Cross-strand coupling of a β-hairpin peptide stabilized with an Aib-Gly turn studied using isotope-edited IR Spectroscopy

Isotope-edited IR spectroscopy was used to study a series of singly and doubly C-13=O-labeled beta-hairpin peptides stabilized by an Aib-Gly turn sequence. The double-labeled peptides have amide I' IR spectra that show different degrees of vibrational coupling between the C-13-labeled amides due to variations in the local geometry of the peptide structure. The single-labeled peptides provide controls to determine frequencies characteristic of the diagonal force field (FF) contributions at each position for the uncoupled C-13=O modes. Separation of diagonal FF and coupling effects on the spectra are used to explain the cross-strand labeled spectral patterns. DFT calculations based on an idealized model beta-hairpin peptide correctly predict the vibrational coupling patterns. Extending these model results by consideration of frayed ends and the hairpin conformational flexibility yields an alternate interpretation of details of the spectra. Temperature-dependent isotopically labeled IR spectra reveal differences in the thermal stabilities of the individual isotopically labeled sites. This is the first example of using an IR-based isotopic labeling technique to differentiate structural transitions at specific sites along the peptide backbone in model beta-hairpin peptides.