Electronic factor in biological electron transfer systems. Role of aromatic side groups

The electronic factor for electron transfer (Δ) is calculated in proteins and in model systems using the CNDO / S and ab initio methods. The decrease of |Δ| with distance for oligoglycins in different conformations is compared to the decrease in other oligomers. The rate of an intramolecular ET reaction in azurin from a disulphide bridge to Cu(II) is calculated with satisfactory agreement with the experiments. For a pathway with three aromatic side groups we obtain a slightly larger electronic factor than one without such groups but the latter do not seem to be essential for the ET process in this case. The primary electron transfer step in the bacterial photosynthetic center of Rhodopseudomonas viridis is studied. The calculated |Δ| for a direct electron transfer process P*BChl L BPh L P + BChl L BPh   L −is very small and inconsistent with the high rate measured. The accessory chlorophyll, being almost perpendicular to the special pair and pheophytin chromophores, does not seem to have good bridging properties and a much larger coupling is provided by the Tyr M208 phenol side group. However, this coupling is also too small to explain the rapid primary charge separation. We believe that the energy splitting between the charge separated singlet and triplet states caused by this coupling is essential to prevent wasteful triplet formation.