Proton-Coupled Electron Transfer Reduction of a Quinone by an Oxide-Bound Riboflavin Derivative

The redox properties of a surface-bound phosphate flavin derivative (flavin mononucleotide, FMN) have been investigated on planar-FTO and nanoITO electrodes under acidic conditions in 1:1 CH₃CN/H₂O (V:V). On FTO, reversible 2e^–/2H⁺ reduction of FTO|-FMN to FTO|-FMNH₂ occurs with the pH and scan rate dependence expected for a 2e^–/2H⁺ surface-bound couple. The addition of tetramethylbenzoquinone (Me₄Q) results in rapid electrocatalyzed reduction to the hydroquinone by a pathway first order in quinone and first order in acid with k_H = (2.6 ± 0.2) × 10⁶ M^–1 s^–1. Electrocatalytic reduction of the quinone also occurs on derivatized, high surface area nanoITO electrodes with evidence for competitive rate-limiting diffusion of the quinone into the mesoporous nanostructure.