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 (P865•+QB•−) of the primary donor P865, a bacteriochlorophyll dimer, and the terminal acceptor, QB, a...