Optically active particles defined

There is great interest in the electrical and optical properties of materials confined within small particles known as nanoparticles. These are materials made up of clusters (of atoms or molecules) that are small enough to have material properties very different from the bulk. Most of the atoms or molecules are near the surface and have different environments from those in the interior—indeed, the properties vary with the nanoparticle s actual size. These are key players in what is hoped to be the nanoscience revolution. There is still very active work to learn how to make nanoscale particles of defined size and composition, to measure their properties, and to understand how their special properties depend on particle size. One vision of this revolution includes the possibility of making tiny machines that can imitate many of the processes we see in single-cell organisms, that possess much of the information content of biological systems, and that have the ability to form tiny computer components and enable the design of much faster computers. However, like truisms of the past, nanoparticles are such an unknown area of chemical materials that predictions of their possible uses will evolve and expand rapidly in the future. 

One of the rare examples for the use of immobilized oxynitrilases has been published by Degussa [146]. The company investigated the asymmetric synthesis of (i )-cyanohydrins and used (i )-oxynitrilase, which had been cross-linked and subsequently polyvinyl alcohol-entrapped. The obtained immobilized lens-shaped biocatalysts were much more satisfying in terms of long-term stability and activity compared to the free enzyme and also showed less catalyst leaching than other enzyme supports. Moreover, the immobilization method is cheap, efficient, feasible on an industrial scale, and gives particles of defined size. The utility of these entrapped enzymes could be shown, as indicated in Scheme 57, in the synthesis of (i )-mandelonitrile (R)-175) from aldehyde 174. No catalyst deactivation was observed even after 20 cycles of reuse and yields as well as optical purities of (R)-175 remained constant within normal limits. 

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