α-Helix formation in the S-peptide (residues 1–19 of ribonuclease A) was studied in detail by use of two-dimensional 1H nuclear magnetic resonance to monitor the effects of 2,2,2-trifluoroethanol (TFE) at 0 °C and pH* 2.07. TFE stabilizes the S-peptide α-helix. Helix formation by a particular amino acid was monitored by the chemical shifts of the Cα, Cβ, and Cγ protons while increasing the concentration of TFE: large changes in chemical shift of a particular residue indicate that it is induced to go helical, whereas small chemical shift changes indicate little helix formation. Residues Thr-3 to Met-13 undergo chemical shift changes consistent with helix formation, whereas the other residues do not. Earlier work [Kim, P.S., & Baldwin, R. L. (1984) Nature 307, 329–334] reported that residues Thr-3 to His-12 become helical in aqueous solution. The existence of a “helix stop signal” was inferred from this behavior. We thus conclude that this helix stop signal persists in TFE solutions.
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