Structures of the visual chromophores and related pigments: a conformational basis of visual excitation.

A detailed conformational analysis of the known crystal structures of vitamin A, carotenoid, and xanthenoid derivatives has been made. These studies suggest that the visual chromophore 11-cis retinal and its isomers may each occur in several different conformational states, which would be expected to have different lightabsorbing characteristics. The presence of such states for 11-cis retinal and all-trans retinal may contribute to the observed rhodopsin intermediates that have been characterized by different lambda(max) values, and to the breadth of spectral response in scotopic and color vision.