Temperature-Dependence of the Amide-I Frequency Map for Peptides and Proteins†

In our recent work [Phys. Chem. Chem. Phys. 11, 9149 (2009)], a molecular-mechanics force field-based amide-I vibration frequency map (MM-map) for peptides and proteins was constructed. In this work, the temperature dependence of the MM-map is examined based on high-temperature molecular dynamics simulations and infrared (IR) experiments. It is shown that the 298-K map works for up to 500-K molecular dynamics trajectories, which reasonably reproduces the 88 ◦C experimental IR results. Linear IR spectra are also simulated for two tripeptides containing natural and unnatural amino acid residues, and the results are in reasonable agreement with experiment. The results suggest the MM-map can be used to obtain the temperature-dependent amide-I local mode frequencies and their distributions for peptide oligomers, which is useful in particular for understanding the IR signatures of the thermally unfolded species.