Promotion of the electrochemical response of some negatively charged proteins at an edge-plane graphite electrode by various redox inert cations: an electrochemical manifestation of Frumkin adsorption

Abstract

Promotion of the cyclic voltammetric response of several small negatively charged metalloproteins P at an edge-plane graphite (EPG) electrode E by various inorganic cations M at pH 8 has been interpreted in terms of two equilibria: P + M K → PM E + PM K M* → EPM The first equilibrium, where a 1 : 1 protein-cation complex PM is formed, occurs in the bulk, while the second takes place at the electrode surface. The value of the equilibrium constant K depends on the cation charge and the charge on the cation binding site of the protein. A simple model for understanding the interactions in the inorganic ion pairs in an aqueous medium has been developed. The model is extended to compare the association of the inorganic ions with various proteins in an aqueous medium and is used to estimate K. The adsorption of the protein-metal ion complex on the electrode surface (the second equilibrium) is assumed to be weak (i.e. the equilibrium constant K M* is small) and reversible. It is found that only EPM, the PM species adsorbed on the electrode surface, is electroactive. The experimental consequences of the two equilibria are discussed. The adsorption pattern is found to obey the Frumkin isotherm. The standard Gibbs energies of adsorption ΔG ads 0 have been calculated specifically for 2[4Fe-4S] ferredoxin in the presence of various cations. The magnitude of the ΔG ads 0 values, which are found to lie within the range −13.4 to 22.2 kJ mol −1 for 2[4Fe-4S] ferredoxin, suggest physical adsorption of the protein. The ΔG ads 0 values for the various cations are found to vary linearly with z 2/ r where z is the charge on the cation r is its ionic radius. It is concluded that the role of the cation is to induce a reversible weak adsorption of a negatively charged protein suitable for electron transfer on an EPG surface at pH 8. Speculations are made on the nature of the adsorption.