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.

The low-frequency spectra of the amino acids L-alanine and glycine and their peptides were studied using terahertz (THz) time-domain spectroscopy (TDS) at room temperature. In a previous work (Yamamoto et al., Biophys. J. 89, L22-L24 (2005)), the low-frequency spectra of amino acids (glycine and L-alanine) and their polypeptides (polyglycine and poly-L-alanine) were studied by THz-TDS, and it was found that there is a clear difference in low-frequency dynamics between the amino acids and the polypeptides. In the present study, amino acids and short peptides were chosen in order to investigate the effect of polymerization on low-frequency spectra. We focus on two physical quantities to represent the spectral features: (1) the intensity of the reduced absorption cross section (RACS), which we define from the absorption coefficient and refractive index, and (2) the exponent in the power law behavior of the RACS. We found that the two physical quantities show different dependences on peptide chain length, suggesting that the two physical quantities reflect different dynamics and interactions. The change in RACS intensity may be due to intermolecular or intrachain motion. The validity of the assumption of constant IR activity in the investigated frequency region is critical to understanding the origin of the variation in the exponent with chain length.

No abstract

Energy transfer among three dye components, namely, p-2,4,5,7-tetrakis(N-methylpyridinium-4-yl)-6-potassium-oxy-3-fluorone (Fluorone), meso-tetra(N-methyl-3-pyridyl) porphine (m-TMPyP), and meso-tetra(N-methyl-4pyridyl)porphine (p-TMPyP), was investigated for the purpose of utilizing a wide range of sunlight wavelengths for clay nanosheets. In this paper, we define a new parameter, the enhancement ratio of the excitation frequency (Γ 380−780 nm ) of the dye, in order to comprehensively represent the performance of the light-harvesting system (LHS). Γ 380−780 nm is defined as the enhancement ratio of the excitation frequency of dye that has the lowest excitation energy in the system (p-TMPyP in this system) in terms of the frequency of p-TMPyP without the use of light-harvesting dyes (Fluorone and m-TMPyP), when the visible-light region of sunlight (380−780 nm) is used for irradiation light. Γ 380−780 nm was calculated from the absorption spectra of dyes, the results of energy transfer experiments, and the sunlight spectrum (380−780 nm). We found that Γ 380−780 nm in our LHS is 2.4, which implies enhancement of the excitation frequency of p-TMPyP by 2.4 times relative to that of p-TMPyP without a light harvesting system. We believe that Γ 380−780 nm is an important standard parameter of the performance of artificial LHSs.

By terahertz (THz) time-domain spectroscopy we have measured low-frequency spectra of amino acid (glycine; Gly), short peptides ((Gly)3and (Gly)4), six globular proteins and bacteriorhodopsin (BR). From the analysis of the THz spectra we defined and obtained the reduced absorption cross sections for these cases, which are proportional to the vibrational density of states. We observed anharmonic behaviors in the low-frequency modes of the short peptides. The globular proteins we investigated show a universal feature in the low-frequency spectra. BR shows the dynamical transition in the temperature dependence of the THz spectrum when the sample is hydrated.