| Abstract: | Synchrotron ir radiation was used to collect far‐ir spectroscopy data on amino acid homopolymers, including poly‐L ‐phenylalanine, poly‐L ‐alanine, poly‐L ‐leucine, poly‐L ‐valine, and poly‐L ‐tryptophan. The intensity of the broadband synchrotron ir source, coupled with the use of a Michelson interferometer and Fourier transformed data, provides uniquely high signal to noise far‐ir data, which can be analyzed by deconvolution techniques that are well established. The temperature dependence of the spectra reveals consistent trends, with the bands shifting to higher frequencies and intensities as the temperature is lowered. A careful analysis of poly‐L ‐phenylalanine, as a function of temperature from 10 to 295 K, reveals consistent (monotonic) changes with temperature. An explanation for these effects is presented. Since the observed modes are of low frequency and low energy, a substantial fraction of the observed transitions result from a combination of excited state absorption and stimulated emission at ambient temperature. Anharmonicity of the vibrational potentials, with the resultant decrease in spacing of higher energy levels, is consistent with the frequency up shifts as temperature is decreased. The results emphasize that these excited state effects likely govern much of the behavior of low‐frequency modes in all biomolecules. © 1999 John Wiley & Sons, Inc. Biopoly 49: 591–603, 1999 |
