Origin of the Cotton Effect of Helical Polypeptides

A theory of the 225-mμ Cotton effect is presented in which the optical rotation is produced by the static helical electric field with the large quadrupole moment of the nπ transition of the peptide link. As a result of the helical perturbation, a component of electric transition moment is borrowed from the π—π transition moments and from the charge transfer associated with the nπ transition. This component is parallel to the large magnetic moment of the nπ transition and accordingly is associated with an induced rotatory strength as well as enhanced absorption. Both effects disappear in the random chain form. The theory is in sufficiently general form that it may be applied to other nπ transitions. In particular, the theory predicts that the contribution of a perturbing atom to the rotatory strength is proportional to the quantity XY where X and Y are the Cartesian coordinates of the perturbing atom relative to the nucleus of the heteroatom. X and Y are oriented such that X=0 defines the nodal plane of the π system and Y=0 defines the nodal plane of the nonbonding orbital.