Photo-induced dynamics of proteins

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The early stage in the photoinduced dynamics of photoactive yellow protein in aqueous solution has been investigated at room temperature by the femtosecond luminescence spectroscopy using an optical Kerrgate technique. Remarkable oscillatory components have been directly observed with the time resolution of 180 fs and the spectral resolution of 5 nm. Mapping the phase of oscillations in time at each wavelength reveals that each oscillation is restricted in the respective narrow spectral region, the period and amplitude of which are 500-800 fs and 100-200 cm^<-1>, respectively. Since the mean spectral position of each component seems to be related to the molecular vibrational energy, the oscillatory components are possibly attributed to the combination of the intramolecular vibrations of the chromophore and the surrounding protein motionsIn order to investigate the dynamics of protein motions in the course of the photo cycle more quantitatively, a time-domain THz spectroscopic system has been constructed. Femtosecond laser pulses from a Ti;Sapphire oscillator (MaiTai, Spectra Physics) are used to emit and detect THz radiation. A bowtie-shaped gold-paint antenna on a GaAs wafer is used for the emitter, while a dipole-shaped gold-paint antenna on a GaAs wafer is used for the detector. A 2 KHz, ±20V ac bias voltage is applied to the emitter. A pre-amplified THz signal is detected by a lock-in-amplifier. In order to evaluate the THz spectroscopic system, reflection spectra of water-methanol mixture and waterethanol mixture in a wide range of the concentration ratio have been measured. We have successfully obtained the concentration dependence of dielectric amplitude and phase for the mixture solutions. Time-domain THz spectroscopy for photoactive proteins in the course of the photo cycle triggered by a visible laser pulse is now progressing.