Krishna Kumar
Department of Chemistry, Tufts University, Medford, MA 02155-5813

The project aimed to create proteins with fluorous (highly fluorinated) interiors by incorporating fluorinated amino acids (building blocks) into a coiled coil model protein. We are happy to report the successful design, synthesis and characterization of such peptide materials and their characterization. The funds were used mainly to purchase trifluoroleucine and trifluorovaline and these were resolved by enzymatic deacylation (prepared for use in peptide synthesis). The peptides was assembled on the solid phase by manual synthesis and characterized by circular dichroism spectroscopy and sedimentation equilibrium analysis.  The sequence was based on the well studied coiled coil region of the yeast transcription factor GCN4. All four leucine residues (a position) and three valine residues (d position) were replaced by the unnatural amino acids trifluoroleucine and trifluorovaline respectively.  The peptide is highly a—helical at low micromolar concentrations as judged by circular dichroism spectra, sediments as a dimeric species in the 5—30 mM concentration range and exhibits a dimer melting temperature that is 15 °C higher than a control peptide with a hydrocarbon core. Furthermore, the fluorous core is more stable towards denaturation by GdnÆHCl and urea. We conclude that additional stability is derived from sequestering the more hydrophobic trifluoromethyl groups from aqueous solvent. These studies have introduced a new paradigm in design of molecular self-assembling systems, one based on orthogonal solubility properties of liquid phases.

Figure 1. Computer generated model of a coiled coil structure decorated with fluorinated side chains in the fluorous core. Left: Figure highlights the close packing achieved by the trifluoromethyl groups in the dimeric form of the peptide. The side chains on the backbone are omitted for clarity. Right: A view from the top of the helix illustrating the superhelical twist and the fluorous core.

Publications acknowledging FRAC funds:
(1) "Fluorophobic Acceleration of Diels-Alder Reactions"
Kathleen E. Myers; Krishna Kumar J. Am. Chem. Soc. 2000, 122, 12025-12026.
(2) "A Coiled Coil with a Fluorous Core"
Basar Bilgicer; Alfio Fichera; Krishna Kumar J. Am. Chem. Soc. 2001, In Press.