Stabilization of the ribonuclease S‐peptide α‐helix by trifluoroethanol

The effects of trifluoroethanol (TFE) on the stability of the α‐helix formed by ribonuclease S‐peptide, residues 1–19 of ribonuclease A, were studied by measuring circular dichroism as a function of TFE concentration, pH, and temperature. The S‐peptide forms an unusually stable α‐helix, which is known to be stabilized by TFE. The magnitude of the effect of charged groups on the peptide, manifested by the change in α‐helix stability as a function of pH, was not altered significantly by either TFE concentration or temperature, indicating that the lower dielectric constant of TFE is not important in the stabilization of this α‐helix. This suggests that the α‐helix might be stabilized by many interactions in addition to the effects of charges. The titration curve of circular dichroism vs. TFE concentration appears to be cooperative at 0°C, but becomes progressively less cooperative at temperatures between 25 and 75°C. The properties of the TFE stabilization indicate that TFE might be a useful probe with which to measure the stability of marginally stable peptides and small proteins.