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 vertical bar-FMN to FTO vertical bar-FMNH2 occurs with the pH and scan rate dependence expected for a 2e(-)/2H(+) surface-bound couple. The addition of tetramethylbenzoquinone (Me(4)Q) 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) x 10(6) M-1 s(-1). Electrocatalytic reduction of the quinone also occurs on derivatized, high surface area nanolTO electrodes with evidence for competitive rate-limiting diffusion of the quinone into the mesoporous nanostructure.