glutathione a chelate-like coordination with an involvement of at least one carboxyl functionality can be assumed. Since uric acid coordinates as a bidentate ligand and is suggested to undergo a chelation with U(VI) as well, specifically by its 7-amine and 6-carbonyl group, it follows that chelate formation must not necessarily result in strong complexation.
In general, these results contribute to a better understanding of the intricate U(VI) interactions in biological systems on a molecular level.