Femtosecond Spectroscopy of Primary Processes of Photoisomerizations in Rhodopsin and Retinoid Proteins

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The visual systems with rhodopsin and other retinoid proteins with retinal have high sensitivity, low noise, and wide dynamic range, In this research, the primary processes of photoisomerization of rhodopsins have been investigated in femtosecond to picosecond time region.The time-resolved spectroscopy apparatus with using tunable femtosecond light pulses in the spectral region of 500-570nm region was constructed, The tunable pulses are obtained by using two-stage dye amplifiers of wavelengthselected femtosecond continuum generated by self-phase modulation. The experiment has improves the data dramatically in time resolution in comparison with previous works. The time resolution of the system is about 300 fs.The detailed mechanism of the excited state dynamics of octopus rhodopsin is clarified. The lifetime of the excited state absorption in 700 nm is about 140 fs. Primerhodopsin (Prime) is generated within 400 fs and the absorption spectrum resembles the spectrum of Bathorhodopsin (Batho), Prime is more likely unthermalized state of Batho, Another state is observed as a gain in the 800 nm region with 2-4 ps lifetime.An analog of bacteriorhodopsin (BR), 14-fluorobacteriorhodopsin (14-F-BR), was also investigated to clarify the relationship between the electric structure and the dynamics of ultrafast structure changes. The feature of the absorbance changes are quite similar to those of BR. The fact that an introduction of fluorine atom does not affect the primary relaxation dynamics indicates that the electronic structure is not an important factor for the dynamics of trans-cis isomerization.