On the mechanism of cytochrome oxidation in bacterial photosynthesis. Quantum tunnelling effects revisited

We propose that the unique temperature dependence of the Chance-De Vault cytochrome oxidation reaction in Chromatium is not due to a transition from low-temperature nuclear tunnelling to a high-temperature activated electron transfer (ET), but rather originates from two parallel ET processes from two distinct low-potential cytochromes to the bacteriochlorophyll dimer cation. These involve a slow activationless process, which dominates at low temperatures (T ≤ 120 K) and an activated process, which is practically exclusive at high temperatures. This conjecture provides plausible nuclear and electronic coupling terms and structural data for the two cytochrome oxidation reactions.