Improved Theoretical π−π* Absorption and Circular Dichroic Spectra of Helical Polypeptides Using New Polarizabilities of Atoms and NC‘O Chromophores

Polarizability and π−π* transition parameters for the amide NC‘O chromophore are reoptimized from optical properties of amides and peptides using the dipole interaction model and updated atom polarizabilities. The parameters differ somewhat from those of a similar optimization in 1979 due to the inclusion of the new atom values and additional experimental target data. The effects of shifting the NC‘O center within a small “optimal” neighborhood of the NC‘ bond center are examined. The new parameters are used to calculate π−π* absorption and circular dichroic spectra of several helical polypeptide structures, including the α-helix, the parallel and antiparallel β-sheets, the 3 10 -helix, and the poly(proline) I and II helices. The results are in a form that permits estimation of properties for any choice of NC‘O center within the optimal neighborhood. New findings include the following: (i) the predicted CD intensities of the undistorted parallel and antiparallel β-sheets are sensitive to the choice of NC‘O center and are comparable to experiment for suitable choices of that center; (ii) the CD spectrum of the poly(proline) II helix agrees best with experiment when the proline ring conformation is near the predicted energy minimum, a result which is more reasonable than the higher energy conformations predicted previously with the 1979 parameters; and (iii) the predicted CD spectrum for the right-handed 3 10 -helix is consistent with the observed spectrum for a decapeptide believed to be in this form. The predicted spectra of the α-helix and the poly(proline) I helix show small improvements over previous calculations in the agreement with experiment.