Effects of n-Propyl Alcohol and Detergents on the Optical Rotatory Dispersion of α-Chymotrypsinogen, β-Casein, Histone Fraction F1, and Soybean Trypsin Inhibitor

The optical rotatory dispersions of α-chymotrypsinogen, β-casein, the histone fraction, F1, and soybean trypsin inhibitor were studied in the spectral zone of 190 to 350 mµ. The Cotton effects indicated that these proteins are nonhelical in the native state and that they can be converted in part to the α-helical conformation by treatment with n-propyl alcohol or anionic detergents. Tetradecyl sodium sulfate was more effective than dodecyl sulfate, and the latter was more potent than decyl sulfate in ordering the polypeptide chains. In the presence of high concentrations of propanol, the spectrum of the β conformation appeared, especially in chymotrypsinogen and soybean trypsin inhibitor; i.e. negative minima were found in the rotatory dispersion curves at 228 to 232 mµ and positive maxima at 200 to 206 mµ. Low pH, 1.8 to 3.5, enhanced the effects of the detergents and propanol. Although the transitions were practically instantaneous at room temperature, the amount of the β form, as estimated from the positive extremum at 203 to 206 mµ, increased slowly on long standing. Native chymotrypsinogen, β-casein, histone, and soybean trypsin inhibitor, according to optical rotatory dispersion data, were found to be devoid of the β conformation. Circular dichroism measurements confirmed the conclusion that the β form appeared in soybean trypsin inhibitor on treatment with propanol.