Quantitative aspects of the photochemistry of isomeric retinals and visual pigments

Abstract

Abstract: The photoisomerization quantum efficiencies of all-trans-, 9-cis-, 1 1-cis-, 13-cis-, and 9-cis, 13-cis-retinal were de-termined upon direct excitation in polar and nonpolar solvents and biacetyl triplet sensitization in a polar solvent. The four cis-retinals had quantum yields of 20 % in nonpolar solvents and 4-5 % in polar solvents. The biacetyl triplet sensitized quantum yields of the cis isomers were also ca. 20%. while all-trans-retinal underwent no detectable triplet isomerization. High-pressure liquid chromatography was used to analyze the photoproducts. It was found that only two types of carbon double bonds isomer-ized, the cis bonds and the terminal carbon double bond. Photobleaching of rhodopsin, isorhodopsin, and 9.1 3-isorhodopsin (isorhodopsin 11) gave stereospecifically all-trans-retinal. When rhodopsin was treated with the unique triplet sensitizer, tri-methyl- 1,2-dioxetane, the protein was denatured and the chromophore isomerized. Based on product studies and quantum yield measurements, it is concluded that in the cis-retinals the photoisomerization occurred from the singlet and triplet states. In visual pigments it is not possible to assign the state from which isomerization occurred.