Thermal stability and electrode reaction of chlorella ferredoxin embedded in artificial lipid bilayer membrane films on a graphite electrode

Chlorella ferredoxin was incorporated from aqueous solution in cast films of cationic artificial lipids on solid substrates. The thermal stability of [2Fe-2S] clusters in ferredoxin in the films was much more improved in comparison with that of the clusters in ferredoxin dissolved in aqueous solution. The diffusion coefficient, D, and the heterogeneous electron-transfer rate constant, k°', at 25 °C for ferredoxin embedded in lipid films of dioctadecyldimethylammonium bromide (2C₁₈N⁺Br^-) on a graphite electrode were determined to be 4 (±2) x 10^-10 cm² s^-1 and 1.4 (±0.2) x 10^-4 cm s^-1, respectively, by analysis of the experimentally obtained and simulated cyclic voltammograms for the modified electrode. Compared to the electrode reaction of ferredoxin in an aqueous solution, the electrode reaction of ferredoxin embedded in a lipid film of 2C₁₈N⁺Br^- possesses the following points: (i) the redox potential of the embedded ferredoxin is somewhat (~100 mV) positive with respect to that in an aqueous solution; (ii) the k°' value obtained in this study is ~1 order of magnitude smaller than that obtained from the direct electron-transfer reactions of ferredoxin in a solution at a graphite electrode in the presence of multivalent cations such as Mg²⁺, Ca²⁺, or Cr(NH₃)6³⁺ or at an indium oxide electrode modified with poly(L-lysine) or aminosilane; (iii) the D value for the embedded ferredoxin in this study is ~3 orders of magnitude smaller than that obtained for ferredoxin dissolved in an aqueous solution.