FT-IR Spectroscopy and Density Functional Theory Calculations of 13C Isotopologues of the Helical Peptide Z‑Aib6‑OtBu

Isotope-edited FT-IR spectroscopy is a combined synthetic and spectroscopic method used to characterize local (e.g., residue-level) vibrational environments of biomolecules. We have prepared the 310 helical peptide Z-Aib6-OtBu and seven 13C-enriched analogues that vary only in the number and position(s) of 13CO isotopic enrichment. FT-IR spectra of these eight peptides solvated in the nonpolar aprotic solvent dichloromethane have been collected and compared to frequency, intensity, and normal mode results of DFT calculations. Single 13C enrichment of amide functional groups tends to localize amide I vibrational eigenmodes, providing residue-specific information regarding the local environment (e.g., hydrogen bonding or solvent exposure) of the peptide bond. Double 13C enrichment of Z-Aib6-OtBu allows for examination of interamide coupling between two labeled amide functional groups, providing experimental evidence of interamide coupling in the context of 310 helical structure. Although the calculated and observed interamide couplings of Z-Aib6-OtBu are a few cm–1 and less, the eight peptides exhibit distinct infrared spectra, revealing details of interamide coupling and residue level vibrational environments.