Primary electron transfer in modified bacterial reaction centers: optimization of the first events in photosynthesis

Pigment exchange in bacterial reaction centers (RC) of Rb. sphaeroides allows the systematic variation of the free energies of the primary (BA) and secondary (HA) electron acceptors. The effect of energetically elevated acceptor levels on the initial electron transfer (ET) step, on the P+BA− population and on the quantum yield of charge separation is explained within the stepwise ET model. Slower and less efficient charge separation with enhanced non-exponential character is found in the modified samples indicating that the native RC is a highly optimized system where small changes of an energy level may cause a significant reduction in efficiency.