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

Abstract

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 (UQH2) was investigated in phosphate and borate 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 kinetion approach. This permitted us to conclude that the reduction of UQ to UQH2 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 UQH2. 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.

Electrochemistry of LB films of mixed MGDG:UQ on ITO

“…According to the literature [7,22,36], the global electrochemical reaction of the UQ/UQH2 redox couple proposed for process I and II in our experimental conditions is described by eq. (1), which is similar than that of benzoquinone in aqueous buffered solution.…”

“…One of the first attempts to study the electrochemistry of UQ used lipid monolayer and bilayer systems supported on Hg [7][8][9][10]. On solid electrodes, different methods have been applied to modify the surface of the electrode with a thin layer simulating the structure of the lipid layer [11], where the quinone molecules can be embedded.…”

Electrochemical impedance spectroscopy and fluorescence lifetime imaging of lipid mixtures self-assembled on mercury

“…Lipid monolayers were selfassembled on mercury as described earlier. [23][24][25] Briefly, the appropriate lipid mixture in pentane was spread on the surface of the working 0.1 M KCl aqueous solution and the pentane was allowed to evaporate. The HMDE was then immersed slowly in the solution across the lipid film, under the control of an oleodynamic system.…”

Ubiquinone-10 in gold-immobilized lipid membrane structures acts as a sensor for acetylcholine and other tetraalkylammonium cations

“…As the molecule is very insoluble in water, electrochemical measurements are performed either with UQ adsorbed directly on an electrode surface [13] or embedded onto supported lipid mono-and bilayers [14][15][16][17][18][19][20]. The latter option is preferred, as the UQ properties can then be determined in an environment similar to the one in which it is found in nature.…”

“…The accepted redox mechanism is rather complex, as the electron transfer reactions are coupled with proton transfers from and to the surrounding media [19]. The formation of the semiubiquinone radical (UQH • ) as an intermediary in most UQ redox reaction pathways has been shown [17]. The behaviour and stability of this radical and the products that can be expected from its oxidation and reduction are strongly dependent on the surrounding conditions, especially on the pH of the solution [19].…”