Genetic Encoding of para -Pentafluorosulfanyl Phenylalanine: A Highly Hydrophobic and Strongly Electronegative Group for Stable Protein Interactions.

SF ₅ Phe, para -pentafluorosulfanyl phenylalanine, is an unnatural amino acid with extreme physicochemical properties, which is stable in physiological conditions. Here we present newly developed aminoacyl-tRNA synthetases that enable genetic encoding of SF ₅ Phe for site-specific incorporation into proteins in high yields. Owing to the SF ₅ moiety's dichotomy of strong polarity and high hydrophobicity, the unnatural amino acid forms specific and strong interactions in proteins. The potential of SF ₅ Phe in protein research is illustrated by (i) increasing the binding affinity of a consensus pentapeptide motif toward the β subunit of Escherichia coli DNA polymerase III holoenzyme by mutation of a phenylalanine to a SF ₅ Phe residue, (ii) site-specifically adhering β-cyclodextrin to the surface of ubiquitin, and (iii) selective detection of ¹⁹ F- ¹⁹ F nuclear Overhauser effects in the Escherichia coli peptidyl-prolyl cis / trans -isomerase B following mutation of two phenylalanine residues in the core of the protein to SF ₅ Phe. With increasing use of the SF ₅ moiety in pharmaceutical chemistry, this general method of functionalizing proteins with SF ₅ groups opens unique opportunities for structural biology and in vivo studies.