Theoretical study of the opsin shift of deprotonated retinal schiff base in the M state of bacteriorhodopsin.

The origin of the opsin shift, which deprotonated Schiff base (DPSB) shows in the M state of the bacteriorhodopsin (bR) photocycle, was theoretically investigated for the first time using a combined quantum mechanical and molecular mechanical (QM/MM) method. From the QM(SAC-CI)/MM(AMBER99) results, the chromophore conformational effect was found to be the main factor, whereas the Coulombic interaction with the protein environment gave a non-negligible contribution. The present result revised the conclusion drawn by previous studies and provided a new interpretation of the opsin shift mechanism of DPSB. To test the computational models for taking into account the electronic polarization and charge redistribution effects of the surrounding environment, the size of the QM region was expanded up to 5-7 Å from DPSB, which decreased the excitation energies in solution and in protein by 0.08-0.13 eV and 0.21-0.26 eV, respectively. We also found that the rCAM-B3LYP functional significantly improves the B3LYP results when calculating the potential energy curve for the C(6)-C(7) twisting.