Uranium(VI) Complexation with Pyoverdins and Related Model Compounds Studied by EXAFS

Synchrotron-based EXAFS spectroscopy is a powerful technique to obtain structural information on radionuclide bioligand species in solution. As an example pyoverdin-type siderophores are a unique class of bioligands, with a high potential to dissolve, bind, and thus transport uranium in the environment. Pyoverdins are secreted from fluorescent Pseudomonas species which are ubiquitous soil bacteria. The functional groups of the pyoverdin molecule, LH4, participating in metal binding are the catechol group of the chromophore and two ligand sites in the peptide chain, i.e., one or two hydroxamate groups and one or two -hydroxy acid moieties [1]. The formation of complexes of UO22+ with pyoverdins released by the groundwater bacterium Pseudomonas fluorescens (CCUG 32456) isolated at a depth of 70 m in the Äspö Hard Rock Laboratory, Sweden, was investigated in our previous study [2]. Two UO22+-P. fluorescens pyoverdin species, UO2LH2 and UO2LH-, could be distinguished on the basis of UV-vis spectroscopy and fs-TRLFS.

Little structural information is available regarding the U(VI) pyoverdin species formed in aqueous solutions. We therefore performed U LIII-edge EXAFS measurements of test solutions containing 5x10-4 or 0.001 M UO22+ and pyoverdins or related model compounds at an ionic strength of 0.1 M NaClO4. The pH was varied between 2 and 8 depending on the bioligand. EXAFS measurements were carried out on the Rossendorf Beamline (ROBL) BM20 at the ESRF [3]. The samples were measured at room temperature using a water-cooled Si(111) double-crystal monochromator in channel cut mode (5-35 keV). The spectra were collected either in fluorescence mode using a 13-element Ge solid-state detector or in transmission mode using Ar filled ionization chambers. The model compounds simulate the hydroxamate function (simple hydroxamate and a trihydroxamate compound) and the chromophore of the pyoverdin molecule. The obtained structural information for the axial and equatorial oxygen atoms surrounding the uranium atom will be presented. Estimations of the near order surrounding of uranyl in pyoverdin complexes are based on the comparison with those found in uranium(VI) model ligand species.