The Effect of Ionic Strength on the Electron-Transfer Rate of Surface Immobilized Cytochrome c

Abstract

Horse heart cytochrome c was immobilized on four different self-assembled monolayer (SAM) films. The electron tunneling kinetics were studied in the different assemblies as a function of the ionic strength of the buffer solution using cyclic voltammetry. When cytochrome c is electrostatically immobilized, the standard electron exchange rate constant k0 decreases with the increase of the solution's ionic strength. In contrast, the protein covalently attached or ligated has a rate constant independent of the ionic strength. The inhomogeneity of electrostatically immobilized cytochrome c increases with the increase of the solution's ionic strength whereas that of the covalently attached protein is independent of the ionic strength. A comparison of these different electron-transfer behaviors suggests that the thermodynamically stable geometry of cytochrome c in the electrostatic assemblies is also an electron transfer favorable one. It suggests that the surface charges of cytochrome c are capable of guiding it into geometries in which its front surface faces the electron-transfer partner. The inhomogeneity observed in this study indicates that a distribution of cytochrome c orientations and thus a distribution of electron transfer rate constants exists.

Characterization of Nanoporous Gold Electrodes for Bioelectrochemical Applications

“…A distribution of formal potentials may occur and, as described below, a distribution of heterogeneous electron transfer rate constants (k et ) may be observed. [50][51][52] No changes in the voltammetric response with continuous potential cycling (30 scans) in aqueous buffer (not shown) were observed, demonstrating the stability of the adsorbed protein.…”

Direct Electrochemistry of Redox Enzymes as a Tool for Mechanistic Studies

“…The effect of the immobilization on the properties of redox proteins has been discussed, often with emphasis on cytochromes adsorbed onto modified gold electrodes [275][276][277] 202 Noncatalytic data was also reported for a laccase covalently attached to a monolayer of thiol self-assembled on a gold electrode, 77 for Photosystem-I solubilized in detergent and adsorbed on modified gold, 286 and for a number of enzymes embedded into alternate layers of charged polymers or layers of surfactants. 258 This occurred for a bacterial reaction center, 287 photosystems I 288 and II, 289 bovine milk xanthine oxidase, 290 Rhodobacter capsulatus xanthine dehydrogenase 183 and DMSO reductase, [291][292][293] For adsorbed enzymes, a preliminary diagnosis of health can come from noncatalytic electrochemical studies themselves, since the conformity of the peaks to ideal predictions (section 2.1) reports on the homogeneity of reduction potentials, and the values of the measured reduction potentials can be compared to those measured under potentiometric, equilibrium titrations.…”

Electron tunneling through Pseudomonas aeruginosa azurins on SAM gold electrodes

“…There are fewer charged groups on the azurin surface than on cyt c, a protein where the heme/electrode ET rate constant decreases with increasing ionic strength. 7,8,21 The rate/ionic strength profile for cyt c can be interpreted in terms of binding-site electrostatic interactions screened by the added salt that would increase the distance between protein lysine residues and the SAM surface. 21,26…”