Thermal unfolding of helices of a C‐peptide analogue of ribonuclease A in sodium dodecyl sulfate solution

The conformation of a 13‐residue C‐peptide analogue of ribonuclease A, \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm suc - A}\mathop {\rm E}\limits^ - {\rm T - AAA}\mathop {\rm K}\limits^{\rm + } {\rm FL}\mathop {\rm R}\limits^{\rm + } {\rm A}\mathop {\rm H}\limits^{\rm + } {\rm A - CONH}_2 $\end{document} , in NaDodSo 4 solutions with respect to temperature was studied with CD. The equilibrium constant of unfolding yielded a straight line in a van't Hoff plot. In 10 m M NaDodSo 4 , Δ G u = 120 cal/mol, Δ H u = 700 cal/mol, and Δ S u = 2.0 entropy units all on per helical residue. These values compared fairly well with the thermodynamic parameters of the uncharged helix‐coil transition of (Glu) n in 0.1 M NaCl based on the theories of Zimm and Bragg and Zimm and Rice. The peptide was not unfolded at 75°C completely. Even in water without surfactant it was not a “random coil.”