SYNERGY IN THE SPECTRAL TUNING OF RETINAL PIGMENTS - COMPLETE ACCOUNTING OF THE OPSIN SHIFT IN BACTERIORHODOPSIN

UV-visible and solid-state NMR studies of a series of 6-s-trans proton ated Schiff bases of retinal with aniline show that the bathochromic s hift induced by weakening the imine counterion is significantly greate r in the 6-s-trans conformation than in the 6-s-cis conformation. Base d on the observed magnitude of this coupling between the electronic ef fects of 6-s isomerization and imine counterion strength in the model compounds, the large opsin shift and unusual chemical shifts in light- adapted bacteriorhodopsin can be fully explained. These phenomena ther efore do not require a negative point charge or polarizability effects in the chromophore binding pocket. The results are consistent with an effective center-to center distance between the Schiff base and its c ounterion of about 4 Angstrom in light-adapted bacteriorhodopsin.