Traditional Library Screening

The most common tool for discovering hits is library screening. The library may consist of traditional compounds with potentially high activity molecules, smaller fragments of less activity, or even virtual molecules tested through molecular modeling simulations. 

Compounds are normally stored as a stock solution to be dispensed as needed by robotic equipment. Dimethylsulfoxide (DMSO) is a preferred solvent for several reasons. First, in low concentrations, DMSO is well tolerated in most assays. Second, the low melting point of DMSO (18 °C) allows samples to be easily frozen. Compounds that are stored in the solid state are less prone to decompose. Third, DMSO is less volatile than most organic solvents. Decreased volatility minimizes solvent evaporation, so concentrations remain nearly constant over prolonged storage. Maintaining a known concentration is vital. The activity of each screened molecule is related by a concentration-effect relationship. If the concentration of a stock solution is not accurate, then any subsequent assessment of activity will also be incorrect.1 

The purity of compounds in the library is an important factor. Reaction side products (e.g., triphenylphosphine from a Wittig reaction) should be completely removed from a compound in a library. Similarly, compounds should be free of any solvents used in their synthesis. Unfortunately, not all impurities are easily removed. Impurities can sometimes interfere with an assay and lead to inaccurate results.2 

SCHEME 10.1 Isomeric products often found in libraries 

Compound libraries may be bought and sold individually. After the dissolution of the Soviet Union in 1991, laboratories that had been formerly well funded by the Soviet government suddenly became essentially broke. As a means of generating funds, some research groups began to sell portions of their in-stock compounds. The samples were readily purchased by Western companies, including the pharmaceutical industry. The value of the compounds depends on the novelty of the structures and their purity. 

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Traditionally, libraries of enzymes - either naturally occurring or deliberately created mutants - are created in bacterial cells and the resulting clones are screened (Fig. 2, left). This is a time-tested strategy, but it suffers from two... 

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