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

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Abstract
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: π0π*(NV1), π+π* (NV2), nπ*, and n′π*. Inclusion of the π+π*transition (λ0 = 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 α‐, αII‐, ω‐, π‐, 310‐, 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 α‐helix has been diminished.