Nature and dynamics of the charge-separated intermediate in reaction centers in which bacterichlorophyll replaces the photoactive bacteriopheophytin. 2. The rates and yields of charge separation and recombination

The primary photochemistry of the (M)L214H and (M)L214H/(L)E104V mutant bacterial reaction centers (RCs) from Rhodobacter sphaeroides has been investigated at room and cryogenic temperatures. In both mutants the native bacteriopheophytin electron acceptor (BPh_L) is replaced with a bacteriochlorophyll (BChl) molecule denoted by B; in the double mutant a hydrogen-bonding interaction of B-is removed. The initial stage of charge separation, formation of an intermediate P⁺I^-, is slowed somewhat in both mutants but without a detectable loss in yield. However, the yield of the subsequent stage of charge separation, P⁺I^- → P⁺Q_A^-, is significantly reduced due to the combination of slower electron transfer from I^- to Q_A and enhanced charge recombination of P⁺I^- to the ground state. Models are considered in which P⁺B^- and P⁺BCh1_L are quantum-mechanically mixed or in thermal equilibrium. It is concluded that P⁺B^- and P⁺BCh1_L are very close in energy in the mutants and that P⁺BCh1_L is very close in energy to the primary electron donor, P^*, in both the mutant and wild-type RCs. We also propose that the energy of P⁺BCh1_L is important not only in determining the dynamics of the initial stage of charge separation but also in dictating the rates and yields of the subsequent electron transfer and charge recombination processes.