Chromcrphore-Environment Interaction of Visual Pigments in Model System

SEVERAL laboratories^1–6 have recently been concerned with the mechanism of the bathochromic shift of about 120 nm which results when 11-cis retinal (λ _max 380 nm) combines with the protein opsin to form rhodopsin (λ_max 498 nm). A red shift of up to 186 nm is involved in the formation of iodopsin from 11-cis retinal and cone opsin^7,8. The active site of bovine rhodopsin consists of the 11-cis retinylidene chromophore attached to a primary amine group of the protein forming a Schiff-base linkage of the type shown in Fig. 1, Ia. On the basis of the chemical reactions of rhodopsin and its derivatives it has been suggested that an interaction between a protonated form of the chromophore (structure of the type Ib) and a lipophilic environment contributes¹¹ to the red shift.