Doubly resonant two-dimensional three-wave-mixing spectroscopy of polypeptides: Structure-spectrum relationships

Doubly resonant (DR) two-dimensional (2D) three-wave-mixing (3WM) spectroscopy of a coupled multi-chromophore system in an isotropic condensed phase was theoretically studied recently [J. Chem. Phys. 126 (2007) 054505], where the signal was found to be sensitive to chirality of the coupled multi-chromophore system. This method is shown to be a useful technique for directly measuring the A- and E-symmetric amide I vibrations of three representative helical polypeptides. Furthermore, the 2D sum-frequency-generation spectra of anti-parallel and parallel β-sheet polypeptides are found to be distinctively different due to the symmetric natures of the one- and two-exciton states of these two different β-sheet polypeptides. Numerical simulations of DR 2D SFG spectra of these representative secondary structure polypeptides have been performed to demonstrate its potential use for discriminating different secondary structure polypeptides and for investigating chiral properties of vibrational one- and two-exciton states of coupled multi-chromophore systems in general.