On the Relationship between the Characteristics of Bilirubin Oxidases and O₂ Cathodes Based on Their “Wiring”

The characteristics of a novel cathode on which O 2 is electroreduced to water under physiological conditions (pH 7.4, 0.15 M NaCl, 37.5 °C) are reported. The cathode was made by electrically connecting (“wiring”) redox centers of bilirubin oxidase (BOD) from Trachyderma tsunodae ( Tt ) to carbon electrodes through a redox polymer. The cathode was more stable than that made by wiring BOD from Myrothecium verrucaria ( Mv ) (N. Mano et al., J. Am. Chem. Soc . 2002, 124, 6480−6486) (5% loss of current per day of operation vs 10%), and its operating potential at 3 mA cm -2 current density was −140 mV vs the potential of the reversible O 2 /H 2 O electrode, 50 mV higher than that of the Mv -BOD cathode. The improved stability is attributed to stronger electrostatic bonding in the adduct of Tt -BOD with the “wiring” redox polymer, and the higher operating potential to the replacement of methionine by phenylalanine in the axial position of the type 1 Cu +/2+ center, which upshifts the center's potential.