Incoherent Oscillations Accompanying Charge Separation in Photosynthetic Reaction Centers

Early events of charge separation in reaction centers (RCs) of bacterial photosynthesis are modeled by kinetic equations with time-dependent rate constants. An illustrative case of regular motion along a “slow” coordinates leading to oscillations in the kinetics is examined. Different schemes of charge separation are investigated. A good fitting of experimental kinetics of native Rba. sphaeroides RCs is achieved in the five states model P*1BAHA ↔ P*2BAHA ↔ I ↔ P A B − HA ↔ PBA A H − with two excited states * 1 P BAHA and * 2 P BAHA and three charge separated states I, P + A B − HA and PBA A H − (P is a primary electron donor, bacteriochlorophyll dimer, BA and HA are an electron acceptor, monomeric bacteriochlorophyll and bacteriopheophytin in active A-branch, respectively). In the model only the first excited state is directly populated by optical excitation. The emission of the two excited states is assumed to be at 905 and 940 nm, respectively. The intermediate state I is assumed to absorb at 1020 nm as well as the P A B − HA state. The model explains the deep oscillations in the kinetics of the * 1,2 P stimulated emission and of the A B − absorption. In the simpler schemes without the I state or with only one excited state the accordance with the experiment is achieved at unreal parameter values. A possible nature of the I and * 2 P BAHA states and a possible incoherent nature of the oscillations are discussed.