The calculated ultraviolet optical properties of polypeptide beta-configurations.

Abstract

Straightforward application of the current theory of optical activity to the [beta]-pleated sheets leads to the conclusion that in these structures, in contrast to the [alpha]-helix, the exciton contribution to rotational strength is not large. A model has been used in which the major source of optical activity is attributed to interactions within the polypeptide backbone. The calculated absorption spectra and circular dichrosim of the antiparallel pleated sheet show the same major features as the experimental spectra of poly-L-lysine and Bombyx mori silk fibroin. The major [pi] - [pi] absorption band is expected at 195 m[mu] a weaker band at 198 m[mu] representing a separation of only 770 cm-1. In this region of the spectrum the [beta]-sheet is expected to exhibit hyperchromicity. From an evaluation of the complete expression for rotational strength, the circular dichroism band at 195-198 m[mu] is expected to be positive with a magnitude on the order of 10 x 10-40 erg cm3-rad. The n-[pi] rotational strength is expected to be negative but smaller in magnitude than in the [alpha]-helix. The calculated absorption spectrum and circular dichroism of the parallel sheet are expected to be sufficiently different to allow differentiation between the 2 configurations in solution. The major [pi] - Cotton effect in the parallel sheet is expected at 181 m[mu] with a much smaller negative Cotton effect near 216 m[mu].