Dependence of Quinone Electrochemistry on Solvent Polarity and pH

We reduced quinones in aqueous/organic mixtures with dielectric constants (ε) from 78.4 to 47, using protic or aprotic organic co-solvents. The increase in half-wave potential (E 1/2 ) with declining ε was fit to the Born equation down to ε ≈ 55. Cu 2+ and ubiquinone-0 were reduced in a two-electron step that was more ε-sensitive with aprotic co-solvent. Naphtho- and anthraquinone were reduced in two one-electron steps that were influenced identically by protic or aprotic co-solvents. The first reduction wave produced neutral semiquinone (·QH), which was protonated to ·QH 2 + before reduction in the second wave. Most radii derived from fitting E 1/2 vs. ε data to the Born equation were too small; non-electrostatic effects destabilized the oxidized species, enhancing the Born electrostatic increase in E 1/2 . Additionally, for ε < 55, deviation from the Born equation was probably due to changes in solvent structure, dynamics, and solvent-solute interactions. Finally, quinones were incorporated into phosphatidyl choline sonicated vesicles: Ubiquinone-0 had two irreversible two-electron reduction waves, one from a membrane surface-bound population, and another (whose E 1/2 was negatively shifted by 150 mV) localized in the membrane interior. Ubiquinone-10 had a single irreversible two-electron reduction wave that was 250 mV more negative than the UQ-0 membrane-interior population.