Circular Dichroism and Optical Rotatory Dispersion of Trypsin Inhibitors*

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Abstract
The circular dichroism (CD)** and the optical rotatory dispersion (ORD) of soybean Kunitz and 1.9S trypsin inhibitors, three fractions of limabean inhibitors and ovomucoid were studied at various pH values and in 2-chloroethanol-water mixtures. Ovomucoid showed a negative Cotton effect with a trough at 230 mμ in the ORD curve and negative extrema at 222 and 210 mμ in the CD spectrum. This suggests that ovomucoid is a helical protein. On the other hand, soybean and limabean inhibitors showed a trough at 210 mμ in the ORD curves and a negative extremum at 206 mμ in the CD spectra. Thus, it was found that soybean and limabean inhibitors are nonhelical proteins. At longer wavelengths, the CD spectrum of Kunitz inhibitor gave negative bands at 294, 278 270, and 246 mμ and a large positive maximum at 226 mμ. 1.9S inhibitor exhibited a large negative CD maximum centered at 280 mμ and a positive maximum at 234 mμ with a shoulder at 245 mμ. The CD spectra of limabean inhibitors also showed a negative band at 280 mμ with almost the same intensity as that of 1.9S inhibitor. Limabean inhibitor, fraction I gave a positive CD maximum at 248 mμ. Both fractions II and IV showed two positive CD maxima at 248 and 233 mμ. Ovomucoid showed negative CD extrema at 292 and 264 mμ and a positive maximum at 243 mμ. The origin of these CD bands is discussed. The effects of pH and 2-chloroethanol on the CD and ORD of the inhibitors were also studied.