A non‐perturbation method for the optical properties of helical polymers

The optical absorption and circular dichroism (CD) of poly‐L‐alanine in an α‐helical conformation are computed in terms of monomer properties by use of Bogoliubov exciton theory (a non‐perturbation method). It was shown earlier that a proper formulation of the theory of CD for large molecular systems permits the use of periodic boundary conditions, a feature which greatly simplifies calculations as well as conceptual understanding. If is found that the shape of the observed CD spectrum can be explained in terms of the two rotational strength components and an electric dipole strength component referred to as the helix term. Comparisons of the results of perturbation theory and Bogoliubov exciton theory show that perturbation theory is valid for absorption but is inadequate for CD if the coupling among monomers is sufficiently large lo predict the observed hypochromism.