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SynopsisA ~netliod of calculation of the circular dichroism (CD) of random-coil polypeptides hw been developed by means of a illonte-Carlo approarh to the treatment of statistical systems and exciton theory of optical activity of polymers. The contribution of r-r* and n-r* amide transitions to the CII has been taken into account. The r-r* transition gives rise to two CD bands: a negative short-wavelength band and a weak positive long-wavelength one. The n-r* transition gives rise to one negative CD band.Theoretical treatments of the optical activity of polypeptides have mainly been concerned with regular polypeptide conformations.However, experiments that random-coil polypeptides have optical rotatory dispersion (ORD) and circular dichroism (CD) which are sharply different from those of the a-helix and the @ form. Therefore, a theoretical investigation of the optical rotation of the polypeptide random coil is desirable.The steric arrangement of chromophores in the random coil is not fixed. Nevertheless, the random coil retains some elements of short-range order due to a limited freedom of the internal rotation. The observed ORD and CD of the polypeptide random coil must be deeply affected by the short-range order in the chain, since the optical activity depends on the interaction of chromophores.We have carried out a calculation of the CD of the random-coil polypeptide for the region of the amide -R--R* (X 220 mp) transitions. For this purpose we have used the Monte-Carlo method which is a method of numerical calculation of statistical systems. We generated a random-coil polypeptide chain according to existing data on its short-range order. Then the CD calculation was carried out for this chain by means of the exciton theory of optical activitylo*ll which previously had been successfully applied for the CD and ORD calculations of regular polypeptides.The calculation was repeated successively for other similarly generated chains until the mean of results for these Rlonte-Carlo trials had converged within limits which were acceptable.190 mp) and n--R* (X 20.51

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