State conjugated side chains

The most significant amino acids for modification and conjugation purposes are the ones containing ionizable side chains aspartic acid, glutamic acid, lysine, arginine, cysteine, histidine, and tyrosine (Figure 1.6). In their unprotonated state, each of these side chains can be potent nucleophiles to engage in addition reactions (see the discussion on nucleophilicity below). 

The formation of an aldehyde group on a macromolecule can produce an extremely useful derivative for subsequent modification or conjugation reactions. In their native state, proteins, peptides, nucleic acids, and oligonucleotides contain no naturally occurring aldehyde residues. There are no aldehydes on amino acid side chains, none introduced by post-translational modifications, and no formyl groups on any of the bases or sugars of DNA and RNA. To create reactive aldehydes at specific locations within these molecules opens the possibility of directing modification reactions toward discrete sites within the macromolecule. 

Polythiophenes (PTs) have received a great deal of attention due to their electrical properties, environmental stability in doped and undoped states, non-linear optical properties, and highly reversible redox switching [1]. Thiophene possesses a rich synthetic flexibility, allowing for the use of several polymerization methods and the incorporation of various side chain functionalities. Thus, it is of no great surprise that PTs have become the most widely studied of all conjugated polyheterocycles [184]. 

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