Factors Affecting the Absorption Maxima of the Acidic Forms of Bacteriorhodopsin. A Study with Artificial Pigments

The absorption maximum (568 nm) of light-adapted bacteriorhodopsin bR568 undergoes reversible changes after acidification. At pH 2.9, the absorption shifts to 605 nm (forming bR605) and it blue shifts to 565 nm, after further acidification to pH approximately 0.5 (forming bR565). Molecular models accounting for such acid-induced changes are relevant to the structure and function of bacteriorhodopsin. In the present study we approached the problem by applying artificial bR pigments based on selectively modified synthetic retinals. This may allow direct identification of the specific regions in the retinal binding site where the above changes in the protein-retinal interactions take place. We investigated the spectroscopic effects of acid in a variety of artificial pigments, including cyaninelike retinals, retinals bearing bulky groups at C4, short polyenes, and retinals in which the beta-ionone ring was substituted by aromatic rings. The results provide direct evidence for the hypothesis that the generation of bR605 is due to changes in polyene-opsin interactions in the vicinity of the Schiff base linkage. The second transition (to bR565) was not observed in artificial pigments bearing major changes in the ring structure of the retinal. Two approaches accounting for this observation are presented. One argues that the generation of bR565 is associated with acid-induced changes in retinal-protein interactions in the vicinity of the retinal ring. The second involves changes in polyene-opsin interactions in the vicinity of the Schiff base linkage. For both bRw and bRN5, our results do not discriminate between the direct titration of negative or dipolar protein groups in the binding site and changes in the retinal-protein interactions induced by changes in the protein structure outside of the binding site.