Bacterial mediated formation of Pd-nanoclusters for the development of biocatalysts

The bacterial strain Bacillus sphaericus JG-A12 was isolated from a uranium mining waste pile near the town of Johanngeorgenstadt, Germany. The cells of this strain are capable of selective and reversible accumulation of U, Cu, Pb, Al, and Cd from the U waste waters [1]. Further, they are enveloped by a surface layer (S-layer) which differs significantly in its primary structure from the other B. sphaericus S-layers studied up to date [2]. The highly ordered crystalline S-layers are one of the most common cell envelope structures of bacteria and archaea [3]. They are composed of protein monomers which possess the ability to self-assemble into two-dimensional paracrystalline lattices. The highly regular structure of the S-layers with many pores of identical size offers good binding sites for different metals and nucleation sites for the formation of metal nanoclusters or minerals.
EXAFS and IR analyses of the interactions of purified and recrystallised S-layer with Pd(II) demonstrated, that Pd(II) is bound to the carboxyl groups of the protein. In accordance to these results the C-terminal domain of the protein was found to contain aspartate and glutamate rich stretches which we assume to be responsible for the deposition of Pd in the pores of the protein lattices. Interestingly, the Laser Induced Infrared spectroscopic analysis showed that the deposition of the Pd complexes stabilizes the structure of the protein. The bound Pd(II) is reduced to metallic Pd nanoclusters in the presence of reducing agents which is in agreement with previous studies on another S-layer [4]. In addition, Pd nanoclusters are formed on the intact cells of B. sphaericus JG-A12 via a two step process consisting of biosorption (I) and of consequent metal reduction (II) in the presence of molecular H2 as an electron donor. The metallic nature of the clusters was confirmed by EXAFS and their size was estimated to be about 9-10 Ǻ. TEM analysis showed that the clusters are localised at specific sites of the bacterial cell wall, indicating the primary role of the S-layer in their formation.
The Pd nanoclusters formed on S-layer and bacterial cell templates are highly interesting for technical applications, e. g for the development of novel catalysts. Genetic engineering of the S-layer is applied for optimising its capability to form Pd nanoclusters with increased catalytic activity and also in order to extend its binding capacity to other metals of industrial interest.

1. Selenska-Pobell, S. et al. (1999) FEMS Microbiol. Ecol. 29, 59-67.
2. Raff, J. (2002) “Wechselwirkungen der Hüllproteine von Bakterien aus Uranabfallhalden mit Schwermetallen“ PhD Thesis, FZR-Report No. 358.
3. Sleytr, U. et al. (1997) FEMS Microbiol. Rev. 20, 47-97.
4. Wahl et al. (2001) Advanced Materials 13, 736-740.