Electron Transfer between Quinones in Photosynthetic Reaction Centers

The mechanism of electron transfer (ET) from the primary to the secondary quinone of bacterial photosynthetic
reaction centers is discussed on the basis of theoretical computations of the minimum energy nuclear
configurations and ET coupling elements, and quantum dynamic simulations of elementary reaction steps.
For ET to occur via tunneling, unreasonably high values of the electronic coupling elements or very stringent
energy conditions, i.e., tight degeneracy (within a few cm-1) between the initial and final vibronic states, are
necessary, both for the direct and through-bridge (superexchange) routes. The assumption of tight degeneracy
significantly slows down the process so that other competitive processes, such as proton transfer from the
H-bonded HisM219 to the primary quinone, can take place. All these results suggest that the iron−histidine
bridge can play an important role in the ET mechanism.