A simple model for the optical properties of chiral amides

✍ Scribed by Robert W. Woody

• Publisher: Wiley (John Wiley & Sons)
• Year: 1983
• Tongue: English
• Weight: 787 KB
• Volume: 22
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360220128

✦ Synopsis

The intramolecular charge-transfer model for the electronic structure of amides has been extended to include the effects of torsion about the peptide bond (Au = w -180) and pyramidalization at the peptide nitrogen (ON). The model is used to calculate the absorption and CD spectra of nonplanar amides as a function of the angles Au and ON. For cis-amides, the rotational strengths of the nr* and rr* transitions are much more sensitive to 8~ for a given value of Au than is the case for trans-amides. The results from the model are compared with available experimental and previous theoretical treatments of such systems. The contributions of nonplanarity in amides to the CD of various polypeptide conformations is discussed. The effects should be negligible for a-helixes but may be quite significant in P-sheets and P-turns. The CD spectrum of unordered peptides would be readily explained if there were a significant bias toward negative Aw values in polypeptides of L-amino acids. However, the evidence currently available does not indicate such a bias.