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

SFPhe, -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 SFPhe for site-specific incorporation into proteins in high yields. Owing to the SF moiety's dichotomy of strong polarity and high hydrophobicity, the unnatural amino acid forms specific and strong interactions in proteins. The potential of SFPhe in protein research is illustrated by (i) increasing the binding affinity of a consensus pentapeptide motif toward the β subunit of DNA polymerase III holoenzyme by mutation of a phenylalanine to a SFPhe residue, (ii) site-specifically adhering β-cyclodextrin to the surface of ubiquitin, and (iii) selective detection of F-F nuclear Overhauser effects in the peptidyl-prolyl /-isomerase B following mutation of two phenylalanine residues in the core of the protein to SFPhe. With increasing use of the SF moiety in pharmaceutical chemistry, this general method of functionalizing proteins with SF groups opens unique opportunities for structural biology and studies.