Proton Transfer versus Hydrogen Bonding: The Reduction of Ubiquinone Q2 Incorporated in a Self‐Assembled Monolayer in Unbuffered Aqueous Solution

Abstract

AbstractThe electrochemical reduction of ubiquinone Q2 (UQ2) in unbuffered aqueous media was investigated over a pH range of 2 to 11, and the results were compared to those obtained in our previous work in buffered solutions. The short isoprene chains of UQ2 were incorporated in a self‐assembled monolayer. Analysis of the mechanism and a study in D2O solutions allowed us to demonstrate the important role of hydrogen bonding in the stabilization of the reduced species, also in water. If the concentration of H+ is low relative to that of the quinone, the electrochemical reaction does not proceed through proton‐coupled electron transfer, as it does in buffered medium or at low pH values (pH<4.4). We proved that the reduction of UQ2 is a two‐electron process followed by a first‐order reaction. Unlike that observed for most quinones in unbuffered media, UQ2 used the labile hydrogen of the isoprene chain to compensate for the lack of protons through a tautomeric reaction.