The properties of metallic arrays critically depend on the size and shape of the colloidal metallic particles they are made from as well as on their one, two or three dimensional arrangement. It is believed that biomacromolecular assemblies offer a great potential to develop novel inorganic materials useful for electronics, information processing, and catalytic processes. One interesting direction is the involvement of biotemplating methods which take advantage of the characteristic nanometre-scaled structuring of the biological specimens to build up defined solid nanoparticle patterns. The usability of biological templates for such purposes has already been proved with DNA, viruses, antibodies, S-layers, and microtubules.
In the present study, we used microtubules as templates. Microtubules are highly ordered protein polymers  consisting of regularly arranged alpha-beta tubulin dimers which have a length of about 8 nm and a diameter of 4 5 nm. We describe the formation and the binding of nanometre-scaled noble metal particles, especially Pd, on the microtubule surface. Dependent on the reaction conditions used the mean particle diameter was found between 1.5 nm and 3.5 nm. Regarding their neighbourhood relationship, the immobilized particles were mainly equally spaced distributed with maxima in the frequency distribution at distances near 5 nm. The particles formed organized arrays, ordered chains, and defined patterns, reflecting the regular arrangement of the alpha- and beta-tubulin subunits within the microtubule.
The surface of the tubulin molecules exposes a defined pattern of amino acid residues that provides a wide variety of active sites for nucleation, organization, and binding of metal particles. The metallic character of the nanometre-sized particles grown on the microtubule surface has been shown by high resolution transmission electron microscopy.
Taking the crystal structure of tubulin, derived from electron crystallography data of taxol-stabilized Zn-induced two-dimensional tubulin sheets , it was shown that histidines are centrally located on both alpha-tubulin (at positions 309, 393, and 406) and beta-tubulin (at position 309) at the microtubule outer surface. These histidines are easily accessible and correspond to the pattern of the Pd particles deposited. Thus, we assume that these histidines are candidates for the binding of the Pd particles. Histidines are known to be also common ligands coordinating transition metals. For tubulin Zn was found to bind to his192 and his283 of the alpha-subunit .
Depending on the kind of reducing agent and the metal concentration used during synthesis, palladium nanoparticles with different sizes were synthesized. By further particle growth, a quasi-continuous microtubule coating was obtained resulting in Pd nanowires.
We have shown that microtubules are efficient bioorganic templates for the support of noble metal particles to form structurally defined nanoparticle arrays. Under appropriate conditions, every tubulin molecule is able to nucleate and to bind a palladium nanoparticle thus forming regular arrays that reflect the tubulin pattern in an isomolecular fashion. Such an organization of nanoparticles into functional 2D and 3D structures is a potential approach to develop electronic devices with novel superior properties.
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