Direct electrochemistry of thioredoxins and glutathione at a lipid bilayer-modified electrode

By using direct electrochemical analysis we have established that the reduction of Escherichia coli thioredoxin (EcT), T4 thioredoxin (T4T), and glutathione (GSSG) occurs at a self-assembled lipid bilayer-modified gold electrode via two separate one-electron processes. The first electron transfer has half-wave potentials of -0.05 +/- 0.01, -0.07 +/- 0.01, and -0.06 +/- 0.01 V, whereas the second one has values of -0.48 +/- 0.01, -0.39 +/- 0.01, and -0.45 +/- 0.01 V, for EcT, T4T, and GSSG, respectively. The scan-rate dependence of the cyclic voltammetry indicates, for both waves, that the process of electron transfer is dominated by a bulk diffusion of free species to and from the electrode, and that strongly adsorbed species do not significantly contribute at the scan rates used. The voltage separation of the peak currents indicates a quasi-reversible electron transfer process with an electrochemical rate constant which is larger for the second (lower potential) electron than for the first one. Using the above half-wave potentials of the one-electron steps, one can calculate a thermodynamic half-wave potential for the two-electron reduction processes. The values of these potentials are -0.265, -0.23, and -0.25 V for EcT, T4T, and GSSG, respectively. These are in excellent agreement with literature values obtained from equilibrium measurements of enzyme-catalyzed reactions involving these species. It is quite clear from these results that lipid bilayer-modified electrodes provide a biocompatible and direct means of efficiently carrying out electrochemical reactions with sulfur-based redox systems, as we have previously shown to be the case with metalloproteins.