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 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 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.