Protein Supported Metallic Nanostructures as Catalysts
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several micrometers [3].
ýZ~2 37 0 C -b-
aIp tubulin 25 nm microtubule Figure 1: Microtubules are formed by a self assembly process at physiological pH and temperature.
Figure 2: Scanning electron micrograph of microtubules, negatively stained with 1% uranyl acetate solution, 20 kV, m=100000 x 65
Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
EXPERIMENTS Microtubule assembly The microtubule (MT) samples were prepared by an in vitro self-assembly process of pure tubulin heterodimer isolated from porcine brain. The final protein concentration was about 1 mg/ml. The MTs were assembled in a buffer solution of 20 mM PIPES (1,4-piperazine diethane sulfonic acid, pKa 6.8), 80 mM NaCl, 0.5 mM MgC12, 1 mM EGTA (ethylene glykole-bis-(2-aminoethyle)-tetraacetic acid) by adding of 0.25 M GTP (guanosin-5'triphosphate) and 10 mM taxol (from taxus brevifolia) and warming the sample up to 37 'C. The MT formation was accompanied by turbidity measurements at 340 nm wavelength. The steady state level, at which the tubublin mass in the polymerized state shows no further increase, was usually observed after 20 min. The assembled microtubules were chemically fixed by vigorous stirring in 0.05% and then after 5 min in 3% glutaric aldehyde. The samples were dialyzed against buffer/H 20 to eliminate excess glutaric aldehyde.
Synthesis of nanoparticles 200 gIl of an aqueous Na2 PdCl4 (0,0125 M, pH 6.8) solution were added to 100 gl of the assembled microtubules and 280 gl of H2 0. The Pd 2 - ions were reduced with trisodium citrate (0.25 M in H 20) at 90'C in 1 h. The sample was dialyzed against buffer/H 2 0 to eliminate excess Pd2 +. 15 ml of a commercial gold colloid (5 nm, 10 nm, 20 nm and 60 nm in diameter) were concentrated, each added to 100 gl of the assembled microtubules and immobilized on the microtubules within 30 min at room temperature. 2 nm gold particles were synthesized on microtubules by reduction of a HAuC14 solution (1 g/ml in H20) with NaBH 4 (0.5 eq in H2 0) at 0' C. These gold particles served as nucleation seeds for the following reduction of 400 ll Na2 PdC14 (0.0125 M in buffer/H 20 1:10, pH 6.8) with 40 ll trisodium citrate (0.25 M in H 20) at 70'C within 30 min. The sample was dialyzed against buffer/H 20 to eliminate excess Pd2+ Catalytic Activity The catalytic activity for the hydrogenation of crotonic acid was determined by measuring the hydrogen consumption per mass of noble metal and time in a standard test reactor at 25 °C and atmospheric pressure. The total metal content was analyzed by ICP measurements. RESULTS Following a bottom-up approach noble metal particles in the nanometer size range are obtained by the reduction of the corresponding metal salts in the presence of the protein assemblies. By reduction of an aqueous Na2PdCI 4 solution, palladium particles (1 - 5 nm) could be nucleated and immobilized on the tubulin lattice of microtubules (figure 3). The catalytic activity of these microtubule-immobilized palladium nanoparticles in the hydrogenation reaction of crotonic aci
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