ubiquinone-q2 and 1-2-oleoylphosphatidylcholine

The redox properties of ubiquinone-10 (UQ) were examined in monolayers of mixtures of dioleoylphosphatidylcholine, palmitoylsphingomyelin, and cholesterol of different compositions, self-assembled on a mercury electrode, over the pH range from 7.5 to 9.5. A detailed analysis of the cyclic voltammograms of UQ in the above lipid environments points to a mechanism consisting of an elementary electron transfer step followed by two protonation (or deprotonation) steps in quasiequilibrium and by a further electron transfer step. In a lipid environment of solid-ordered (s(o)) microdomains in a liquid-disordered (l(d)) matrix, electron transport across the lipid monolayer takes place in the l(d) phase. In a pure s(o) phase, UQ tends to segregate into UQ-rich pools, exhibiting reversible electron transfer steps. In a lipid environment consisting of liquid-ordered (l(o)) microdomains (lipid rafts) in an l(d) matrix, UQ molecules tend to localize along the edge of the lipid rafts. However, in a lipid environment consisting exclusively of l(o) and s(o) microdomains, UQ molecules tend to segregate into UQ-rich pools. In all lipid environments, electron transport by UQ occurs with the quinone moiety localized on the solution side with respect to the ester linkages of the dioleoylphosphatidylcholine molecules.

Topics: Cholesterol; Electrochemical Techniques; Gels; Hydrogen; Kinetics; Membrane Microdomains; Mercury; Oxidation-Reduction; Phosphatidylcholines; Sphingomyelins; Thermodynamics; Ubiquinone

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.

Topics: Biophysical Phenomena; Biophysics; Electrochemistry; Electron Transport; Free Radicals; Hydrogen-Ion Concentration; In Vitro Techniques; Membranes, Artificial; Models, Chemical; Phosphatidylcholines; Protons; Ubiquinone