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 ≈2 ps, thereby catalyzing the photoreaction.