Screened substrates

Adapted input window and screen substrate allowing transmission of low energy X-rays. For very low energy (typically below 30 keV) a thin Be window is used. Such a window in combination with a Be screen substrate will make possible imaging down to 3 keV. 

Figure 10.39 Synthesis of a novel N-hydroxypyrrolidine and a fluorogenic screening substrate for transaldolases based on stereospecific transketolase catalysis.

Even considering only the example of the proline family of aldol catalysts, it is dear that there will soon be hundreds of cases of organocatalysts described in the literature. Direct, organocatalytic aldol reactions do not yet have the generality of traditional stoichiometric methods, which can offer predictable results for a wide variety of substrates. However, companies already offer to screen substrates against panels of up to 200 enzymes to find the optimum biocatalyst for a reaction, and the same approach could be applied to identify rapidly the best organocatalyst for a process. 

Woodhall T, Williams G et al (2005) Synthesis of screening substrates for the directed evolution of sialic acid aldolase towards tailored enzymes for the preparation of influenza A sialidase inhibitor analogues. Org Biomol Chem 3 1795-1800... 

Substrate synthesis, reaction parameters, enantioenrichment via biocatalysis, catalyst handling, economic considerations Catalyst screening, substrate design, process optimization, application of Josiphos ligands, ligand synthesis, scale-up Production considerations, comparison with classical methods... 

A high surface energy substrate is most easily wet with a low surface tension liquid. In order to obtain good print resolution, the ink should not excessively wet the screen substrate and emulsion. The screen material can be treated to modify the surface energy so that the ink does not wet the screen. If the ink wets the screen but the ink cohesive forces are sufficiently strong to overcome ink-screen adhesion, well-defined ink columns are deposited. Ceramic substrates used in thick-film applications have sufficiently high surface energy and the ink wets the substrate. An excessive amount of surfactant can lower... 

Table 8.2 Critical Surface Tension of Commonly Screened Substrates and Diluting Acrylates...

Hiraoka C, Matsuda M, Suzuki Y, Fujieda S, Tomita M, Fuhshuku K, Obata R, Nishiyama S, Sugai T. Screening, substrate specificity and stereoselectivity of yeast strains, which reduce sterically hindered isopropyl ketones. Tetrahedron Asymmetry 2006 17 3358-3367. 

Derivatization of channel walls has also been reported for the successful incorporation of biocatalytic surfaces in microreactors. Such a reactor has been used in the chiral resolution of a variety of substrates (Scheme 6.20). Such methodology may be used to screen substrates rapidly for enzymatic evaluation. In a manner comparable with chemical catalysts, the use of immobilized enzymes is a cost-effective method for their recycle and reuse. Other examples of continuous flow resolutions have been demonstrated... 

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