Amide I two-dimensional infrared spectroscopy of β-hairpin peptides

Smith, Adam W.; Tokmakoff, Andrei
January 2007
Journal of Chemical Physics;1/28/2007, Vol. 126 Issue 4, p045109
Academic Journal
In this report, spectral simulations and isotope labeling are used to describe the two-dimensional IR spectroscopy of β-hairpin peptides in the amide I spectral region. 2D IR spectra of Gramicidin S, PG12, Trpzip2 (TZ2), and TZ2-T3*T10*, a dual 13C′ isotope label, are qualitatively described by a model based on the widely used local mode amide I Hamiltonian. The authors' model includes methods for calculating site energies for individual amide oscillators on the basis of hydrogen bonding, nearest neighbor and long-range coupling between sites, and disorder in the site energy. The dependence of the spectral features on the peptide backbone structure is described using disorder-averaged eigenstates, which are visualized by mapping back onto the local amide I sites. β-hairpin IR spectra are dominated by delocalized vibrations that vary by the phase of adjacent oscillators parallel and perpendicular to the strands. The dominant ν⊥ band is sensitive to the length of the hairpin and the amount of twisting in the backbone structure, while the ν∥ band is composed of several low symmetry modes that delocalize along the strands. The spectra of TZ2-T3*T10* are used to compare coupling models, from which we conclude that transition charge coupling is superior to transition dipole coupling for amide groups directly hydrogen bound across the β strands. The 2D IR spectra of TZ2-T3*T10* are used to resolve the redshifted amide I band and extract the site energy of the labeled groups. This allows the authors to compare several methods for calculating the site energies used in excitonic treatments of the amide I band. Gramicidin S is studied in dimethyl sulfoxide to test the role of solvent on the spectral simulations.


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