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 S-2 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 (similar to 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(1,131), and in this case, the rate of quenching g decreases significantly.