Electrochemical modeling of electron and proton transfer to ubiquinone-10 in a self-assembled phospholipid monolayer

Ubiquinone-10 (UQ) was incorporated at concentrations ranging from 0.5 to 2 mol% in a self-assembled monolayer of dioleoylphosphatidylcholine (DOPC) deposited on a mercury drop electrode, and its electroreduction to ubiquinol (UQH(2)) was investigated in phosphate and berate buffers over the pH range from 7 to 9.5 by a computerized chronocoulometric technique. The dependence of the applied potential for a constant value of the faradaic charge due to UQ reduction upon the electrolysis time t at constant pH and upon pH at constant t was examined on the basis of a general kinetic approach. This permitted us to conclude that the reduction of UQ to UQH(2) in DOPC monolayers takes place via the reversible uptake of one electron with the formation of the semiubiquinone radical anion UQ(.-), followed by the rate-determining protonation of this anion with UQH(.) formation; this neutral radical is more easily reduced than UQ, yielding the ubiquinol UQH(2). In spite of the very low concentration of hydrogen ions as compared with that of the acidic component of the buffer, the only effective proton donor is the proton itself; this strongly suggests that the protonation step takes place inside the polar head region of the DOPC monolayer, which is only accessible to protons.