Low-frequency dynamics of bacteriorhodopsin studied by terahertz time-domain spectroscopy.

We measured low-frequency spectra of bacteriorhodopsin (BR) by terahertz (THz; 1 THz approximately = 33 cm(-1)) time-domain spectroscopy. Both the absorption coefficient and the refractive index were obtained simultaneously in the THz frequency region. The dependence of the THz spectra on hydration and temperature was studied in detail. We defined the reduced absorption cross-section (RACS) which was estimated from the product of the absorption coefficient and the refractive index. RACS exhibited power-law behavior in the frequency region from 7 cm(-1) to 26 cm(-1), and we investigated the hydration and temperature dependence of spectral features such as the magnitude of RACS and the exponent in the power law for RACS. For the dried BR sample, the observed spectral dependence on hydration and temperature suggests that anharmonic coupling between low-frequency modes was relatively weak. For the hydrated BR sample, the temperature dependence of the spectral features was similar to that of the dried sample in the temperature range from -100 degrees C to around -40 degrees C. The THz spectra of the hydrated BR markedly changed at about -40 degrees C, similar to an inelastic neutron scattering experiment which indicates that the mean-square displacement of BR substantially changes at -40 degrees C. We discuss the relationship between the THz spectra, the inelastic neutron scattering spectra, and the function of BR.