Tsai and Weber never determined the rate of the homogeneous reaction but assumed it would be relatively fast since no reduction peaks were observed in voltammograms, even at 2 Vs -1 scan rate. To further investigate the EC mechanism, proposed on the basis of the Tsai and Weber article , we carried out additional experiments at increased scan rates. For 5 V/s a standard electrode was used, but carbon fibre microelectrodes (33 μm Ø) were employed to investigate the quenching process at higher scan rates (50 and 100 V s-1). These measurements were performed specifically around the pH range (pH 6 -7) at which tyrosine deprotonation appeared to have a marked effect on the presence of the cathodic peak.