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 polypept ides, but two anomalous sidebands of the NH stretch with a similar tem perature dependence as that of the anomalous amide-I vibrational mode at 1650 cm(-1) of crystalline ACN were detected. Fermi resonance combi ned with the appearance of a red-shifted sideband of NH stretch throug h coupling to lattice modes seems to explain this band structure. Obse rvations are indicative of excitons that may occur in polypeptides as well as in single crystals of ACN. (C) 1997 Elsevier Science B.V.