Primary Process in Vision Studied by Ultrafast Spectroscopy

Primary Process in Vision Studied by Ultrafast Spectroscopy 

Absorption of a photon by the chromophore induces primary photoreaction, followed by conformational changes of protein, and eventually activates transducin. This is called the bleaching process because rhodopsin loses its color. To investigate the primary photoreaction processes in rhodopsin, two spectroscopic approaches have been applied low-temperature and time-resolved spectroscopies 

The first challenge to the isomerization model was made in 1972 by picosecond laser photolysis [34], The first picosecond laser photolysis observed formation of bathorhodopsin within 6 ps after excitation of bovine rhodopsin at room tempera- 

In the case of 5-membered rhodopsin, only a long-lived excited state (r = 85 ps) was formed without any ground-state photoproduct (Fig. 4.5D), giving direct evidence that the CTI is the primary event in vision [39]. Excitation of 7-membered rhodopsin, on the other hand, yielded a ground-state photoproduct with a spectrum similar to photorhodopsin (Fig. 4.5C). These different results were interpreted in terms of the rotational flexibility along the C11-C12 double bond [39]. This hypothesis was further supported by the results with an 8-membered rhodopsin that possesses a more flexible ring. Upon excitation of 8-membered rhodopsin with a 21 ps pulse, two photoproducts - photorhodopsin-like and bathorhodopsin-like products - were observed (Fig. 4.5B) [40], Photorhodopsin is a precursor of bathorhodopsin found by picosecond transient absorption spectroscopy [41]. Thus, the picosecond absorption studies directly elucidated the correlation between the primary processes of rhodopsin and the flexibility of the Cl 1-02 double bond of the chromophore, and we eventually concluded that the respective potential surfaces were as shown in Fig. 4.5 [10,40]. 

The structure of the intermediate states in Rh7 and Rh8 has been studied recently by theoretical investigation [42], Regarding the proton translocation model, it should be noted that the excitation photon density was extremely high in the low-temperature picosecond experiments [10,35]. Therefore, the non-Arrhenius dependence of the formation rate of bathorhodopsin on temperature and the deuterium isotope effect may be results which could be detected only under intense excitation conditions. In fact, a deuterium isotope effect was not observed in the process from photorhodopsin to bathorhodopsin under weak excitation conditions [43], 

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