Stabilization of bzip Peptides through Incorporation of Fluorinated Aliphatic Residues

Two fluorinated amino acids, 5,5,5‐trifluoroisoleucine (5TFI) and (2S,3R)‐4,4,4‐trifluorovaline (4TFV), which have been shown to serve as isoleucine surrogates in protein synthesis in Escherichia coli, have been incorporated in vivo into basic leucine zipper (bzip) peptides derived from GCN4. The extents of residue‐specific incorporation of 5TFI and 4TFV were 90 and 88 %, respectively, of the encoded isoleucine residues, as evidenced by MALDI mass spectrometry and amino acid analysis. Both circular dichroism and equilibrium sedimentation studies of the fluorinated bzip peptides indicated preservation of secondary and higher‐order protein structure. Thermal‐denaturation experiments showed an increase of 27 °C in melting temperature when isoleucine was replaced by 5TFI. However, the Tm of the peptide containing 4TFV was increased by only 4 °C over that of the peptide containing valine. Similar trends were observed in chemical denaturation studies in which ΔΔGunfold in water was determined to be 2.1 or 0.3 kcal mol−1 upon incorporation of 5TFI or 4TFV, respectively. When the fluorinated peptides were tested for DNA binding, both their affinity and specificity were similar to those of the respective hydrogenated peptides. These results suggest that fluorinated amino acids, even when introduced into the same positions, can have markedly different effects on the physical properties of proteins, while having little impact on secondary and higher‐order structure.