Helical formation of a 13-residue C-peptide analogue of ribonuclease a in sodium dodecyl sulfate solution

The conformation of a 13‐residue C‐peptide analogue of ribonuclease A——in surfactant solutions was studied by CD. The CD spectrum of the peptide in excess NaDodSO4 solution was typical for a helical conformation; the spectrum appeared to be virtually independent of pH (2.5–6) and temperature (3–25°C). Analysis of the CD data indicated a helicity of about 65–70% with no α‐sheet and β‐turn; this corresponded to 8 or 9 residues in the helical form or slightly more than two turns of α‐helix. This compares with an average of about one turn of α‐helix for the C‐peptide analogue in water at pH 4.7 and 7°C. The conformation of the peptide in cationic surfactant, dodecyl ammonium chloride, and nonionic surfactant, dodecyl heptaoxyethylene ether, solution resembled that in water. We concluded that the C‐peptide analogue can develop a maximum helicity close to the corresponding segment in ribonuclease A in hydrophobic environment provided by the clustering of NaDodSO4 molecules to the cationic side groups of the peptide, except that the end effects may destabilize two or three residues each at both ends of the helix. Thus, in the interior of a protein molecule this hydrophobic effect may overshadow the charged‐group effect than can be explained by the helix dipole model for the helical segments on the exterior of the protein molecule.