The dependence of reversible potentials on the form of modification of edge plane pyrolytic graphite electrodes in voltammetric studies on rubredoxin and ferredoxin from Clostridium pasteurianum

Reversible voltammetry of the anionic rubredoxin and ferredoxin proteins at a pyrolytic graphite edge plane electrode can be observed either by ex situ modification of the electrode with multivalent cations (poly(L-lysine), Cr(III) complexes) or by addition of these cations to the solution. However, poly( L-lysine) binds irreversibly to pyrolytic graphite and voltannmograms obtained at an ex situ form of this modified electrode provide convenient reference points for estimation of the thermodynamic significance of solution phase interactions between the cationic modifiers and the proteins. Results obtained show that significant shifts in reversible potential are induced by the presence of free modifier in solution. Examination of the recombinant proteins and certain mutant forms suggests that association of the anionic proteins with cationic modifiers in solution is the primary cause of this shift. The effect of background ionic strength is minor in comparison. The conclusion is that the modifiers added to the solution phase are not thermodynamically innocent. (C) 1999 Elsevier Science S.A. All rights reserved.