FIGURE 8.1 (A) A display of the block diagram of the "analytical construct" designed by Diversity Sciences. The keys represent the nonspecific version and the general version used for library production. (B) Displays the molecular structure of the commonly used construct during library production and reaction optimization.
A generic representation of the analytical construct designed by GlaxoSmithKline is shown in Figure 8.1. The diagram shows the construct has a linear format that consists of three linker groups, two of which have the same cleavage mechanism, and a third linker that has an orthogonal cleavage condition. In addition, there are two groups (mass-spectrometric levelers) that ionize readily for mass-spectral analysis, a signature peak element (peak splitter), and a mass-coding region. Both the mass-code block and the peak-splitting elements are incorporated into the construct with isotopically stable enriched reagents. The ionizable group or mass-spectrometric leveler can be positive or negative in nature, but is usually positive, because it has a higher sensitivity by mass-spectral analysis. Production of the analytical construct uses amide-bond formation chemistries with nearly 100% yields.
The standard analytical construct used for library production in Diversity Sciences at GlaxoSmithKline is shown in Figure 8.1. The analytical construct uses the common Knorr acid cleavable linkers at the first and second linker positions. The terminal linker is photocleavable and is released upon exposure to 350-nM light [12-15]. The construct contains the mass-code block and uses isotopically labeled Gly as a peak splitting element to facilitate compound identification by mass spectrometry. Also, the construct has two lysine amino acid groups to aid the ionization process of both the code block and the ligand block during mass spectrometry. The side chain of the lysine in the ligand block is used to provide a chemical spacer between the signature peak and the photocleavable linker. The N-terminal amine is butoxycarbonyl (Boc) protected during synthetic chemistry and serves as the ionization site upon acid cleavage. This spacer ensured that the desired library-specific organic chemistry is not hindered by the construct elements. At the completion of the analytical construct, the library-specific chemistry is carried out on the photocleavable link, as illustrated in Figure 8.1.
The initial concept and practical use of analytical constructs was designed by Diversity Sciences to facilitate mass-spectral analysis during combinatorial processes . Several internal and external groups adopted the original concept and published similar results [16-20]. Each element of the analytical construct developed by Diversity Science is outlined below in more depth in the following sections.
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