Still et al. reported the first encoding method utilising such chemically stable tagging moieties. The tag consisted of haloaromatic reagent linker to a carboxylic acid though an internal photochemically cleavable linker. Amide bond chemistry served to attach the tag to the beads. These haloaromatic reagents acylated the same synthesis sites used for ligand synthesis (Figure), but due to the sensitivity of tag detection this competition could be minimised. Once the haloaromatic analyte was attached to the bead it could be selectively detached into solution upon photolysis with UV light. The liberated tag could then be resolved and detected as subpicomole concentrations using electron capture capillary gas chromatography EC-GC.
Chemically "hard" haloaromatic tag suitable for encoding applications where the beads will be exposed to rigorous synthetic conditions the tag are released photochemically and then detected via EC-GC.
A: haloaromatic tags incorporated via amine bond chemistry at the expense of the ligand synthesis sites. B: tags incorporated via carbene insertion. In both A & B tag concentration are minimised to prevent chemically derivation of the encoded ligands or the quenching of their synthesis sites.
While hard tagging strategies have been successfully used to encode a variety of different synthetic
chemistries, a common limitation remains – the requirement for parallel synthesis (ligand and encoding tags).
Since the robust preparation of a large combinatorial library is frequently a difficult synthesis challenge, it would be desirable to obviate the need for tag cosynthesis and instead delineate individual compounds by other physical means. Recent approaches that can replace or minimise the need to cosynthesise a surrogate are described in the following methods: Radio Frequency Encoding & Mass Encoding.