Far-infrared spectroscopy analysis of linear and cyclic peptides, and lysozyme

The far-infrared spectra of lysozyme, alanine-rich peptides and small cyclic helical peptides were studied. Both lysozyme and the alanine-rich peptides had a dome in the spectral background centred on 180–220 cm−1 consistent with either structural collective modes or an ensemble of hydrogen bond vibrational modes associated with the peptide backbone. Molecular dynamics simulation of the alanine-rich peptide's infrared spectrum produced bands with similar positions to the experimental data and vibrational density of states simulation was able to attribute several of these bands to backbone and side chain vibrational modes. Evidence is presented that peaks at 333 and 375 cm−1 are associated with alpha-helices in lysozyme and the alanine-rich peptides, and the peak at 445 cm−1 is associated with beta-pleated sheet. Also, results suggest that peaks at 385, 402 and 470 cm−1 are associated with the secondary structure of the cyclic helical peptide KARAD. This supports the hypothesis the low energy vibrational modes between 300 and 500 cm−1 are diagnostic of the presence of secondary structures in (poly)peptides.