Theoretical pi-pi* absorption and circular dichroic spectra of cyclic dipeptides

The dipole interaction model, treated by the partially dispersive normal mode method, is used to calculate circular dichroic spectra of cyclo(Gly-Gly), cyclo (Ala-Gly), cyclo(Ala-Ala), cyclo(Pro-Gly), cyclo(Pro-Ala), cyclo(Pro-Val), cyclo (Pro-D-Val), and cyclo(Pro-Pro) in the amide pi-pi* absorption band near 190 nm. Assuming a standard backbone geometry, spectra which are in fair to good agreement with experiment are obtained for these molecules. The spectra are predicted to be sensitive to conformations of Pro and Val side chains. The effects of dipeptide ring folding on calculated CD spectra are mostly consistent with those found by other workers, except that it is found that a planar ring conformation of cyclo (Ala-Ala) and cyclo (Ala-Gly) gives predicted spectra comparable to experiment. The same model gives theoretical absorption spectra consistent with available experimental data.