Two-dimensional infrared spectroscopy of a model peptide homodimer

A pair of peptide groups in space, as modeled by formamide dimer, was used to evaluate vibrational coupling between the amide-I modes and the spatial behavior of the coupling using ab initio quantum chemical calculations. It was found that the coupling between two C=O groups, which is electrostatic in nature, is still quite significant as the intermolecular distance reaches 10 angstrom. One- and two-dimensional infrared spectra of the dimer at several configurations were calculated using a vibrational exciton model that utilizes the ab initio computation-obtained parameters. The distance dependence of the coupling is dramatically shown in both the ID and 2D infrared spectral features. The results suggest that the C=O stretching modes in polypeptide can be coupled and their states can be delocalized over quite a long distance in space.