pKa of ubiquinone, menaquinone, phylloquinone, plastoquinone, and rhodoquinone in aqueous solution

Quinones can accept two electrons and two protons, and are involved in electron transfer and proton transfer reactions in photosynthetic reaction centers. To date, the pKa of these quinones in aqueous solution have not been reported. We calculated the pKa of the initial protonation (Q·− to QH·) and the second protonation (QH− to QH2) of 1,4-quinones using a quantum chemical approach. The calculated energy differences of the protonation reactions Q·− to QH· and QH− to QH2 in the aqueous phase for nine 1,4-quinones were highly correlated with the experimentally measured pKa(Q·−/QH·) and pKa(QH−/QH2), respectively. In the present study, we report the pKa(Q·−/QH·) and pKa(QH−/QH2) of ubiquinone, menaquinone, phylloquinone, plastoquinone, and rhodoquinone in aqueous solution.