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 protonated Schiff bases of retinal with aniline show that the bathochromic shift induced by weakening the imine counterion is significantly greater in the 6-s-trans conformation than in the 6-s-cis conformation. Based on the observed magnitude of this coupling between the electronic effects 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 therefore 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 counterion of about 4 A in light-adapted bacteriorhodopsin.