Theoretical pi-pi absorption and circular dichroic spectra of beta-turn model peptides

The dipole interaction model, treated by the partially dispersive normal mode method, is used to calculate pi-pi absorption and circular dichroic spectra of beta-turn model peptides in certain conformations. These include Ac-Gly-Gly-NHMe, Ac-L-Ala-L-Ala-NHMe, and Ac-L-Ala-Gly-NHMe in the standard beta-turn conformations I, II, and III of Venkatachalam and cyclo(L-Ala-Gly-epsilon-aminocaproyl), cyclo(L-Ala-L-Ala-epsilon-aminocaproyl), and cyclo(L-Ala-D-Ala-epsilon-aminocaproyl) in the minimum-energy conformations of Nemethy et al. Boltzmann average circular dichroic spectra of the cyclic compounds agree with experimental spectra in most respects. The results are compared with previous theoretical CD spectra for these molecules and with conformational assignments based on other evidence. Absorption spectra in the pi-pi band are predicted to be moderately sensitive to conformation.