A full polarizability treatment of the π–π* absorption and circular dichroic spectra of α-helical polypeptides

Abstract

The absorption and circular dichroic spectra associated with the π–π* transition near 190 nm are calculated for the right-handed α-helical forms of (Gly)n, (l-Ala)n, and (d-Ala)n, n?12, using NC′O polarizability and transition parameters found for simple amides and isotropic polarizabilities of all other atoms in a partially dispersive normal mode analysis. Good agreement with experiment is found when NC′O parameters centered on the NC′ bond, with a π–π* oscillator strength twice that found for amides, are used. The calculated CD spectrum for (Gly)12 shows a negative peak at 204 nm and a positive peak at 191 nm; the negative peak usually predicted near 180 nm, but not found experimentally, is absent from these results. The effects of methyl side chains in (Ala)n are found to be small. The hypochromicity is calculated to be 0.26–0.40 (exptl. 0.30–0.36) and the splitting of the π–π* band is 4080 cm−1 (exptl. 4370 cm−1). When the n–π* transition at 222 nm is incorporated with an oscillator strength of 0.0025, a rotational strength of −0.10 DBM is calculated for the transition (exptl. −0.24 DBM).