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 o f cationic artificial lipids on solid substrates, The thermal stability of [2Fe-2S] clusters in ferredoxin in the films was much more improved in comp arison with that of the clusters in ferredoxin dissolved in aqueous solutio n. The diffusion coefficient, D, and the heterogeneous electron-transfer ra te constant, k degrees', at 25 degrees C for ferredoxin embedded in lipid f ilms of dioctadecyldimethylammonium bromide (2C(18)N(+)Br(-)) on a graphite electrode were determined to be 4 (+/-2) x 10(-10) cm(2) s(-1) and 1.4 (+/ -0.2) x 10(-4) cm s(-1), respectively, by analysis of the experimentally ob tained and simulated cyclic voltammograms for the modified electrode. Compa red to the electrode reaction of ferredoxin in an aqueous solution, the ele ctrode reaction of ferredoxin embedded in a lipid film of 2C(18)N(+)Br(-) p ossesses the following points: (i) the redox potential of the embedded ferr edoxin is somewhat (similar to 100 mV) positive with respect to that in an aqueous solution; (ii) the k degrees' value obtained in this study is simil ar to 1 order of magnitude smaller than that obtained from the direct elect ron-transfer reactions of ferredoxin in a solution at a graphite electrode in the presence of multivalent cations such as Mg2+, Ca2+, Or Cr(NH3)(6)(3) or at an indium oxide electrode modified with poly(L-lysine) or aminosila ne; (iii) the D value for the embedded ferredoxin in this study is similar to 3 orders of magnitude smaller than that obtained for ferredoxin dissolve d in an aqueous solution.