pK a 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 pK a of these quinones in aqueous solution have not been reported. We calculated the pK a 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 pK a(Q·-/QH·) and pK a(QH-/QH2), respectively. In the present study, we report the pK a(Q·-/QH·) and pK a(QH-/QH2) of ubiquinone, menaquinone, phylloquinone, plastoquinone, and rhodoquinone in aqueous solution.