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

Abstract

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.

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