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\u2550O\ub7\ub7\ub7H 12N hydrogen bond, the coupling can provide direct evidence for single helical turn formation, a proposed key step in coil 12helix 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\u255018O-Leu monolabeled and 13C\u255018O-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\u255018O/15N labeling is a useful structural method for probing local peptide conformation with residue-level specificity