Reaction of uranium(VI) with lipopolysaccharide - a laser spectroscopic study

Bacteria in the environment have a significant influence on the transport of heavy metals in nature. To estimate the mobility of actinides in natural systems, studies on the molecular level are essential to understand the accumulation and metabolism processes. The bacterial cell surfaces consists of various components which represent a wide range of reactive groups like phosphonate, carboxylate and hydroxyl groups, which are able to bind heavy metals, such as lipopolysaccharides [1].
We investigated the complex behaviour of the lipopolysaccharide (LPS) of pseudomonas aeruginosa serotype 10 with uranium(VI) in the aqueous system at pH 2 to 9 by time-resolved laser-induced fluorescence spectroscopy (TRLFS). The complexation of uranium(VI) by LPS effects a strong red shift of the fluorescence emission bands, connected with an increase of the fluorescence intensity, compared to the free uranyl ion, even at low pH ranges.
The shift of the emission bands shows clearly two different types of complexes. One complex appears only at very low pH (2 to 3) with a red shift of 8-9 nm compared to UO22+(aq). At higher pH the emission bands are even shifted 11-12 nm. This kind of red shift is clearly caused by phosphate groups [2,3]. The complex with the lower red shift at very low pH range can be assigned to a hydrogen phosphonate species, and the complex with the stronger red shift can be dedicated to a dehydrogenated phosphonate species.
The evaluation of the time-resolved measurement shows at least three different lifetimes, indicating more than two uranyl-LPS complex types, which are not to distinguish by the shift of the emission bands, but only by lifetime. Two lifetimes of about 8 and 15 µs can be associated with the two uranyl phosphonate complex types.
The complexation of the uranyl ion was completed even at a lesser molar extent of phosphonate groups over a wide pH range (2 to 7). This indicates a participation of carboxyl or hydroxyl groups, which can describe a further form of uranyl-LPS complex type, associated with a lifetime of about 100 µs.

[1] T.J. Beveridge, R.J. Doyle, Metal Ion and Bacteria, John Wiley & Sons Inc., New York, NY, 1989.
[2] A. Koban, G. Bernhard, J. Inorg. Biochem., in press, DOI: 10.1016/j.jinorgbio.2007.01.001 (2007).
[3] S. Scapolan, E. Ansoborlo, C. Moulin, C. Madic, J. Alloys Comp. 271-273, 106-111 (1998).