Pure bioactive compound for further development
Figure 2 The two different approaches for discovery of novel discrete compounds from plant sources.
cation procedures, which are still performed at a low rate of automation. In addition, lack of standardization with every secondary product being present in different—and sometime insufficient—concentration remains a major bottleneck.
Strategy 2 is, in principle, a further development of the chemical screening approach originally developed by H. Zahner in close cooperation with the chemists W. Keller-Schierlein and A. Zeek during the 1970s (8). In the original procedures, compounds were first separated by TLC, isolated, and their structure determined. Purified compounds were applied for screening, or screening was performed directly on TLC plates.
This concept has now being further developed based on advances in high-performance liquid chromatography (HPLC), MS, and other analytical methods. Based on preparative HPLC, the company AnalytiCon AG (Berlin) and E. Merck KGA (Darmstadt) have developed a preparative HPLC (SepBox) that is able to separate gram quantities of crude extracts into hundreds of pure or almost pure compounds. The concentration of the com pounds could be standardized using general detection methods such as evaporating light scattering (ELSD) or MS. Replicate samples can be eliminated prior to the screening by their MS profile, and all new structures can be applied to the screening process. However, with the further development of miniaturized screening devices and assay volumes in the very low microliter or even nanoliter range, each fraction from even a standard preparative HPLC separation can now be tested directly with sufficient material left for at least initial structural elucidation.
A different approach could be screening performed on line with the separation. Such an approach has been developed by the company Screentec in The Netherlands (www.screentec.nl).
Although very powerful, strategy 2 has not yet been implemented on a large scale. Reasons may be its novelty but also the very high investment and running costs. However, as soon as more efficient on-line screening protocols and cost-effective automation become available, this may change and create a new era of natural product discoveries.
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