Electrochemical Oxidation and Reduction of Flavin–Adenine Dinucleotide Adsorbed on a Mercury Electrode Surface

Abstract

Abstract The electrochemical redox reaction of flavin–adenine dinucleotide (FAD) adsorbed on a hanging mercury drop electrode was studied in a pH 6.9 phosphate buffer by means of cyclic d.c. and a.c. voltammetry. Both the oxidized and reduced forms of FAD were strongly adsorbed on the mercury electrode surface. At the surface concentrations of FAD, Γ, lower than 5.0×10−11 mol cm−2, the cyclic d.c. and a.c. voltammetric behavior of adsorbed FAD was explained by the theory for a two-step one-electron surface redox reaction. The formal redox potential, semiquinone formation constant, and charge transfer rate constant for the surface redox reaction of FAD and the interaction parameter of adsorbed FAD species were determined. Cyclic d.c. voltammetry and differential capacity measurement showed that, as Γ exceeded 5×10−11 mol cm−2, the reorientation of adsorbed FAD molecules occurred. At Γ>5.8×10−11 mol cm−2, the semiquinone formation constant was very small and the behavior was interpreted by the theory of a single-step two-electron surface redox reaction. The electrochemical surface redox properties of FAD adsorbed on a mercury electrode surface appear to depend on the orientation mode of adsorbed FAD molecules.