Structural Influences on the Electrochemistry of Ubiquinone

Summary

Ubiquinone, 2,3-dimethoxy-5-methyl-6-polyprenyl-l,4- benzoquinone is almost ubiquitous in living organisms; why? One possibility is the presence of the 2,3 dimethoxy motif. Methoxy groups, in isolation, have two separate effects on the electrochemistry of the quinone nucleus; an inductive electron withdrawing effect, independent of orientation, and a resonant, electron donating effect where the lone pair of electrons on the methoxy oxygen becomes delocalized over the quinone enone system. This latter effect is very dependent on the orientation of the methoxy group relative to the ring, and cannot be achieved by both methoxy groups in the 2,3 — dimethoxy motif. This paper examines the possibility that the different binding sites for ubiquinone might modulate the relative orientations of the methoxy groups, and thereby impose different specificities and energetics on the redox properties of the quinone.