Cis-Trans Isomerization in the Photochemistry of Vision

Various models proposed for the primary photoevent in vision are critically discussed. It is concluded that the classical picture of a single cis-trans isomerization step is the only one which satisfactorily accounts for all the available experimental data. Experiments are performed showing that this process is temperature independent over a range of 200°C. Photoisomerization yields for the free protonated Schiff base of 11-cis (and all-trans) retinal are measured as a function of the excitation wavelength. In contrast to the efficient and wavelength independent photobleaching of rhodopsin, the yields of the ll-cis↠*all-trans isomerization of the free chromophore are small, exhibiting a marked dependence on the excitation wavelength. Potential energy curves for both ground and excited states of rhodopsin are derived from the analysis of the accumulated experimental data. In variance with the behavior of model compounds, photoisomerization in the pigment proceeds via the quantitative population of a common, barrierless, thermally relaxed excited state along the 11–12 torsional coordinate separating the 11-cis (rhodopsin) and all-trans (bathorhodopsin) configurations. In the ground state, interactions with the protein destabilize the all-trans isomerization product, leading to storage of a significant fraction of the photon's energy in the primary step.