Quantum Chemistry of the Color Tuning Mechanism in the Photobiological System

In visual systems and fluorescent proteins, controlling the photo-absorption/emission energy (color tuning) of the chromophore is the essentials to furnish a protein with the photo-functionality. Depending on the protein environment, the chromophores show a variety of colors, which are relevant to the character of the excited states and to the interactions between the chromophore and the environment. Here we summarize our recent studies on the spectral tuning mechanism of the human visual cone pigments and the fluorescent proteins. These studies elucidated a common feature in the color tuning, which also suggests a strategy to artificially control the color of proteins. We also explain our recent progress in developing the symmetry-adapted cluster-configuration interaction (SAC-CI) method and hybrid quantum mechanical/molecular mechanical (QM/MM) method particularly for studying the photo-functional proteins.