INFRARED INTENSITIES OF VIBRATIONAL-MODES OF AN ALPHA-HELICAL POLYPEPTIDE - CALCULATIONS BASED ON THE EQUILIBRIUM CHARGE CHARGE FLUX (ECCF)MODEL

Equilibrium charge/charge flux (ECCF) mode] calculations are performed on the infrared (IR) intensities of alpha-helical CH3CO-(L-Ala)46-NHC H3. The ECCF parameters used in the calculations are transferred from those of N-methylacetamide (which are obtained by an ab initio molecul ar orbital calculation), and are refined to some extent in order to ob tain better agreement with the experimental data. By comparing the IR intensities calculated by the ECCF and FPC (fixed partial charge) mode ls, a large transition dipole of the amide-I IR band is shown to origi nate mostly from charge fluxes in the CO and CN bonds of the peptide g roups. The intensities and dichroic ratios of other characteristic ban ds of the peptide groups are also discussed.