Mechanism of color tuning in retinal protein: SAC-CI and QM/MM study

The SAC-CI and QM/MM methods were applied to calculations of the absorption peaks of the five retinal proteins, bovine rhodopsin (Rh), bacteriorhodopsin (bR) in BR, K, and KL states, and sensory rhodopsin II (sRII). The results nicely agree with the experimental excitation energies and provided an insight into the mechanism of the large blue shifts in Rh and sRII from BR: geometric distortion in Rh and protein electrostatic effect in sRII. These results indicate that the present approach is useful for studying the excitation spectra and the mechanism of the color tuning in the retinal proteins.