Theoretical CD studies of polypeptide helices: examination of important electronic and geometric factors.

An improved model for calculating the CD of polypeptides has been developed. Excited state wavefunctions were derived from CNDO/S (complete neglect of differential overlap, spectroscopic) calculations on N-methylacetamide. Four discrete peptide-localized transitions were employed: pi 0 pi* (NV1), pi* + pi* (NV2), n pi*, and n' pi*. Inclusion of the pi + pi transition (lambda 0 = 140 nm) significantly improves the accuracy of the calculated CD spectra in the 180-250-nm region. Spectra were computed for various helical structures, including right-handed alpha-, alpha II-, omega-, pi-, 3(10-), and poly (proline) I-helices, and the left-handed poly (proline) II-helix. Sensitivity to changes in the peptide backbone geometry and chain length are examined. Electronic factors such as ground-state charge distribution, hybridization effects, and basis set deorthogonalization have been investigated. The nonconservative nature of the poly (Pro) I and II CD spectra is reproduced, and the helix band present in earlier exciton calculations on the alpha-helix has been diminished.