Decisive role of electronic polarization of the protein environment in determining the absorption maximum of halorhodopsin.

It is known that the absorption maximum of halorhodopsin is red shifted by 10 nm with the uptake of a chloride ion Cl(-). According to the X-ray structure, the ion is located at the position of the counterion of the chromophore, protonated retinal Schiff base. Thus, the direction of the observed spectral change is opposite to that expected from the pi-electron redistribution (an increase in the bond alternation) induced by the counterion. The physical origin of this abnormal shift is never explained in terms of any simple chemical analogues. We successfully explain this phenomenon by a QM/MM type of excitation energy calculation. The three-dimensional structure of the protein is explicitly taken into account using the X-ray structure. We reveal that the electronic polarization of the protein environment plays an essential role in tuning the absorption maximum of halorhodopsin.