Vibrational circular dichroism of tetraphenylporphyrin in peptide complexes? A computational study

The Raman and absorption spectra of tetraphenylporphyrin (TPP) were calcula ted and compared to experiment. The computation was based on the harmonic m olecular force field and electric tensors obtained ab initio at the BPW91/6 -31G* level. Good agreement was found between experimental and calculated f requencies and intensities. In order to estimate whether induced optical ac tivity in chiral complexes interferes with the signal of peptide vibrations , the vibrational circular dichroism (VCD) spectra of TPP were simulated. T he magnetic field perturbation theory (MFP) and the gauge-invariant atomic orbitals (GIAO) were used for the simulation. Such spectra were compared to theoretical VCD intensities of a model tripeptide as well to experimental spectra of a complex of the peptide and tetrakis (p-sulfonatophenyl) porphy rin (TSPP). No significant contribution to VCD signal from the TPP residue was found in experimental spectra. Thus, possible peptide conformational ch anges occurring during the complexation can be monitored directly in the am ide I frequency region. (C) 2000 Wiley-Liss,Inc.