THE DYNAMICS OF THE PRIMARY EVENT IN RHODOPSINS REVISITED

The conclusions obtained from an early simulation study of the primary event in the vision process are reexamined by simulating the dynamics of the primary photoisomerization reaction in bacteriorhodopsin using a detailed molecular model for the protein-chromophore complex. The calculated photoisomerization time is of the same order of magnitude as that predicted by the early simulation and found in recent experimental studies. Several simulations with different conditions indicate that the isomerization process involves an efficient transfer of energy from the reaction coordinate to other degrees of freedom and is better described by a damped-motion model than by an inertial model. The damped motion is expected to give a significant quantum yield for both the cis --> trans and trans --> cis cases only if the minimum of the excited state potential surface is located above the ground state maximum (unless the excited state surface is extremely shallow or if the inertial effects associated with the surface crossing process are very large). The present study suggests that the observed quantum yield should not be analyzed by one-dimensional models but by multidimensional microscopic simulations that consider the surface crossing process and the subsequent ground state relaxation processes.