Efforts towards the design of 'teflon' proteins: in vivo translation with trifluorinated leucine and methionine analogues.

In vivo incorporation of monofluorinated noncanonical amino acids into recombinant proteins has been well-established for decades. Proteins fluorinated in this way proved to be useful tools for many practical applications. In contrast, trifluorinated amino acids have been incorporated in only a few peptides and relatively small proteins by using expression systems in living cells. A novel class of proteins with a fluorous core can be envisaged only if full replacement of the core-building hydrophobic and aliphatic amino acids such as leucine or methionine with the related analogues trifluoromethionine and trifluoroleucine would be feasible. However, our systematic efforts to introduce these amino acids in larger proteins (over 10 Da) that contain different structural motifs clearly show that only partial substitutions are possible. The reasons are high toxicity of these substances and difficulties to accommodate them into the compact cores of natural proteins without adverse effects on their structural integrity. Therefore, engineering of such three dimensional 'Teflon'-like structures would require, besides an expansion of the amino acid repertoire of the genetic code, a de novo protein design as well.