Quantum Dynamics of Electron Transfer from Bacteriochlorophyll to Pheophytin in Bacterial Reaction Centers

We extend our previous works on the early electron-transfer steps in bacterial photosynthetic reaction centers to the dynamics of electron transfer from the bacteriochlorophyll anion to pheophytin. The approach employed here takes into account the whole set of normal coordinates of the acceptor and donor groups, in order to reliably account both for shifts and mixing of the normal coordinates and for changes in vibrational frequencies upon electron transfer. It is shown that intramolecular modes provide not only a discrete set of states strongly coupled to the initial state but also a quasi-continuum of weakly coupled states, which account for faster dephasing effects. Detuning effects are accounted for by averaging the computed probability over a small range of the electronic energy difference. The computed transition probability is of the same order of magnitude, a few picoseconds, as the observed one.