Pigment exchange in bacterial reaction centers (RC) of Rb. sphaeroides allows the systematic variation of the free energies of the primary (B-A) and secondary (H-A) electron accepters. 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.