B-Branch Electron Transfer in the Photosynthetic Reaction Center of a Rhodobacter sphaeroides Quadruple Mutant Q- and W-Band Electron Paramagnetic Resonance Studies of Triplet and Radical-Pair Cofactor States

The directionality of light-induced charge transfer in bacterial photosynthetic reaction centers (RCs) with respect to their A and B cofactor branches is still poorly understood on the electronic level A prominent example is primary electron transfer in the RCs from the purple bacterium Rb sphaeroides Site-directed mutants with specific alterations of the cofactor binding sites with respect to the native system can deliver useful information toward a better understanding of the directionality enigma Here we report on electron paramagnetic resonance (EPR) studies of the LDHW quadruple mutant, HL(M182)/GD(M203)/LH(M214)/AW(M260), which contains crucial mutations in the electron-transfer pathway The directionality of the charge separation process was studied under light- or dark-freezing conditions first directly by 95 GHz (W-band) high-field EPR spectroscopy examining the charge-separated radical pairs (P-865 (+)Q(B) (-)) of the primary donor P-865, a bacteriochlorophyll dimer, and the terminal acceptor, Q(B), a ubiquinone-10 Second, it was studied indirectly by 34 GHz (Q-band) EPR examining the triplet states of the primary donor (P-3(865)) that occur as a byproduct of the photoreaction At 10 K, the triplet state has been found to derive mainly from an intersystem crossing mechanism, indicating the absence of charge-separated radical-pair states with a lifetime longer than 10 ns B-branch charge separation and formation of the triplet-state P-3(865) via a radical-pair mechanism can be induced with low yield at 10 K by direct excitation of the bacteriopheophytins in the B-branch at 537 nm At this wavelength, charge separation most probably proceeds via hole transfer from bacteriopheophytin to the primary donor The triplet state of the primary donor is found to be quenched by the carotenoid cofactor present in the RC The light-induced transient EPR signal of P (+)Q(B) (-) is formed in a minor fraction of RCs (<1%) for RCs frozen in the dark In contrast, about 70% of RCs illuminated upon freezing are trapped in the long-lived (tau > 10(4) s) charge-separated-state P (+)Q(B) (-) The temperature dependence of the EPR signals from P (+)Q(B) (-) points to two factors responsible for the forward electron transfer to the terminal acceptor Q(B) and for the charge-recombination reaction The first factor involves a significant protein conformational change to initiate P (+)Q(B) (-) charge separation, presumably by moving the quinone from the distal to the proximal position relative to the iron The second factor includes protein relaxation, which governs the charge-recombination process along the B-branch pathway of the LDHW mutant