Figure 1.

Molecular structure of retinal in the dark state (11-cis) and the Meta II intermediate (trans). Structures of model compounds used are 1: 2-methyl-1,3-butadiene; 2: 3-methyl-1,3E,5-hexatriene; 3: propane; 4: propene; 5: 4-methyl-1,3E,5E,7-octatetraene (4MOT) (trans); 6: 4-methyl-1,3E,5Z,7-octatetraene (cis); 7: 3,7-dimethylnon-1,3E,5Z,7E-tetraen-1-ylidene (N-methyl)amine (37DM+) (cis); 8: 37DM+ (trans); 9: 37DM; and 10: 1,3,3-trimethyl-2-[1′-(3′-methyl-1′,3′-butadienyl)]-cyclohexene. (Equivalent methyl positions corresponding to retinal are enclosed in circles and by parentheses in text.)