Proton coupled electron transfer of ubiquinone Q2 incorporated in a self-assembled monolayer

Abstract

We present a complete study of the reduction of ubiquinone Q₂ (UQ₂) in simpler aqueous medium, over a pH range of 2.5 to 12.5. The short isoprenic chain ubiquinones (UQ₂) were incorporated in a self-assembled monolayer. Under these conditions, the global 2e⁻ electrochemical reaction can be described on the basis of a nine-member square scheme. The thermodynamic constants of the system were determined. The global 2e⁻ process is controlled by the uptake of the second electron. The elementary electrochemical rate constants obtained by fitting of the experimental rate constant were k_s4 = 1.5 s⁻¹ for QH˙⁺₂ ↔ QH₂, k_s5 = 1.5 s⁻¹ for QH˙ ↔ QH⁻ and k_s6 = 1 s⁻¹ for Q˙⁻ ↔ Q²⁻. The three electrochemical reactions QH˙⁺₂ ↔ QH₂, QH˙ ↔ QH⁻ and Q˙⁻ ↔ Q²⁻ are successively involved when increasing the pH. Protonations can occur or not, before or after the electron uptake and the reaction paths are, from low to high pH: e⁻, H⁺e⁻, e⁻H⁺, H⁺e⁻H⁺, H⁺e⁻ and e⁻H⁺.