Kinetics of electron and proton transfer to ubiquinone-10 and from ubiquinol-10 in a self-assembled phosphatidylcholine monolayer

Abstract

Upon incorporating from 0.5 to 2 mol% ubiquinone-10 (UQ) in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) supported by mercury, the kinetics of UQ reduction to ubiquinol-10 (UQH 2) as well as that of UQH 2 oxidation to UQ were investigated in borate buffers over the pH range from 8 to 9.5 by cyclic voltammetry. A general kinetic approach was adopted to interpret the dependence of the applied potential upon the scan rate at constant pH and upon pH at constant scan rate, while keeping the initial reactant concentration and the faradaic charge constant. The oxidation of UQH 2 to UQ in DOPC monolayers occurs via the reversible release of one electron with formation of the semiubiquinone radical cation UQH 2 ⋅+, followed by its rate-determining deprotonation by hydroxyl ions with formation of the UQH ⋅ neutral radical; the latter is then instantaneously oxidized to UQ. Analogously, the rate-determining step in UQ reduction to UQH 2 consists in the protonation by hydrogen ions of the semiubiquinone radical anion UQ ⋅− resulting from the reversible uptake of one electron by UQ. However, a non-negligible fraction of UQ ⋅− uptakes protons very slowly, and hence, retains its intermediate oxidation state during the recording of the cyclic voltammetric peak for UQ reduction.