| Abstract
| - The 12-residue tryptophan zipper β-hairpin (SWTWENGKWTWK) and two 13C-isotopomers were examinedin the amide-I region using FTIR and femtosecond two-dimensional infrared (2D IR) spectroscopies.Spectroscopic features of the labeled transitions with 13C-substituted amide unit present in the terminal orturn region of the hairpin, including their frequency shifts and distributions, line broadenings, orientations,and anharmonicities of diagonal peaks, allow the peptide local structure and local environment to be examined.The results suggest a larger structure fluctuation in the terminal region than in the turn region as a result ofthe side chain effect and solvent−peptide interaction. The results also suggest that the uncoupled amide-Imodes are not degenerate and that this is likely to be a common situation for solvated polypeptides. In addition,the amide-I states in the terminal and turn regions were found to be delocalized over several neighboringamide units. Cross-peaks between the various labeled and unlabeled structural regions were clearly observedin the 2D IR correlation spectra, allowing them to be characterized for monitoring structural changes. Theseresults illustrate the sensitivity of 2D IR to the local environment of solvated peptides. The simulated 1D and2D IR spectra of the hairpin, obtained by using the vibrational exciton model incorporating coupling constantsfrom quantum chemical computations and semiempirical calculations, were found to reproduce the essentialfeatures of the experimental results.
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