Primary Process in Bacterial Photosynthesis and Light Sensor Studied by Ultrafast Spectroscopy

Primary Process in Bacterial Photosynthesis and Light Sensor Studied by Ultrafast Spectroscopy 

Unlike visual rhodopsins that bleach upon illumination, archaeal rhodopsins exhibit photocycle. This is highly advantageous in ultrafast spectroscopic studies and these techniques have been extensively applied in addition to low-temperature spectroscopy [2,12,13]. In particular, bacteriorhodopsin has been regarded historically as the model system to test new spectroscopic methods. As in visual rhodopsins, the light absorption of archaeal rhodopsins causes formation of red-shifted primary intermediates [68]. The primary K intermediate can be stabilized at 77 K. Time-resolved visible spectroscopy of bacteriorhodopsin reveals the presence of the precursor, called the J intermediate [12,13]. The J intermediate is more red-shifted (7.max -625 nm) than the K intermediate (2rn ix -590 nm). The excited state of bacteriorhodopsin possesses blue-shifted absorption, which decays nonexpo-nentially. The two components of the stimulated emission decay at about 200 and 500 fs [69]. The J intermediate is formed in 500 fs, and converted to the K intermediate within 3 ps [12,69]. 

As for visual rhodopsins, spectroscopic studies of the protonated Schiff base of all-trans-retinal in solution are important for understanding the isomerization mechanism. We first reported the excited state dynamics of the protonated Schiff base of all-trans-retinal in methanol solution [81], and found that the kinetics is very similar to that of the ll-cis form (Fig. 4.6B). The only difference was that the lifetimes are 1.2-1.4 times longer in the all-trans form than in the ll-cis form [53,81], Slightly faster decay of the ll-cis form may be reflected by their molecular structures, namely the initial steric hindrance between C10-H and C13-CH3 in the ll-cis form (Fig. 4.3) that accelerates the fluorescence decay. Interestingly, it was found that the all-trans-locked 5-membered system, which prohibits both C11=C12 and 03=04 isomerizations, exhibits similar kinetics to those of the all-trans form in solution [82], These results are entirely different from those of the 11-cis-locked 5-membered system, in which the excited-state lifetime is 5-times longer (Fig. 4.6B,C) [53]. This suggests more complex excited-state dynamics for the all-trans form. Observation of the J-like state in protein [70-72] might be correlated with such properties of the protonated Schiff base of the all-trans form. 

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