A significant role for high‐energy transitions in the ultraviolet circular dichroism spectra of polypeptides and proteins

The nπ* and ππ* transitions of polypeptides mix significantly with very high energy transitions in the poly(Pro)II conformation, as evidenced by the strongly nonconservative CD spectrum in the 170–250 nm region. Because of this, the exciton model, the standard quantum mechanical model for predicting absorption and CD spectra of polypeptides, gives poor results for the poly(Pro) II (P II ) conformation, although it works well for the α‐helix, β‐sheet, and β‐turns. The exciton theory has been extended to include the effects of mixing of discrete peptide transitions near 200 nm with the large number of uncharacterized transitions in the deep ultraviolet. These latter transitions dominate the polarizability, and their mixing with the discrete transitions can be described via bond and lone‐pair polarizability tensors, derivable by ab initio methods. This extended exciton method gives a good description of the CD spectrum of (Ala) n oligomers in the P II conformation. For this conformation, the polarizability contributions lead to a strong negative band near 200 nm that dominates the calculated and observed CD spectrum. The model does not give a good description of the CD of (Pro) n oligomers, probably because of conformational heterogeneity or nonadditive contributions of the Pro side chains. The model improves the calculated CD spectra of α‐helical (Ala) n oligomers. Although the high‐energy transitions make only a small net contribution to the CD of the α‐helix in the 200 nm region, they enhance the negative exciton band at ∼205 nm and largely cancel the negative exciton band near 175 nm, substantially improving agreement with experiment. Chirality, 2010. © 2010 Wiley‐Liss, Inc.