Isorhodopsin II: artificial photosensitive pigment formed from 9,13-dicis retinal

We have found that in addition to the 11-cis and 9-cis isomers of retinal which are known to couple with the visual pigment apoprotein opsin to form pigments, a third isomer 9,13-dicis retinal also will form a pigment. That this isomer is indeed bound to opsin has been shown unequivocally by removing the chromophore without isomerization and subsequent identification by high-speed liquid chromatography. Using similar techniques, we have shown that the product of bleaching by light of all three pigments in Trition X-100 is the all-trans isomer. This specificity in the product of bleaching, as with many other properties of visual pigments, is not shared by the free chromophore. Of particular interest is that when 9,13-dicis retinal is combined with opsin to form a pigment, a single photon can isomerize it about two double bonds, to the all-trans isomer.