Theory of Optical Activity of Helical Structures and its Application to the α-Helix

The theory of optical activity presented in an earlier paper has been applied to helical structures, obtaining a compact formula for the residue rotation [m′]λ0. In particular, the theory has been applied to the α-helix, and this theory consists of two parts: firstly the calculation of the magnitude and orientation of the polarisability tensor ellipsoid of the peptide residue from the locations of the atoms in it and secondly the calculation of optical activity. The calculated value of [m']D for a right handed α-helix is about +100° which is in excellent agreement with the experimental value for most L polypeptides. Also the sign of the optical rotation does not change on varying the polarisability tensor over a wide range. Consequently the absolute configuration of the α-helix in those L polypeptides which have a positive value for [m′]D is shown to be right handed.