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The discovery by Louis N. M. Duysens in the 1950s that illumination of photosynthetic purple bacteria can cause oxidation of either a bacteriochlorophyll complex (P) or a cytochrome was followed by an extended period of uncertainty as to which of these processes was the `primary' photochemical reaction. Similar questions arose later about the roles of bacteriopheophytin (BPh) and quinones as the initial electron acceptor. This is a personal account of kinetic measurements that showed that electron transfer from P to BPh occurs in the initial step, and that the oxidized bacteriochlorophyll complex (P+) then oxidizes the cytochrome while the reduced BPh transfers an electron to a quinone.

Oscillations in the electrostatic energy gap [ΔVelec(t)] for electron transfer from the primary electron donor (P) to the adjacent bacteriochlorophyll (B) in photosynthetic bacterial reaction centers are examined by molecular-dynamics simulations. Autocorrelation functions of ΔVelec in the reactant state (PB) include prominent oscillations with an energy of 17 cm−1. This feature is much weaker if the trajectory is propagated in the product state P+B−. The autocorrelation functions also include oscillations in the regions of 5, 80 and 390 cm−1 in both states, and near 25 and 48 cm−1 in P+B−. The strong 17-cm−1 oscillation could involve motions that modulate the distance between P and B, because a similar oscillation occurs in the direct electrostatic interactions between the electron carriers.