Bacteriochlorophyll excited-state quenching pathways in bacterial reaction centers with the primary donor oxidized

One striking feature of bacterial reaction centers is that while they show a high degree of structural symmetry, function is entirely asymmetric: excitation of the primary electron donor, P, a bacteriochlorophyll (BChl) dimer, results almost exclusively in electron transfer along one of the two symmetric electron transfer pathways. Here another functional asymmetry of the reaction center is explored; i.e., the two monomer BChl molecules (B_A and B_B) have distinct interactions with P in the oxidized state, P ⁺. Previous work has suggested that the excited states of both B _A and B_B were quenched via energy transfer to P ⁺ within a few hundred femtoseconds. Here, it is shown that various excitation wavelengths, corresponding to different initial B_A and B_B excited states, result in distinct reaction pathways, and which pathway dominates depends both on the initial excited state formed and on the electronic structure of P⁺. In particular, it is possible to specifically excite the Q_X transition of B_B by using excitation at 495 nm directly into the carotenoid S₂ state which then undergoes energy transfer to B_B. This results in the formation of a new state on the picosecond time scale that is both much longer lived and spectrally different than what one would expect for a simple excited state. Combining results from additional measurements using nonselective 600 or 800 nm excitation of both B_A and B_B to the Q_X or Q_Y states, respectively, it is found that B_B* and B_A* are quenched by P⁺ with different kinetics and mechanisms. B_A* formed using either Q_X or Q _Y excitation appears to decay rapidly (∼200 fs) without a detectable intermediate. In contrast, B_B* formed via Q _X excitation predominantly generates the long-lived state referred to above via an electron transfer reaction from the Q_X excited state of B_B to P⁺. This reaction is in competition with intramolecular relaxation of the Q_X state to the lowest singlet excited state. The Q_Y excited state of B_B appears to undergo the electron transfer reaction seen upon Q_X excitation only to a very limited extent and is largely quenched via energy transfer to P ⁺. Finally, the ability of P⁺ to quench B _B* depends on the electronic structure of P⁺. The asymmetric charge distribution between the two halves of P in the native reaction center is effectively reversed in the mutant HF(L168)/LH(L131), and in this case, the rate of quenching decreases significantly.