Fig. 2. Current–time traces for electrochemical experiments on apo-MoFe protein (a); wild type MoFe protein (b); β-98^Tyr→His variant (c); and β-99^Phe→His variant (d), each in the presence of Eu–L mediator mixture. Potential steps were applied at the times indicated, and throughout the experiment, N₂-flushed buffer (Tris–HCl, pH 7.4) flowed through the cell.