Adsorbed lumiflavin at mercury electrode surfaces

The adsorption of lumiflavin (LF) at a mercury electrode surface has been studied by means of cyclic voltammetry in phosphate buffer solutions of pH 6, 7 and 8. At low to medium coverages, LF, which is redox-active, is suggested to adsorb in an orientation parallel to the electrode surface, independent of potential. At higher coverages, concerted re-orientations of the LF monolayer occur as the potential is changed. At potentials positive of ca. -0.35 V and negative of -1.0 V, adsorbed LF is likely in the parallel configuration, while two perpendicular orientations exist at potentials between these limits, consistent with higher attainable coverages. One of these is particularly stabilized in pH 6 solutions, when reduced LF is known to be uncharged, while it is likely to be anionic (and hence de-stabilized) at pH 7 and 8. A second set of small redox peaks observed just negative of the main redox peaks have been assigned to lumiflavin dimers, formed when the molecules are oriented in the perpendicular orientation in the negative potential range. The electrochemical behaviour of LF shows striking differences from that of FAD, although the common electrochemically active moiety is the isoalloxazine ring system. (C) 1998 Elsevier Science S.A. All rights reserved.