ADSORBED LUMIFLAVIN AT MERCURY-ELECTRODE SURFACES

The adsorption of lumiflavin (LF) at a mercury electrode surface has b een studied by means of cyclic voltammetry in phosphate buffer solutio ns of pH 6, 7 and 8. At low to medium coverages, LF, which is redox-ac tive, is suggested to adsorb in an orientation parallel to the electro de surface, independent of potential. At higher coverages, concerted r e-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 o rientations exist at potentials between these limits, consistent with higher attainable coverages. One of these is particularly stabilized i n pH 6 solutions, when reduced LF is known to be uncharged, while it i s likely to be anionic (and hence de-stabilized) at pH 7 and 8. A seco nd set of small redox peaks observed just negative of the main redox p eaks have been assigned to lumiflavin dimers, formed when the molecule s are oriented in the perpendicular orientation in the negative potent ial range. The electrochemical behaviour of LF shows striking differen ces from that of FAD, although the common electrochemically active moi ety is the isoalloxazine ring system. (C) 1998 Elsevier Science S.A. A ll rights reserved.