The Cotton Effects in the Optical Rotatory Dispersion of Proteins as New Criteria of Conformation.

In a recent study on the optical rotatory dispersion of proteins with improved spectropolarimetric techniques, the negative Cotton effect with the trough in the optical rotatory dispersion curves at 233 rnp was investigated (1). According to Simmons, Blout, et al. (2, 3), the amplitude of this effect is related to the amount of the a-helical conformation. The same seems to be true of the positive peak at 198 rnp (3, 4). The present communication deals with the Cotton effects of a large group of globular proteins, chiefly enzymes. Since more improvements in the technique were introduced, it was possible to obtain reliable measurements in the 195 to 210 rnp spectral zone. The Moffitt constants b0 were determined from optical rotatory dispersion measurements in the 240 to 350 rnp zone. This was necessary because of the small amounts of the proteins available. Also, the instruments were designed and the monochromators calibrated for the highest accuracy in the far ultraviolet region. It was noticed that by using the 212 rnp value for the parameter X0, the Moffitt curves turned downward when the low wave length range of 260 or 255 rnM was reached. Better straight line plots in this spectral zone, however, were obtained with higher X0 values of 216 or 220 m/*, as observed also by others (5-7). Although most of the proteins studied were of the ordinary a-helical type, our interest was focused chiefly on the proteins which showed abnormalities regarding the b. values and the Cotton effects.