Bacterial S-layers as template for the formation of Pd nanoclusters

Nano-catalysts made from palladium and other noble metals promise to accelerate chemical reactions even at low temperatures. Critical for the efficiency of such nano-catalysts is a perfect control of their cluster-size. By using the surface protein layer (so-called S-layer) of a bacterium, particles with a uniform size distribution of 50 to 80 Pd atoms can be generated. For the first time, the bonding sites between the metal and the S-layer protein have been characterized. Hereby, the prerequisite is given to manipulate this protein by genetic engineering enabling the design of materials with new optic, magnetic and catalytic properties.
The uranium uptake mechanism were investigated by EXAFS performed at the Rossendorf Beamline. The measurements revealed that the purified S-layer proteins of this bacterium coordinate uranium through phosphate groups of phosphorylated serine and threonine in a monodentate mode as well as through carboxyl groups of aspartic and glutamic acids in a bidentate mode.
The heavy metal binding capacity of the S-layer led to the idea to use it as template for the formation of Pd nanoclusters. Within the pores of the S-layer, isolated from vegetative bacteria, Pd(II) solutions are reduced to metallic palladium by the use of hydrogen.
In order to investigate the metal-protein interactions and their impact on the secondary structure, a solution of Pd(II) ions has been sorbed on the S-layer matrix. The combination of Fourier transform infrared spectroscopy and EXAFS spectroscopy revealed that the surface Pd(II) is predominantly coordinated by aspartic and glutamic residues through 4 nitrogen and oxygen atoms at a distance of 2.01 Å. In contrast to U(VI), that binds to carboxyl and phosphate groups, Pd(II) binds exclusively to the carboxyl groups of the S-layer. The topology of nitrogen- and carboxyl-bearing side chains appears to mediate the binding of heavy metals to aspartic and glutamic acids. These side chains are thus targets for the design of engineered S-layer based nanoclusters.