Terahertz spectroscopy of proteins: Viscoelastic damping of boson peak oscillations

The Raman and neutron scattering spectra of proteins generally exhibit a broad central line with a weak shoulder at 15 to 30 cm-1 (2–4 meV) [1]. With decreasing temperature or degree of hydration, the central line narrows while the shoulder develops into a broad peak (fig.1) [2, 3]. The latter, known as ´the boson peak´, appears as a characteristic feature also in the low frequency spectra of molecular liquids and glasses [5]. The temperature and hydration dependence of its line shape could be reproduced qualitatively by molecular dynamic simulations of a single protein molecule, suggesting an assignment to intramolecular modes [6]. Furthermore, normal mode studies suggest that conformational changes in proteins proceed via a few delocalized modes with characteristic frequencies below 1 THz. (30 cm-1) [7, 9]. This frequency range is thus of considerable biological interest. By definition, normal mode analysis does not take into account the anharmonicity of the force field and the interaction with the solvent. The predicted delocalized vibrations, however, exhibit periods of a several picoseconds, which fall in the range of fast dissipative processes, and are thus likely to be overdamped. The corresponding spectra exhibit a maximum at ω = 0, which prohibits a discrimination from those of other relaxation processes. The question of damping is an essential aspect of protein-solvent interactions. We have thus measured the solvent-dependence and viscoelastic damping of protein vibrations in the boson-peak region. Inelastic neutron scattering experiments together with the new approach of far-infrared emission spectroscopy will be discussed.