COMPLEX ELECTROCHEMISTRY OF FLAVODOXIN AT CARBON-BASED ELECTRODES - RESULTS FROM A COMBINATION OF DIRECT ELECTRON-TRANSFER, FLAVIN-MEDIATEDELECTRON-TRANSFER AND COMPROPORTIONATION

Staircase cyclic voltammetry (SCV) and differential pulse voltammetry on fully oxidized flavodoxin from Desulfovibrio vulgaris Hildenborough at the bare glassy carbon electrode give one redox couple at a potent ial of -218 mV (standard hydrogen electrode (SHE)) at pH = 7.0 with an SCV peak current proportional to the scan rate. This response is caus ed by flavin mononucleotide (FMN), dissociated from the protein and ad sorbed onto the electrode. The midpoint potential and the pK of 6.5 ar e equal to the values measured with free FMN in solution. When the cat ionic promoter neomycin is added, one additional and diffusion control led response is observed. The midpoint potential is -413 mV (SHE) at p H 7.0 with a redox-linked pK of 4.8 for the reduced form. The temperat ure dependence is -1.86 mV K-1, yielding Delta S degrees = -179 J mol( -1) K-1 and Delta H degrees = -12.4 kJ mol(-1). Although the starting material was 100% quinone, no response was observed around the midpoin t potential of the quinone to semiquinone reduction of -113 mV (SHE) a t pH 7.0, determined in an EPR-monitored titration with dithionite. Di gital simulation shows that the peak currents of the second reduction couple approach a maximum value after a few cycles if comproportionati on of fully reduced and fully oxidized flavodoxin occurs in solution a nd a small amount of semiquinone is either present initially or is gen erated by mediation of electrode-bound FMN. In the latter case the het erogeneous electron transfer rate between adsorbed FMN and flavodoxin is 6.3 X 10(-6) m s(-1). The implications of this anomalous behaviour for electrochemistry on flavin enzymes like glucose oxidase are discus sed.