Spotting of Small Molecules

As combinatorial chemistry has become an increasingly established method for the production of different sets of small molecules, a strategy was born to combine classic bead-based combinatorial synthesis with the preparation of arrays by spotting (Table 6.2.1). In 1999 Schreiber et al. were the first group to ''print'' small molecules on an inorganic planar support to study protein-ligand interactions [26]. They modified a standard microscope slide by attaching maleimide groups on its surface. They then spotted the dye rhodamine and three molecules with known protein-binding properties and containing free thiol groups on to the glass slide. The thiol groups reacted by 1,4-addition with the maleimide resulting in the desired covalent linkage to the surface. As expected, all three ligands participated in specific binding with their fluorescence-labeled protein partners - biotin (strep-tavidin), digoxigenin (monoclonal antibody DI-22), and a synthetic pipecolyl a-ketoamide (FKBP-12). By ''printing'' lower affinity ligands of FKBP12 they could show that the intensity of fluorescent spots correlated well with the affinity of the protein for the immobilized compounds. Being aware of the limitations imposed by the thiol group Schreiber et al. expanded their methodology to include alcohol-containing small molecules ''printed'' on glass slides (Scheme 6.2.1). With this strategy they could build upon well established solid-phase synthesis on polymeric supports using silyl linkers, cleave the small molecules by addition of HF-pyridine, and spot these stock solutions on activated glass slides [27].


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