Enhanced sensitivity to local dynamics in peptides by use of temperature-jump IR-spectroscopy and isotope labeling

Abstract

Site-specific isotopic labeling of molecules is a widely used approach in IR-spectroscopy to resolve local contributions to vibrational modes. The induced frequency shift of the corresponding IR band depends on the substituted masses, but also on hydrogen bonding and on vibrational coupling. The impact of these different factors was analyzed with a designed three-stranded β-sheet peptide and by use of selected 13C isotope substitutions at multiple positions in the peptide backbone. Single strand labels give rise to isotopically shifted bands at different frequencies depending on the specific sites, demonstrating sensitivity to the local environment. Cross-strand double and triple labeled peptides exhibited two resolved bands, which could be uniquely assigned to specific residues, whose equilibrium IR indicated only weak local-mode coupling. Temperature-jump IR-laser spectroscopy was applied to monitor structural dynamics and revealed an impressive enhancement of the isotope sensitivity to both local positions and coupling between them as compared to equilibrium FTIR. Site-specific relaxation rates were altered upon introduction of additional cross-strand isotopes. Likewise, the rates for the global β-sheet dynamics were affected in a manner dependent on the distinct relaxation behavior of the labeled oscillator. The study demonstrates that isotope labels do not just provide local structural probes, but they rather sense the dynamic complexity of the molecular environment.