Microtiter Platebased Positional Encoding

The 96-well microtiter format has been a common platform for high throughput screening (HTS) for many years, and also offers many advantages as a platform for parallel synthesis. There are multiple liquid-handling devices (multichannel pipettes, robotic systems, etc.) that significantly simplify and accelerate the process of the reagent delivery to the wells on the plate. Moreover, all 96 compounds can be handled as one synthesis entity. Their structures are spatially encoded by their location on the plate. Contemporary analytical instrumentation, such as mass spectrometers and HPLC systems, can also easily handle compounds in the 96-well microtiter format. Combined with the commonly used 96-well-based systems for screening, this eliminates the need for time-consuming redistributing and relabeling of a large number of synthesized compounds for analytical and biological characterization.

Synthesis on multipins In 1984, Geysen and coworkers [23] introduced an alternative to the synthesis on polymeric beads. As a solid support, they proposed the use of ''pins''; these are reusable polyethylene rods, which have a diameter of 4 mm and a length of 40 mm, and are grafted with polyacrylic acid. The pins are attached to a supporting block arranged in an 8 x 12 array to fit into a 96-well mi-crotiter plate. The 96-well plate is used as 96 separate reaction vessels. Each well can be used for the attachment of a different building block. Washing and de-protection steps can be carried out in a separate common reaction vessel. After the synthesis is completed and side-chain protecting groups are removed, peptides or small organic molecules can be assayed while attached to the pins [24, 25]. If a cleavable linker was used, individual peptides are separated from the solid support in a 96-well microtiter plate containing appropriate reagents [26], or by exposing the array to vapor-phase ammonolysis [27, 28]. Originally, the capacity of each pin was 10-100 nmol. Later, a much greater variety of multipins of different capacities with a wide selection of linkers became commercially available from Chiron Technologies Pty. Ltd. In the latest development, the pins can be attached to the radio-frequency tags to allow for encoding in the ''split-and-pool'' mode.

HiTOPS system Over time, original approaches have been developed by many companies to perform the synthesis of organic compounds on the footprint of a 96-well plate. The HiTOPS (high-throughput organic parallel synthesis) system [29] (Scheme 5.11) utilizes a variety of 96-deep-well filtration microtiter plates available from Polyfiltronics/Whatman.

The plates are made of polypropylene and other polymers, and are available with a selection of different filters. The volume of each well is 2 mL and allows the use of up to 50 mg of resin. For larger scale syntheses, several wells or even the entire row or the entire column can be used for the preparation of the same compound. Reactants are retained in wells by the positive pressure of an inert gas.

Scheme 5.11. HiTOPS system. Synthesis device (left) and cleavage device (right).

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