Abstract
11- cis retinal is the chromophore of visual pigments, whose absorption maxima (λ max) lie between approximately 440 and 565 nm. To explain these color differences, Kropf and Hubbard (1958) postulated that retinal forms a protonated Schiff's base with an amino group on opsin and that the colors of the visual pigments depend on secondary interactions between the opsin and the π-electron system of the chromophore. This hypothesis has since been strengthened by: (1) Identification of the ϵ-amino group of a lysine residue as the chromophoric site on opsin; (2) theory and experiments showing that λ max of protonated Schiff bases of retinal can lie at 490 nm and beyond; (3) changes in λ max of rhodopsin resulting from conformational changes of opsin; (4) observations on squid retinochrome, a retinylidene chromoprotein whose λ max can be changed reversibly by reagents which interact with the protein.
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