Uranium(VI) complexation studies with selected aromatic acids - a comparison of different spectroscopic techniques

To understand the actinide interaction processes in biological systems on a molecular level it is necessary to explore the complexation behaviour of actinides with selected bioligands of relevant functionalities as model compounds. We used salicylhydroxamic acid (SHA), benzohydroxamic acid (BHA) and benzoic acid (BA) as model compounds for pyoverdins, which are natural bioligands secreted from Pseudomonas ssp. possessing a high potential to bind actinides [1]. The different U(VI) species, MpLqHr, were characterized by UV-vis spectroscopy and laser-induced spectroscopic techniques (TRLFS, fs-TRLFS).
From the UV-vis measurements different coordination modes of uranyl can be postulated [2]. If uranyl is coordinated to hydroxamic acids this results in a blue shift of the absorption maxima in comparison to the bands of the free uranyl ion of app. 25 nm. Whereas in the case of coordination to the carboxylic acid group of benzoic acid, a red shift of the absorption maxima of app. 2.5 nm was observed.
The U(VI)-TRLFS measurements show, that all uranyl ligand species emit no fluorescence light. In all three systems a static quench process of the uranyl fluorescence due to the complex formation was observed.
In the U(VI)-SHA system the stability constant for the 1:1 complex, UO2[HOC6H4CONHO]+, is log β 0111 = 16.33 0.16 and for the 1:2 complex, UO2[HOC6H4CONHO]2, log β 0122 = 33.43 0.11. In the U(VI)-BHA system the stability constants results to log β 0110 = 7.49 0.03 for UO2C6H4CONHO+ and log β 0120 = 15.40 1.15 for UO2[C6H4CONHO]2. In the U(VI)-BA system the stability constant for the 1:1 complex, UO2C6H4COO+, was determined to log β 0110 = 3.02 0.13. All stability constants were averaged from the measurements by UV-vis and TRLFS.
The higher formation constants of uranyl with the aromatic hydroxamate compounds compared to benzoic acid points to the high potential of the pyoverdins to bind and mobilize uranium(VI) in the environment.

[1] M. Bouby, I. Billard, H. J. Maccordick, Czechoslovak. J. Phys. 49, 769-772 (1999).
[2] M. Glorius, H. Moll, G. Bernhard, Complexation of uranium(VI) with aromatic acids in aqueous solution – A comparison of hydroxamic acids and benzoic acid, Radiochimica Acta, in press (2007).

This work was funded by the BMWi under contract number 02E9985.