Generalized Susceptibility Theory. II. Optical Absorption Properties of Helical Polypeptides

Abstract

The electric dipole polarizability of helical polypeptides is discussed in terms of a previously derived closed‐form approximation of the generalized susceptibilities of molecular systems. Although helical symmetry leads to complications in the formulation of the helix polarizability, the absorption spectrum is determined by considering only the appropriate diagonal element of the polarizability in momentum space. In addition, evidence is given for a new absorption band in substituted amides, located between the $\mathrm{n\to \pi *}$ and $\mathrm{\pi \to \pi *}$ bands. This band is included in the calculation of the absorption spectrum of the α helix and of poly‐l‐proline II. The results for the α helix agree well with the observed spectrum as far as band positions, polarizations, and relative intensities are concerned, but they do not predict the magnitude of the observed hypochromism. The calculated relative intensities of the perpendicular and parallel components of the perturbed $\mathrm{\pi \to \pi *}$ band is in much better agreement with experiment than the results of perturbation theory. Of particular interest is the fact that the calculated perturbed new band (mystery band) corresponds to an observed long‐wavelength band (perpendicular polarization) which previously was given an $\mathrm{n\to \pi *}$ assignment.