Redox properties of ubiquinon (UQ10) adsorbed on a mercury electrode

Abstract

The electrochemical behaviour of ubiquinone 10 (UQ₁₀) adsorbed on mercury in contact with aqueous electrolytes has been investigated at coverages smaller than a monolayer. Stability regions, acid–base ionisation constants and standard potentials for redox equilibria between conjugate stable species were determined and reaction mechanisms proposed. The standard potential for the ubiquinone/ubihydroquinone couple obtained was 0.276 V vs. SCE (–0.138 at pH 7). The values of pK_a obtained for the two acid–base dissociation equilibria for the ubihydroquinone, 12 and 13.6, are higher than those predicted from the Hammett equation and lower than the value obtained in a low-relative-permittivity medium (80 % w/w ethanol). The ubisemiquinone ion redical has been found to be stable at pH > 13.6 with a disproportionation constant of 0.4. The corresponding constant for the protonated radical was estimated to be 10¹⁴, showing the high instability of this form. This fact, together with kinetic considerations, suggests that for pH values lower than 12 the redox chemistry proceeds via dismutation. Two-phase transitions for the reduced and oxidised forms were observed.