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, suc-AET-AAAKFLRAHA-CONH2, in NaDodSO4 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 mM NaDodSO4, delta G mu = 120 cal/mol, delta H mu = 700 cal/mol, and delta S mu = 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 degrees C completely. Even in water without surfactant it was not a "random coil."