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.

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 (Δ E _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 Δ E _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.