Methoxy Dihedral Angles of Ubiquinone Contribute More than 160 mV to the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of the Photosynthetic Reaction Center

Abstract

Ubiquinone is an almost universal, membrane-associated redox mediator. Its redox properties are substantially determined by its environment in the binding sites of proteins and by the dihedral angles of the methoxy groups relative to the ring plane. In this work, we use the photosynthetic reaction center as a model system for understanding the role of methoxy conformations in determining the redox potential of the ubiquinone/semiquinone couple. Despite the abundance of X-ray crystal structures for the reaction center, the quinone site resolution has thus far been too low to provide a reliable measure of the methoxy dihedral angles of the primary and secondary quinones, QA and QB. We have performed HYSCORE on isolated reaction centers with ubiquinones 13C-labeled at the headgroup methyl and methoxy groups, and the isotropic and anisotropic hyperfine coupling constants were estimated. Comparison of our data to quantum mechanically derived models and available crystal structures gives a best fit for the 2-methoxy dihedral angle of QB as 50° more out of plane than in QA. This assignment corresponds to a redox potential gap (ΔEm) between QA and QB of ≈180 mV. This is consistent with the failure of a ubiquinone analog lacking the 2-methoxy group to function as QB in mutant reaction centers with ΔEm ≈160-195 mV. We conclude the 2-methoxy group of ubiquinone provides an essential mechanism by which the reaction center tunes the redox potential difference necessary for electron transfer. The influence of the methoxy groups on the second electron transfer is under investigation through similar EPR studies of the biradical, QA-QB-, generated in mutant reaction centers and at high pH in wild type reaction centers.