Photodynamics of All-trans Retinal Protonated Schiff Base in Bacteriorhodopsin and Methanol Solution

Nonadiabatic ONIOM(CASSCF:AMBER) and CASSCF simulations elucidated different photodynamics of an all-trans retinal protonated Schiff base (RPSB) in bacteriorhodopsin and methanol as well as without an environment. The bR protein matrix holds RPSB tight via specific interactions and promotes bond-specific (along the C13═C14 bond), unidirectional, and ultrafast photoisomerization with a high quantum yield. In contrast, in methanol and for the twisted bare RPSB, photoisomerization is not bond-specific (mainly along the C11═C12 bond), is nonunidirectional, and is ineffective. Therefore, bR efficiently "catalyzes" photoisomerization and stores enough energy to promote the subsequent proton pumping and protein conformational changes.