The Exciton model and the circular dichroism of polypeptides

Exciton coupling of the 190 nm pi pi(*) transition is an important factor in the CD spectrum of peptides and proteins. The CD spectrum of the alpha-helix is dominated by the exciton effect. The spectrum is sensitive to the direction of the pi pi(*) transition dipole moment, especially for short helices. Exciton theory is much less successful in accounting for the CD spectrum of the poly(proline)II (PPII) conformation, an important conformer in collagen and in unordered peptides. Mixing of the pi pi(*) transition with high-energy transitions in the peptide backbone and in side chains must be considered to explain the strong negative CD band near 200 nm of PPII.