Preparation of a New Visual Pigment Analogue of Cattle Opsin using 5,6-Dihydroretinal

Abstract

ONLY two compounds, 11-cis-retinal I and 11-cis-3-dehydroretinal II, have been found to act as chromophores in naturally occurring visual pigments¹. These compounds contain one and two double bonds respectively in the cyclic portion of the structure. The analogous compound, 5,6-dihydroretinal III which is similar in all other respects to retinal except that the ring is saturated, has not been isolated from the retina. Recombination experiments, in which visual pigments are formed from the corresponding visual protein and one of the naturally occurring retinals, show further selectivity on the part of the protein². This selectivity is manifested by the protein's ability to discriminate between the geometrical isomers of naturally occurring retinals. Consequently, the 11-cis and 9-cis isomers combine with visual proteins to produce visual pigments, whereas the all-trans and 13-cis isomers do not. Only the 11-cis isomer combines with a visual protein to form a pigment identical to those which occur naturally. In this article, we report the formation of a new synthetic pigment from 5,6-dihydroretinal III and cattle opsin; we also describe some of its spectral properties and photochemical behaviour. Apparently, this is the first report of a direct recombination of a retinal other than the naturally occurring compounds.