Behavior of flavocoenzymes at the gold electrode/solution interface studied by electro-optical measurement.

Abstract

The behavior of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) at the interface between the solution and a gold electrode has been studied by specular reflectivity measurement and cyclic voltammetry in 0.05 M carbonate buffer (pH 10.0). Current-potential (i-E) curves obtained for FMN and FAD showed redox peaks at about -0.5 V (vs. Ag/AgCl), suggesting that the isoalloxazine moieties of these compounds participate in the electron transport. The reflectivity-potential curves for these compounds measured simultaneously with the i-E curves showed a marked decrease in reflectivity in the potential region more positive than about -0.8 V. Such a trend suggests that adsorption of these compounds, as well as of their reduction products, takes place on the electrode surface. In order to obtain information about the adsorbed states of these molecules, the data were analyzed according to the theoretical treatment of McIntyre and Aspnes. The results suggest that both FMN and FAD are adsorbed with their isoalloxazine rings in contact with the electrode surface. In addition, the adenine moiety of FAD appears to come partly into contact with the surface. The reduced forms of FMN and FAD are also observed to lie in essentially the same states as their oxidized forms.