The two-dimensional IR nonlinear spectroscopy of a cyclic penta-peptide inrelation to its three-dimensional structure

A form of two-dimensional (2D) vibrational spectroscopy, which uses two ult rafast IR laser pulses, is used to examine the structure of a cyclic penta- peptide in solution. Spectrally resolved cross peaks occur in the off-diago nal region of the 2D IR spectrum of the amide I region, analogous to those in 2D NMR spectroscopy. These cross peaks measure the coupling between the different amide groups in the structure. Their intensities and polarization s relate directly to the three-dimensional structure of the peptide. With t he help of a model coupling Hamiltonian, supplemented hy density functional calculations, the spectra of this penta-peptide can be regenerated from th e known solution phase structure. This 2D-IR measurement, with an intrinsic time resolution of less than 1 ps, could be used in all time regimes of in terest in biology.