Proton Transfer versus Hydrogen Bonding: The Reduction of Ubiquinone Q 2 Incorporated in a Self‐Assembled Monolayer in Unbuffered Aqueous Solution

The electrochemical reduction of ubiquinone Q 2 (UQ 2 ) 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 UQ 2 were incorporated in a self‐assembled monolayer. Analysis of the mechanism and a study in D 2 O 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 UQ 2 is a two‐electron process followed by a first‐order reaction. Unlike that observed for most quinones in unbuffered media, UQ 2 used the labile hydrogen of the isoprene chain to compensate for the lack of protons through a tautomeric reaction.