Simulation studies of amide I IR absorption and two-dimensional IR spectra of β hairpins in liquid water

Amide I IR absorption and two-dimensional (2D) IR photon echo spectra of a model β hairpin in aqueous solution are theoretically studied and simulated by combining semiempirical quantum chemistry calculations and molecular dynamics simulation methods. The instantaneous normal-mode analysis of the β hairpin in solution is performed to obtain the density of states and the inverse participation ratios of the one-exciton states. The motional and exchange narrowing processes are taken into account by employing the time-correlation function theory for the linear and nonlinear response functions. Numerically simulated IR absorption and 2D spectra are then found to be determined largely by the amide I normal modes delocalized on the peptides in the two strands. The site-specific isotope-labeling effects on the IR and 2D IR spectra are discussed. The simulation results for the ideal (A₁₇) β hairpin are directly compared with those of the realistic 16-residue (GB1) β hairpin from an immunoglobulin G-binding protein. It was found that the characteristic features in IR and 2D spectra of both the ideal (A ₁₇) β hairpin and the GB1 β hairpin are the same. The simulated IR spectrum of the GB1 β hairpin is found to be in good agreement with experiment, which demonstrates that the present computational method is quantitatively reliable.