The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (Q(A)) and Secondary (Q(B)) Quinones of Type II Bacterial Photosynthetic Reaction Centers

[Image: see text] Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (Q(A)) and secondary (Q(B)) electron acceptors in photosynthetic reaction centers from purple bacteria such as Rb. sphaeroides. (13)C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQ(A) and SQ(B), were compared with DFT calculations of the (13)C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔE(m)) between Q(A) and Q(B) of 175–193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as Q(B) in mutant reaction centers with a ΔE(m) of ∼160–195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from Q(A) to Q(B) in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.