Helix propagation in trifluoroethanol solutions.

Helix propagation of the S-peptide sequence (residues 1-19 of ribonuclease A) in 2,2,2-trifluoroethanol (TFE) solutions has been investigated with CD and nmr Overhauser effect spectroscopies. In this study, the S-peptide helix is covalently initiated at the N-terminus through disulfide bonds to a helix scaffold derived from the N-terminal sequence of the bee venom peptide apamin. The entire S-peptide sequence of this hybrid sequence peptide becomes helical at high proportions of TFE. Residues 14-19 of the S-peptide are not helical in the free peptide in TFE, nor are they helical in ribonuclease A. The "helix stop" signal encoded by the S-peptide sequence near residue 13 does not persist at high TFE with this hybrid sequence peptide. The helix-stabilizing effects of TFE are due at least in part to facilitated propagation of an extant helix. This stabilizing effect appears to be a general solvation effect and not due to specific interaction of the helical peptide with TFE. Specifically these data support the idea that TFE destabilizes the coil state by less effective hydrogen bonding of the peptide amide to the solvent.