Sequential chemical tagging methods

Compounds used for tagging must satisfy a number of criteria  

the concentration of the tag should be just sufficient for its analysis, that is, the majority of the linkers should be occupied by the combinatorial synthesis  

the tagging reaction must take place under conditions that are compatible with those used for the synthesis of the library compound  

it must be possible to separate the tag from the library compound and 

analysis of the tag should be rapid and accurate, using methods that could be automated. 

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Sequential chemical tagging methods uses specific compounds (tags) as a code for the individual steps in the synthesis. These tag compounds are sequentially attached in the form of a polymer-like molecule to the same linker or bead as the library compound at each step in the synthesis (Figure 6.10). The amount of tag used at each step must be strictly controlled so that only a very small percentage of the available linker functional groups are occupied by a tag. At the end of the synthesis both the library compound and the tag compound are liberated from the bead. The tag compound must be produced in a sufficient amount to enable it to be decoded to give the history and hence the possible structure of the library compound. 

Sequential analysis of amino acids in purified peptides and proteins is best initiated by analysis of the terminal amino acids. A peptide has one amino acid with a free a-amino group (NH2-terminus) and one amino acid with a free a-carboxyl group (COOH-terminus). Many chemical methods have been developed to selectively tag and identify these terminal amino acids. 

Direct structure determination methods, where positives are characterized directly via off-bead or on-bead identification of their chemical structure, will be described in detail in this section. Indirect methods that determine the structure of positives from the library architecture will be covered later they use either deconvolutive methods (Section 7.3), where the iterative synthesis of library pools with decreasing complexity via sequential determination of the best monomers leads to the identification of a positive structure, or encoding methods (Section 7.4), where, during the library synthesis, the structure of each component is coupled to a tag that can be read from a single bead after the library screening. 

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