Retinal-protein interactions in bacteriorhodopsin monomers,dispersed in the detergent L-1690

Bacteriorhodopsin monomer dispersed in a solution of the detergent L-1690 could maintain the specific interaction between retinal and protein in the pH range 9.0-0.0 at 25°C. λ_max of the absorbance spectrum was 550 nm at pH 9.0, 556 nm at pH 5.5, 609 nm at pH 2.1 and 570 nm at pH 0.0. Increasing the NaCl concentration in the solution promoted formation of the 609 nm product at pH 5.0-3.0 and also its transition to the 570 nm product at pH 2.5-1.0. Retinal isomer analysis gave a ratio of 13-cis- to all-trans-retinal of 53 : 47 at pH 5.5. When the pH of the solution was reduced, the relative content of all-trans-retinal increased and the ratio of 13-cis- to all-trans-retinal was 14 : 86 at pH 0.0. Illumination of the solution at pH 7.2 yielded a product containing 9-cis-retinal or 9-cis, 13-cis-retinal, which may be due to a reaction other than the photoreaction cycle.