Role of the chlorophyll dimer in bacterial photosynthesis.

The role of a special dimer (D) of bacteriochlorophyll molecules in bacterial photosynthesis was examined by calculations of the rates of electron transfer reactions in a system of the dimer and a bacteriopheophytin (BPh) molecule. It was found that the dependence of the potential surfaces of D on the distance between the monomers allows a fast light-induced electron transfer from D to BPh but only a slow back reaction (reduction of D(+) by BPh(-)). The same potential surfaces allow efficient reduction of D(+) by cytochrome c. Possible advantages of greatly different values of the electronic matrix elements for the forward and back reactions are pointed out. It is suggested that the electrostatic interaction between D(+) and an ionized group of the protein might play an important role in the photosynthetic reaction.