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step 1 (1st cycle) splitting of the beads step 3 mixing of the beads step 1 (2nd cycle) splitting of the beads step 2 J reactions A, B, C

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step 3

mixing of the beads tA^cE

Fig. B.11.1. Basic concept of split synthesis.

pounds. Because each bead reacts with only one set of reagents in each cycle, each bead carries only a single compound (''one-bead-one-compound''); in other words: each compound is located on a different bead [1, 2].

Split-and-mix libraries have found numerous applications in the search for a substrate for a given receptor and vice versa. A typical affinity assay encompasses labeling of the host of interest with, e.g., a dye, a fluorophore, or radioactivity and equilibrating the labeled host with the bead-supported library of potential guests. The labeled host will be concentrated by those beads that carry molecules with affinity for the receptor. These beads are easily identified by visual inspection of the assay under a low-power microscope. Isolation and structural analysis reveals the structure of the active compound [3]. Recently, split-and-mix libraries are also successfully applied in the search for catalysts (see Chapter 5.4).

Because each resin bead carries, typically, only 100 pmol compound the structure of the compound cannot be elucidated by conventional IR, NMR, etc. Although MS is a convenient analytical tool, it fails if the library contains isomers with identical mass. An elegant method for determining the structure of each library compound is molecular encoding. Easily detectable molecular tags are attached to the beads as they proceed through the split-and-mix synthesis, thereby recording the reaction history of each individual bead. After the screening of the library, these molecular tags can be cleaved from each of the selected beads and analyzed to reveal the structures of the library members on these beads. The encoding methods most commonly used employ either microsequenceable oligonucleotides (PCR analysis), oligopeptides (Edman degradation), or small molecules that can be analyzed by gas chromatography, high-performance liquid chromatography, or mass spectrometry [4].

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