Charge Separation and ET

The SP donates the excited electron to the accessory BChl on the A side. The ways of avoiding the B side are very subtle. It seems to be related to a water molecule connecting the SP and the accessory BChl. In terms of evolution and reproduction, the system is created by dimerization of two monomers. However, the precise structure of the protein and the way the subunits are attached to each other is enough to cause great differences in ET properties. 

FIGURE 15.8 Bacterial RC (Rb. sphaeroides) from different angles. Only the cofactors are seen here. The protein strncture was determined by Deisenhofer and Michel. 

The fiPh chromophore is a safe haven for the electron. It cannot return to the accessory Chi because of a low Boltzmann factor. The electron could return to the SP, whose triplet state is at a lower energy. However, in this case, the electronic factor is too low because of a large distance. The average lifetime on BPh is 200 ps. In the next step, it transfers to Qa, a quinone molecule. 

Between and another quinone, Qg, there is an Fe + ion. This ion is not involved in ET, but remains as Fe +. Instead, it appears to be active in connecting the imidazol side chains of the peptides located between the two quinones, Qa and Qg. 

The natural way of operation of a bacterial RC, permits only SP to be excited from the antenna system. It is possible to separate the RC from the antenna, however, and excite directly with higher energies. This way, the accessory BChl may become excited. Such an excitation also leads to charge transfer of type BChlA -BPh or SP BChl- SP BChl BPh-. 

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