Recent developments in the electronic spectroscopy of amides and alpha-helical polypeptides

Recent experimental and theoretical advances in understanding the electronic excited states of simple amides are reviewed. Polarized reflection spectroscopy of single crystals of N-acetylglycine shows that the direction of the first pipi* (NV(1)) transition dipole moment of a secondary amide differs by approximately 15 degrees from that of a primary amide. Ab initio calculations on simple amides support this conclusion. Ab initio studies of di- and tri-amides demonstrate that several inter-amide charge-transfer (CT) transitions occur in the 150-175-nm region, between the NV(1) and NV(2) transitions. When the correct dipole transition moment direction for peptides is used in calculations of the circular dichroism of the alpha-helix, the results are much improved over those from earlier calculations that used the direction for primary amides. Studies that consider the mixing of the NV(1) transition with CT transitions are reviewed. These indicate that such mixing is likely to have a significant effect on the absorption and CD spectra of the alpha-helix and other types of peptide conformation. Nevertheless, the independent systems model gives a reasonable first approximation to the absorption and CD spectra of the alpha-helix.