Ab Initio Simulations of the Vibrational Circular Dichroism of Coupled Peptides

The vibrational circular dichroism (VCD) of peptides in different conformations was simulated using the magnetic field perturbation (MFP) model for the model dipeptide CH3-CONH-CH2-CONH-CH3. The geometry was optimized in the 4-31G basis set with the torsion angles constrained to mimic the a-helical, @-sheet, 310-helical, and polyproline I1 conformations. An effective mass was used for the appropriate H on each C, position to simulate the local chirality of L-alanine residues. The normal modes and VCD spectra were calculated at the ab initio quantum mechanical level with the same basis set using analytical derivatives (CADPAC) and the MFP formulations of Stephens and co-workers. These calculations gave simulated VCD spectra that were quite comparable to experimental results with respect to the VCD sign pattern and relative intensities of both the amide I and I1 modes for polypeptides having these conformations. The MFP results are further shown to be superior to a similar calculation based only on dipole coupling, by comparison to experimental polypeptide spectra.