Anomalous temperature dependence of the IR spectrum of polyalanine

We have studied the temperature dependence of the infrared spectra of acetanilide (ACN), tryptophan–(alanine)15, and tyrosine–(alanine)15. No sidebands of the amide-I vibration were observed in the polypeptides, but two anomalous sidebands of the NH stretch with a similar temperature dependence as that of the anomalous amide-I vibrational mode at 1650 cm−1 of crystalline ACN were detected. Fermi resonance combined with the appearance of a red-shifted sideband of NH stretch through coupling to lattice modes seems to explain this band structure. Observations are indicative of excitons that may occur in polypeptides as well as in single crystals of ACN.