Vision, primary steps

In the case of the retinal Schiff base (6) the efficiency of cis-trans isomerization of the double bond between C-II and C-12 is considerably enhanced by polar solvents on the one hand and by protonation of the Schiff base on the other hand (Becker and Freedman, 1985). This is rather important because 6 is the chromophore of rhodopsin and this isomerization represents one of the primary steps in vision. 

Because the high quantum yield originates from the high-rate isomerization, which competes with other relaxation processes in the excited state of rhodopsin, ultrafast laser spectroscopies were applied to investigate the isomerization process of the retinal chromophore. Picosecond time-resolved spectroscopy was appHed to the photochemistry of rhodopsin, and the formation of the primary intermediates was reported, such as photorhodopsin and bathorhodopsin at room temperature. - - However, the time resolution needed to be improved in order to detect the cis-tram isomerization process in the excited state of rhodopsin. The direct observation of the rhodopsin excited state was reported in 1991, in which the primary intermediate photorhodopsin formed from the excited state of rhodopsin within 200 fs. Later, the effects of oscillatory features with a period of 550 fs (60 cm ) on the formation kinetics of photorhodopsin, were observed, suggesting that the primary step in vision is a vibrationally coherent process. 

A variety of processes and their mechanisms are now being clocked using femtosecond pump and probe experiments. It includes catalysis (to probe mechanism end improve catalysts), liquid phase reactions (to understand reactions in solution), polymers (to improve physical properties of polymers), and biological processes (e.g., to understand the primary photochemical step in vision). 

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