Influence of a steric hindrance in the chromophore of rhodopsin on the quantum yield of the primary photochemistry

Two derivatives of the visual pigment rhodopsin have been prepared using the 11- and the 9-cis isomers of the chromophore derivative 13-demethyl retinal. The demethylated retinal was chosen in order to prove whether the visual chromophore retinal experiences steric hindrance owing to its 11-cis geometry between the methyl group at position 13 and the hydrogen atom at position 10. Such interaction was supposed to accelerate and to improve the photoisomerization compared with other retinal proteins with different cis-trans geometry of the chromophore. The extinction coefficients of the pigment derivatives were determined as about 54 000 M(-1) cm(-1) (11-cis isomer) and 47 000 M(-1) cm(-1) (9-cis isomer). Simultaneous bleaching of the 13-demethyl-retinal-containing pigments and a rhodopsin sample which was prepared and treated identically allowed us to determine the quantum efficiency for the bleaching from a linear correlation between the absorbance changes and the irradiation period. Compared with the known value for rhodopsin (Phi = 0.67), similarly low values were derived for both pigments (0.28 for the 11-cis, and 0.34 for the 9-cis-containing pigment derivative), adding evidence to the proposed mechanism of steric interaction between chromophore substituents in the visual pigment.