C-13=O-18/N-15 Isotope Dependence of the Amide-I/II 2D IR Cross Peaks for the Fully Extended Peptides

We have used a combination of 2D IR spectroscopy with 13C=18O labeled amide-I and 15N-labeled amide-II modes to reveal how vibrational coupling between labeled peptide units depends on secondary structure. Linear and 2D IR measurements and simulations of Calpha,alpha-diethylglycine homotetrapeptide show that this compound adopts the fully extended (2.05-helical) conformation in CDCl3, consistent with previous work on the Ac-capped peptide. The amide-I/II cross peaks of isotopomers exhibit only a marginal isotope frequency shift between labeled modes that are separated by two peptide units, indicating a very weak coupling. This result is in sharp contrast with a large cross-peak shift observed in 3-10-helical peptides, in which the labeled amide-I and -II modes are connected through an inter-residue C=O...H-N hydrogen bond. The discovered 3D-structural dependence indicates that the 13C=18O/15N labeled amide-I/II cross peaks can distinguish the formation of a single 3-10-helical turn from the fully extended polypeptide chain and increase the versatility of 2D IR spectroscopy as a conformational analysis tool of biomolecules.