Photoreactive bicyclic amino acids as substrates for mutant Escherichia coli phenylalanyl-tRNA synthetases

Unnatural amino acids carrying reactive groups that can be selectively activated under non-invasive biologically benign conditions are of interest in protein engineering as biological tools for the analysis of protein-protein and protein-nucleic acids interactions. The double ring system phenylalanine analogues benzofuranylalanine and benzotriazolylalanine were synthesized, and their photolability was tested by UV irradiation at 254, 320, and 365 nm. Although both showed photo reactivity, benzofuranylalanine appeared as the most promising compound because this amino acid was activated by UVA (long wavelength) irradiation. These amino acids were also tested for in vitro charging of tRNA(Phe) and for protein mutagenesis via the phenylalanyl-tRNA synthetase variant alphaA294G that is able to facilitate in vivo protein synthesis using a range of para-substituted phenylalanine analogues. The results demonstrate that benzofuranylalanine, but not benzotriazolylalanine, is a substrate for phenylalanine tRNA synthetase alphaA294G, and matrix-assisted laser desorption ionization time-of-flight analysis showed it to be incorporated into a model protein with high efficiency. The in vivo incorporation into a target protein of a bicyclic phenylalanine analogue, as described here, demonstrates the applicability of phenylalanine tRNA synthetase variants in expanding the scope of protein engineering.