Low frequency vibrations of amino acid homopolymers observed by synchrotron far-IR absorption spectroscopy: Excited state effects dominate the temperature dependence of the spectra

Synchrotron ir radiation was used to collect Sar-ir spectroscopy data on am ino acid homopolymers, including poly-L-phenylalanine, poly-L-alanine, poly -L-leucine, poly-L-valine, and poly-L-tryptophan. The intensity of the broa dband synchrotron ir source, coupled with the use of a Michelson interferom eter and Fourier transformed data, provides uniquely high signal to noise f ar-ir data, which can be analyzed by deconvolution techniques that are well established The temperature dependence of the spectra reveals consistent t rends, with the bands shifting to higher frequencies and intensities as the temperature is lowered A careful analysis of poly-L-phenylalanine, as a fu nction of temperature from 10 to 295 K, reveals consistent (monotonic) chan ges with temperature. An explanation for these effects is presented Since t he observed modes are of low frequency and low energy, a substantial fracti on of the observed transitions result from a combination of excited state a bsorption and stimulated emission at ambient temperature. Anharmonicity of the vibrational potentials, with the resultant decrease in spacing of highe r 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. (C ) 1999 John Wiley & Sons, Inc.