REDOX PROPERTIES OF UBIQUINONE (UQ10) ADSORBED ON A MERCURY-ELECTRODE

The electrochemical behaviour of ubiquinone 10(UQ10) adsorbed on mercu ry in contact with aqueous electrolytes has been investigated at cover ages smaller than a monolayer. Stability regions, acid-base ionisation constants and standard potentials for redox equilibria between conjug ate stable species were determined and reaction mechanisms proposed. T he standard potential for the ubiquinone/ubihydroquinone couple obtain ed 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 equatio n and lower than the value obtained in a low-relative-permittivity med ium (80% w/w ethanol). The ubisemiquinone ion radical has been found t o be stable at pH > 13.6 with a disproportionation constant of 0.4. Th e corresponding constant for the protonated radical was estimated to b e 10(14), showing the high instability of this form. This fact, togeth er with kinetic considerations, suggests that for pH values lower than 12 the redox chemistry proceeds via dismutation. Two-phase transition s for the reduced and oxidised forms were observed.