FT-IR investigation of the uranium S-layer interaction in aqueous solutions

Many bacteria possess so-called surface layer (S-layer) proteins, forming paracrystalline lattice structures on the cell wall (1). Beside its ecological relevance for the retention of toxic metals, they are a good model system for investigations of the interaction of bio-ligands, e.g. proteins, with uranium in aqueous solutions. Several S-layers from different Bacillus strains were proved to have high binding capacity to uranium. But there is still little knowledge of the interactions between the functional groups of the protein with the actinyl ions at a molecular level (2).
In this work we present results of batch experiments where the uranium binding capacities of different S-layer protein from different Bacilllus strains were determined. The experiments were carried out at different pH values (pH 4, 6, and 8) for each isolated S-layer protein. It was found that the uranium binding capacities of each gram S-layer range from 5.0 mg and 63.9 mg, from 11.1 mg and 561.1 mg, and from 14.2 mg and 33.5 mg at pH 4, 6, and 8, respectively, depending on the respective S-layer.
For a deeper understanding of the molecular binding of the uranyl ion to the protein we used Attenuated Total Reflectance Fourier-transform Infrared spectroscopy (ATR-FTIR) which allows vibrational spectroscopic investigations of aqueous solutions containing actinide ions and disolved proteins as well (3, 4). The spectra clearly demonstrate carboxyl groups are the major functional groups which interact with the uranyl ions at pH 4. Additionally, the infrared spectra suggest the formation of different uranium-protein complexes depending on the incubation period (1 h vs. 48 h) which can be observed by a peak shift of the absorption band representing the antisymetric uranyl stretch to lower wavenumbers.
This spectroscopic approach constitutes the foundation of more detailed investigations on the impact of the pH value and of other functional groups (e.g. phosphate or amino groups) on the uranium complexation by bio-systems.

REFERENCES
(1) Sleytr, U.B., Beveridge T.J., Trends Microbiol. 1999 7(6), 253-260
(2) Merroun, M.L., et al. Appl. Environ. Microbiol. 2005, 71(9), 5532–5543
(3) Quilès, F.; Burneau, A. Vib. Spec. 2000, 23, 231-241.
(4) Fahmy, K. et al. Biophys. J. 2006, 91(3), 996-1007