Genetic incorporation of a 2-naphthol group into proteins for site-specific azo coupling.

The 2-naphthol analogue of tyrosine, 2-amino-3-(6-hydroxy-2-naphthyl)propanoic acid (NpOH), has been genetically introduced into proteins in Escherichia coli . This is achieved through the directed evolution of orthogonal aminoacyl-tRNA synthetase/tRNA pairs that selectively charge the target amino acid in response to the amber stop codon, UAG. Moreover, chemoselective azo coupling reactions have been revealed between the 2-naphthol group and diazotized aniline derivatives that are substituted with an electron donating moiety. The coupling reactions required a very mild condition (pH 7) with great reaction rate (less than 2 h at 0 °C), high efficiency, and excellent selectivity.