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 (QH(2)) in aqueous buffer, pH 7.40, was used to calculate a constant for equilibrium (1) Q + QH(2)=Q(.-) + Q(.-) + 2H(+) (k(1); 2k(-1); K-1 = k(1)/2k(-1)). The rate constants for comproportionation between Q and QH(2), k(1), were calculated from the combination of k(1) determined in this work and 2k(-1) reported previously. The Nernst equation was applied to calculate the change in one-electron reduction potential Delta E-1 = E(Q/Q(.-)) - E(Q(/)(.-)QH(2)) in equilibrium (1). The E(Q(.-)/QH(2)) values were calculated from Delta E-1 and the values of E(Q/Q(.-)) known from the literature. The correlations between E(Q(.-)/QH(2)) and E(Q/Q(.-)) as well as between Delta E-1 (k(1)) and E(Q/Q(.-)) are discussed. The values of E, and k(1) are suggested to be the key factors governing the autoxidation of QH(2).