Redox potentials of ubiquinone, menaquinone, phylloquinone, and plastoquinone in aqueous solution

Quinones serve as redox active cofactors in bacterial photosynthetic reaction centers: photosystem I, photosystem II, cytochrome bc (1), and cytochrome b (6) f. In particular, ubiquinone is ubiquitous in animals and most bacteria and plays a key role in several cellular processes, e.g., mitochondrial electron transport. Their experimentally measured redox potential values for one-electron reduction E (m)(Q/Q(center dot-)) were already reported in dimethylformamide (DMF) versus saturated calomel electrode but not in water versus normal hydrogen electrode (NHE). We calculated E (m)(Q/Q(center dot-)) of 1,4-quinones using a quantum chemical approach. The calculated energy differences of reduction of Q to Q(center dot-) in DMF and water for 1,4-quinone derivatives correlated highly with the experimentally measured E (m)(Q/Q(center dot-)) in DMF and water, respectively. E (m)(Q/Q(center dot-)) were calculated to be -163 mV for ubiquinone, -260 mV for menaquinone and phylloquinone, and -154 mV for plastoquinone in water versus NHE.