Library large-mixture

For solution-phase libraries that are composed of mixtures of compounds, the difficulty of analysis escalates with increasing numbers of compounds. Typically, large mixtures of compounds are not analyzed before screening, whereas small ones may be analyzed for reaction completeness using mass spectrometry, HPLC, NMR, or combinations thereof. The identification and analysis of active compounds from these mixtures is painstakingly tedious, and often complete characterization is possible only after deconvolution procedures and resynthesis of the active compound. For solid-phase libraries, the methods currendy developed are discussed below. 

Much of the early work in combinatorial chemistry focused on the preparation of large mixtures of compounds. The most widely used technique for mixture synthesis is the split/recombine method which assures that each component of the mixture is present in approximately equimolar concentrations. The structures of the bound ligands are determined either through an iterative, or reclusive, deconvolution strategy or through the use of encoded libraries. 

There are two approaches to analytical evaluation of OBOC libraries analysis of compounds from individuals bead [51,60] and analysis of mixtures of compounds detached from multiple beads [61, 62], It should be mentioned that analysis of mixtures from OBOC libraries is quite different from that described in section Analysis of Libraries Synthesized in the Format of Small Mixtures of 4-12 Compounds per Mixture. In the mixture of compounds cleaved from a few beads, which are randomly picked for analysis from the OBOC library, the structures of components of the mixture are unknown. Consequently, it is not possible to apply the strategy developed for the analysis of mixtures synthesized in spatially addressed format, which is based on information about the structures of components of each mixture. In the case of OBOC libraries, analysis of only very large mixtures is meaningful when the theoretical distribution of molecular weights within the whole library is compared with that experimentally observed [61, 62], The focus of this section is on characterization of OBOC libraries through analysis of individual beads. 

Techniques for the production of individual cDNAs, cloned in bacteria at the rate of one cDNA per bacterium, were multiplexed to produce cDNA libraries , large collections of cDNAs, simply by repeating the cloning procedure with complex cDNA mixtures. These complex cDNA mixtures, now in bacteria, can be surveyed by retrieving and sequencing each individual cDNA, giving a faithful representation of the abundance of each cDNA in the original mixture if sufficient cDNAs are sequenced. 

Because of concerns about not detecting low concentrations of active compounds in such large mixtures, the current trend is toward much smaller mixtures containing only hundreds or even dozens of compounds. For example, the Affymax group prepared a library containing mixtures of only 540 compounds and identihed potent cyclooxygenase-1 inhibitors after deconvolution (34). The issue of optimum mixture size is not yet settled and discussion will no doubt continue for some time. 

The physical format of the compound library must also be determined before starting an HTOS experiment. The decision as to whether to synthesize mixtures of compounds or discrete chemical entities is related to the purpose for synthesizing the library. Libraries of mixtures allow for the screening of more compounds than discrete libraries, but large numbers of compounds per pool can adversely affect the detection of active compounds. The testing of mixtures also introduces the possibility of signal-to-noise ratio deterioration or the introduction of false positives (30). Within the pharmaceutical industry today, the trend is to produce smaller libraries of well-defined (discrete) compounds using parallel synthesis techniques (31). 

Over the last five years or so there has been an explosion in the use of solid phase synthesis when it was realised that the method could be adopted in combinatorial synthesis to allow rapid, even automated, synthesis of large libraries or mixtures of organic molecules [81-86]. This possibility has been seized upon by drug discovery groups in pharmaceutical companies as a rapid method of preparing and screening large numbers of compounds in the search... 

Type of library random libraries large compound libraries consisting of highly diverse compounds mixtures and single compounds)... 

To make the actual analysis simple and fast, a large amount of precomputed information is utilized. By using library spectra of pure molecular gases together with a special mathematical multicomponent analysis, it is possible to calculate the partial pressures of the gases in a gas mixture. 

Complex optimization of the ligand-protein interactions require to scan large areas of the chemical space. Thus, the combinatorial chemist aims not at the preparation of single compounds but of chemical libraries. Chemical libraries can be produced as collections of single compounds or as defined mixtures. 

Today, analytical evaluation is done on a large scale in a computerized way by means of data bases and expert systems (Sect. 8.3.6). In particular, a library search is a useful tool to identify pure compounds, confirm them and characterize constituents in mixtures. Additionally, unknown new substances may be classified by similarity analysis (Zupan [1986], Hippe [1991], Warr [1993], Hobert [1995]). The library search has its main application in such fields where a large number of components has to be related with large sets of data such as environmental and toxicological analysis (Scott [1995], Pellizarri et al. [1985]). 

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