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

Abstract

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^·−) 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^·−) of 1,4-quinones using a quantum chemical approach. The calculated energy differences of reduction of Q to Q^·− in DMF and water for 1,4-quinone derivatives correlated highly with the experimentally measured E _m(Q/Q^·−) in DMF and water, respectively. E _m(Q/Q^·−) were calculated to be −163 mV for ubiquinone, −260 mV for menaquinone and phylloquinone, and −154 mV for plastoquinone in water versus NHE.