Mechanism of isomerization of rhodopsin studied by use of 11-cis-locked rhodopsin analogues excited with a picosecond laser pulse.

Primary photochemical behaviors of cattle rhodopsin analogues (Rh5 and Rh7) having cyclopenta- and cycloheptatrienylidene 11-cis-locked retinals (Ret5 and Ret7, respectively) were studied by excitation with a picosecond laser pulse (wavelength 532 nm; duration 21 ps). Picosecond absorption and fluorescence measurements of Rh5 showed formation of only a long-lived excited singlet state (tau l/e = 85 ps). The excited state of the retinal analogue having a five-membered ring was stabilized in protein (Rh5) more than in solvent (protonated Schiff base of Ret5; PSB5). Excitation of Rh7 produced two ground-state photoproducts, Rh7 (580) and Rh7 (630). According to the analysis of photon density dependency, Rh7 (580) was a single-photon product of Rh7, while Rh7 (630) was the photoproduct of Rh7 (580). Fluorescence emitted from a seven-membered ring system like Rh7 or a protonated Schiff base of Ret7 (PSB7) was weaker than that in a corresponding five-membered ring system, especially in protein (Rh7). The difference in photoreaction between Rh5 and Rh7 may originate from the difference in fixation of the 11-cis form. On the basis of the spectral and kinetic similarities between Rh7 (580) and photorhodopsin, a precursor of bathorhodopsin, it was proposed that both have twisted all-trans chromophores in the way of the isomerization. The protein moiety of rhodopsin which fixes the chromophore at both ends seems to accelerate the rotation of the C11-C12 double bond and to prevent it from going through relaxation processes other than the isomerization. This may be a plausible reason why rhodopsin has a large quantum yield (0.67).