Spectral Signatures of 3$_{10}$- and $\alpha $-Helices Revealed by Two-Dimensional Infrared Spectroscopy*

Femtosecond two-dimensional infrared (2D IR) spectroscopy is applied to the amide I modes of the homo-octapeptide $Z$-[L-($\alpha $Me)Val]$_{8}$-O$t$Bu in CDCl$_{3}$, TFE and HFIP solutions to acquire 2D spectral signatures that distinguish between 3$_{10}$- and $\alpha $-helix structures. Suppression of diagonal peaks by controlling polarizations of IR pulses clearly reveals cross-peak patterns that are crucial for structural determination. A doublet feature is observed when the peptide forms a 3$_{10}$-helix in CDCl$_{3}$ and TFE, and when it is at the initial stage of 3$_{10}$- to $\alpha $-helix transition in HFIP. In contrast, the 2D IR spectrum shows a multiple peak pattern after the peptide has become an $\alpha $-helix in HFIP. This is the first report on the experimental 2D IR signature of a 3$_{10}$-helical peptide. These results for a model octapeptide demonstrate the powerful capability of 2D IR spectroscopy to discriminate between different helical structures.