Unnatural amino acids with useful chemical functionality can replace phenylalanine in bacterial proteins. Coexpression of a promiscuous phenylalanine-tRNA synthetase mutant enables the synthesis of target proteins bearing iodophenyl, cyanophenyl, ethynylphenyl, azidophenyl, and pyridyl groups (see general structures). Proteins incorporating the analogues have a range of potential applications, including Pd-mediated conjugation (R=CCH), photoaffinity labeling (R=N3), X-ray phasing (R=I), and novel metal coordination (R=pyridyl).
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Kirshenbaum et al. (2002) studied this question.