A large photolysis-induced pK_a increase of the chromophore counterion in bacteriorhodopsin: Implications for ion transport mechanisms of retinal proteins

The proton-pumping mechanism of bacteriorhodopsin is dependent on a photolysis-induced transfer of a proton from the retinylidene Schiff base chromophore to the aspartate-85 counterion. Up until now, this transfer was ascribed to a >7-unit decrease in the pK_a of the protonated Schiff base caused by photoisomerization of the retinal. However, a comparably large increase in the pK_a of the Asp-85 acceptor also plays a role, as we show here with infrared measurements. Furthermore, the shifted vibrational frequency of the Asp-85 COOH group indicates a transient drop in the effective dielectric constant around Asp-85 to ∼2 in the M photointermediate. This dielectric decrease would cause a >40 kJ-mol^-1 increase in free energy of the anionic form of Asp-85, fully explaining the observed pK_α increase. An analogous photolysis-induced destabilization of the Schiff base counterion could initiate anion transport in the related protein, halorhodopsin, in which aspartate-85 is replaced by Cl^- and the Schiff base proton is consequently never transferred.