The kinetics and thermodynamics of quinone–semiquinone–hydroquinone systems under physiological conditions

The steady-state concentration of semiquinones (Q˙–) determined by EPR in the mixtures of eleven alkyl-, methoxy- and chloro-substituted 1,4-benzoquinones as well as 1,4-naphthoquinone (Q) with corresponding hydroquinones (QH2) in aqueous buffer, pH 7.40, was used to calculate a constant for equilibrium (1) Q + QH2⇌Q˙– + Q˙– + 2H+ (k1; 2k–1; K1 = k1/2k–1). The rate constants for comproportionation between Q and QH2, k1, were calculated from the combination of K1 determined in this work and 2k–1 reported previously. The Nernst equation was applied to calculate the change in one-electron reduction potential ΔE1 = E(Q/Q˙–) – E(Q˙–/QH2) in equilibrium (1). The E(Q˙–/QH2) values were calculated from ΔE1 and the values of E(Q/Q˙–) known from the literature. The correlations between E(Q˙–/QH2) and E(Q/Q˙–) as well as between ΔE1 (k1) and E(Q/Q˙–) are discussed. The values of ΔE1 and k1 are suggested to be the key factors governing the autoxidation of QH2.