Only quinones with a 2-methoxy group can act simultaneously as the primary (QA) and secondary (QB) electron acceptors in photosynthetic reaction centers from Rhodobacter sphaeroides. 13C hyperfine sublevel correlation measurements of the 2-methoxy in the semiquinone states, SQA and SQB, were compared with quantum mechanics calculations of the 13C couplings as a function of the dihedral angle. X-ray structures support dihedral angle assignments corresponding to a redox potential gap (ΔEm) between QA and Q B of ∼180 mV. This is consistent with the failure of a ubiquinone analogue lacking the 2-methoxy to function as QB in mutant reaction centers with a ΔEm of ≈160-195 mV.
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