Quantum-mechanical modeling of the femtosecond isomerization in rhodopsin

The photoisomerization of retinal in rhodopsin is modeled by a vibronically coupled electronic two-state system, taking into account a collective reaction coordinate and the ethylenic stretch mode. The model qualitatively reproduces all available spectroscopic information on rhodopsin and accounts for its high reaction efficiency. Quantum simulations of femtosecond time-resolved experiments suggest that the prominent 60 cm(-1) oscillations observed in experiments are due to nonadiabatic wave packet motion along the reaction coordinate. This indicates that the protein is capable of providing an almost friction-free environment for retinal up to approximate to 2 ps, thereby catalyzing the photoreaction.