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Posted: Aug 23, 2006

Amazing survivors: bacteria from radioactive wastes that produce precious metal nanoparticles

(Nanowerk Spotlight) Some microbes are able to tolerate radioactivity and other toxic environments because they developed detoxification mechanisms that allow them to resist adverse environments without being damaged. These protective mechanisms increasingly are of great interest to scientists not only for developing innovative remediation strategies but also for creating novel biotechnological applications. As a recent example, researchers in Germany managed to produce highly stable and regular palladium (Pd) nanoparticles by harnessing the survival mechanism of bacteria found in uranium-polluted waste. These particles showed much improved catalytic activity and other new physical properties, which make them ideal for use as nanocatalysts or nanosensors.
Back in 1997, a team of biologists from the Institute of Radiochemistry at the Forschungszentrum Rossendorf (FZR) in Dresden, Germany, discovered a microbe called Bacillus sphaericus JG-A12 in the waste of an uranium mining site. JG-A12 exhibits a high metal-binding capacity, indicating that it might provide a protective function by preventing the cellular uptake of heavy metals and radionuclides.
The researchers working on interactions of actinides with biomolecular surfaces found that JG-A12 protects itself from the toxicity of uranium by selectively binding large amounts of the toxic metal on its protective protein surface layer (S-layer). The S-layers are surface structures of bacteria that form highly regular protein lattices, covering the whole cell. The lattice pores measure only a few nanometers.
Dr. Sonja Selenska-Pobell, the group leader Molecular Microbiology in the Institute of Radiochemistry at the FZR, explained the recent findings to Nanowerk: "Although JG-A12's properties allowed us to use S-layers as self-assembling organic templates for the synthesis of heavy metal nanocluster arrays, we knew little about the molecular basis of the metal-protein interactions and their impact on secondary structure."
In recent work ("Secondary Structure and Pd(II) Coordination in S-Layer Proteins from Bacillus sphaericus Studied by Infrared and X-Ray Absorption Spectroscopy"), Selenska-Pobell and her colleagues localized the sites on the S-layer protein of JG-A12 that are involved in the metal complexation.
"These sites are the stretches enriched with amino acid residues, such as glutamat and aspartat" says Selenska-Pobell. "The first indications for the complexation of Pd via these carboxylated amino acids were obtained by our colleague Dr. Karim Fahmy, from the Institute of Radiation Physics at the FZR, using FTIR spectroscopy. The exact localization of the metal binding sites was performed combining the use of the endoproteinase Glu-C cleaving after carboxylated amino acids with our knowledge of the primary structure of the protein. The analysis of the atomic structure of the Pd / S-layer protein complexes by x-ray spectroscopy confirmed our estimations."
It is particularly interesting that the Pd nanoparticles make the S-layer extremely stable against higher temperatures as well as highly acidic pH.
Selenska-Pobell notes that it was demonstrated previously that the S-layer selectively binds uranium and several other metals, including platinum and Pd ("Complexation of Uranium by Cells and S-Layer Sheets of Bacillus sphaericus JG-A12").
"On the basis of the knowledge gained in our recent work it would be possible to solve the problem of binding only particular metals through the protein" she says. "By site-specific genetic manipulations of the S-layer one can increase the spectrum of metals bound by it. It would be possible, for instance, to insert additional stretches of carboxylated amino acid or to delete some of them from the native protein. These manipulations can be used also to control the size of the formed nano-particles.
The novel experiments of the Rossendorf researchers make two areas of bacteria-based applications feasible:
1) Modification of the S-layer protein genes and construction of novel S-layers exhibiting affinity to a wide range of metals – Ag, Au, Pb, Fe, etc. – which can be used as nano-catalysts and nano-sensors in industry.
2) Construction of nanoscale bio-ceramics for decontamination of metal polluted industrial liquid wastes. "Until now we have some positive results with cleaning of uranium-contaminated waters ("Biosorption of Uranium and Copper by Biocers") and some preliminary results for enhanced Ni-binding by a genetically modified S-layer" says Selenska-Pobell.
By , Copyright Nanowerk LLC

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