Physical origin of the opsin shift of bacteriorhodopsin. Comprehensive analysis based on medium effect theory of absorption spectra

To elucidate the origin of the opsin shift of bacteriorhodopsin (bR), a self-consistent reaction field method combined with configuration interaction calculation is employed. In addition, the absorption maxima of all-trans-retinal and its Schiff bases are measured in a variety of aprotic solvents. It is shown that the calculation reproduces well the observed solvatochromic shifts. From regression analysis, we obtain an empirical relationship between the absorption maximum of protonated retinal Schiff base and physical parameters of solvent, including dielectric constant and refractive index; On the other hand, based on the crystal structure of bR, we estimate the effective values of such parameters for the retinal-binding pocket. Combining these results, it is shown that the opsin shifts of bR(568) and M-412 can be quantitatively reproduced if the protein matrix acts as a polarizable medium with a high refractive index. From decomposition analysis of the calculated opsin shift, the contributions of (i) ring/chain coplanarization, (ii) separation of a counterion, and (iii) medium effects of the protein are shown to be 2500, 1200, and 1000 cm(-1), respectively. It is revealed that the effects (i) and (ii) are independent of each other, but the effects (ii) and (iii) are significantly correlated. In a polarizable medium, a shift induced by a counterion is almost canceled out by an opposite shift induced by medium effects. In conclusion, the polarizable medium effects play a decisive role in the wavelength regulation of bR.