Couplings between Peptide Linkages across a 3(10)-Helical Hydrogen Bond Revealed by Two-Dimensional Infrared Spectroscopy

Vibrational couplings between the amide modes are keenly dependent on peptide structure. Site-specific couplings can inform us of molecular conformation in detail. For example, when an amide-I mode couples to an amide-II mode that is three residues away because they are brought into proximity in the presence of an intramolecular C═O···H−N hydrogen bond, the coupling can provide direct evidence for single helical turn formation, a proposed key step in coil−helix transition. In this work, we measure 2D IR spectra of a 310-helical hexapeptide, Z-Aib-l-Leu-(Aib)2-Gly-Aib-OtBu, and its 13C═18O-Leu monolabeled and 13C═18O-Leu/15N-Gly bis-labeled isotopomers in CDCl3. The isotope-dependent amide-I/II cross-peaks clearly reveal the existence of vibrational coupling between the second and fourth peptide linkages that are connected through a 310-helical hydrogen bond. Our results demonstrate that the combination of 2D IR and 13C═18O/15N labeling is a useful structural method for probing local peptide conformation with residue-level specificity.