ELECTROCHEMISTRY OF ADSORBED FLAVIN ADENINE-DINUCLEOTIDE IN ACIDIC SOLUTIONS

The adsorption of flavin adenine dinucleotide (FAD) at a mercury electrode surface has been investigated in dilute FAD solutions from pH ca. 2-7. FAD is found to adsorb initially in an orientation in which the isoalloxazine and adenine moieties lie parallel to the electrode surface (Stage I) at all acidic pH values and potentials. At low coverage in Stage I, a persistent separation of the anodic and cathodic peaks is present, independent of sweep rate, perhaps indicative of the expected planar (oxidized) and nonplanar (reduced) conformations of the isolloxazine ring system. As the surface coverage increases, this difference diminishes and a sudden reorientation occurs to a structure in which it is suggested that alternating isolloxazine and adenine moieties are adsorbed perpendicularly to the electrode surface (Stage II). At pH less-than-or-similar-to 4.5 this structure is stable over a very wide potential range. Also, a second vertical highly condensed orientation is observed at positive potentials in these more acidic solutions. These observations may reflect the effects of adenine protonation and the consequent coulombic interactions between it and the charged electrode surface. A third FAD orientation is observed (Stage III) after some time in acidic solutions, possibly involving a bilayer or a multilayer adsorbed on the mercury electrode surface.