AB-INITIO SIMULATIONS OF THE VIBRATIONAL CIRCULAR-DICHROISM OF COUPLED PEPTIDES

The vibrational circular dichroism (VCD) of peptides in different conf ormations was simulated using the magnetic field perturbation (MFP) mo del for the model dipeptide CH3-CONH-CH2-CONH-CH3. The geometry was op timized in the 4-31G basis set with the torsion angles constrained to mimic the alpha-helical, beta-sheet, 3(10)-helical, and polyproline II conformations. An effective mass was used for the appropriate H on ea ch C(alpha) position to simulate the local chirality Of L-alanine resi dues. The normal modes and VCD spectra were calculated at the ab initi o quantum mechanical level with the same basis set using analytical de rivatives (CADPAC) and the MFP formulations of Stephens and co-workers . These calculations gave simulated VCD spectra that were quite compar able to experimental results with respect to the VCD sign pattern and relative intensities of both the amide I and II modes for polypeptides having these conformations. The MFP results are further shown to be s uperior to a similar calculation based only on dipole coupling, by com parison to experimental polypeptide spectra.