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Effect of Fluoro-Amino Acids on Protein Secondary Structure Stability Richard Cheng, Department of Chemistry, University at Buffalo, SUNY Buffalo, NY 14260-3000 Unfolded Monomeric Folded Multimeric Fluorocarbon Protein Stability Hydrocarbon Protein Stability Energy Protein Helix Helix Bundle Protein The stability of enzymes catalysts for application in organic synthesis can be 0.49 improved by incorporating fluorinated amino F3C 1.15 acids. This fluoro-stabilization effect has been attributed to hydrophobics, because the H3N COO Atb difference in hydrophobicity between Leu and Hydrocarbon Fluorocarbon Hfl (0.4 kcal·mol-1·residue-1) is similar to the Stabilization Stabilization change in overall helical protein stability upon H3N COO substituting Leu with Hfl (0.32-0.83 kcal·molPreviously Estimated 0.32-0.83 Leu Fluoro-Stabilization Effect 1·residue-1). This estimation of the fluoroCF3 stabilization effect assumes that the helix F3C propensities are the same for the two amino H3N COO Hfl acids. We aim to measure the -helix and -sheet propensities of fluoro-amino acids. This would be useful for quantitatively predicting the effect of fluoro-amino acids on protein stability. We have measured the helix propensity of fluoro-amino acids such as Hfl, showing that helix propensity decreases significantly upon introducing fluorines. Our results show that Hfl is significantly less favorable than Leu for helix formation. Therefore, the fluoro-stabilization effect is larger than the previous estimation of 0.32-0.83 kcal·mol-1·residue-1 by 1.15 kcal·mol-1·residue-1 for Hfl. These results suggest that the fluorostabilization effect more than overcomes the less favorable helix propensity. Also, the fluoro-stabilization effect may be more than just hydrophobics, but may also include more specific interactions such as those involving the dipole moments of the trifluoromethyl groups. Furthermore, fluoro-amino acids such as Hfl may be more suitable in non-helical structures (i.e. -sheet) to realize the full potential of the fluoro-stabilization effect.