VIBRATIONAL PROPERTIES OF THE PEPTIDE GROUP - ACHIRAL AND CHIRAL CONFORMERS OF N-METHYLACETAMIDE

Vibrational optical activity associated with the peptide group vibrati ons has been examined with N-methylacetamide (NMA) as a model compound . In this process, the structures and vibrational infrared and Raman s pectra of NMA are also examined. The ab initio Hartree-Fock investigat ions with the 6-31G basis set and higher level calculations using ele ctron correlation (MP2/6-31G) and a larger TZP basis set indicate tha t the chiral structures have energies comparable to, and some times sl ightly lower than, the energies of the achiral structures. The predict ed absorption spectra for chiral and achiral NMA conformers are simila r and consistent with the corresponding experimental spectra of NMA. H owever, the predicted Raman spectra of the chiral. and achiral conform ers show significant differences. The predictions for vibrational opti cal activity of the chiral conformers suggest that the vibrations of c arbonyl and of methyl groups are especially sensitive to the conformat ion of peptide groups. These investigations are expected to serve as r eference points for the interpretation of the experimental vibrational optical activity spectra of peptides.