Bacteriochlorophyll Excited-State Quenching Pathways in Bacterial Reaction Centers with the Primary Donor Oxidized

Abstract

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 (∼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.