Unmediated electron transfer

Usual catalytic situation where E iSjP) E A B). First wave corresponds to mediated electron transfer, the second to a combination of both mediated and direct unmediated electron transfer. The general case is solved via numerical resolution of the differential equations at the top of the table. 

Both ellipsometry and cyclic voltammetry were used by Szucs etal to investigate both the adsorption and the mediated and unmediated electron-transfer processes of cytochrome c on gold. The adsorption of cytochrome c was studied by ellipsometry on gold electrodes that had not been treated with surface modifiers. The measurements were carried out at 400 mV (vs. NHE), which was approximately equivalent to the open-circuit potential of the system in the absence of protein in the electrolyte. An irreversibly adsorbed layer formed that completely covered the electrode. They found the value of the calculated thickness of the adsorbed layer on bare gold to be about half that expected if cytochrome c has been in its native form, which suggested that significant unfolding occurred upon adsorption. 

Szucs et alP studied both the adsorption and the mediated and unmediated electron transfer from cytochrome c on a gold electrode using ellipsometry and cyclic voltammetry. They found that the protein formed an irreversibly adsorbed layer that completely covered the electrode. The adsorbed cytochrome c was able to mediate the reduction of cytochrome c in solution via electron transfer through the unfolded protein layer. They did not observe oxidation of cytochrome c in the potassium phosphate... 

Although direct electron transfer has captured the interest of electrochemists for some time, recently it has been observed at a greater variety of surfaces. Extensive studies have been made on mediated and unmediated electron transfer, and these have been discussed in reviews. The present section will focus on those studies which have contributed to the understanding of the interfacial and conformational behavior of cytochrome c at electrode surfaces and on how this behavior contributes to the redox electron-transfer process. 

For the laccase from T. versicolor described herein, in situ XAS measurements were performed at both the Cu k-edge and the Os La-edge for each of the three Os-based redox mediators presented (Figure 15.2). The Os La-edge will be discussed briefly, followed by an in-depth examination of the Cu k-edge analyses for both MET and unmediated electron transfer (uMET). (For the purpose of discussion herein, uMET is considered to be fundamentally different from DET, because of the absence of any specific preferential orientation that ensures efficient DET.)... 

The application of direct electrochemistry of small redox proteins is not restricted to cytochrome c. For example, the hydroxylation of aromatic compounds was possible by promoted electron transfer from p-cresol methylhydroxylase (a monooxygenase from Pseudomonas putida) to a modified gold electrode [87] via the blue copper protein azurin. All these results prove that well-oriented non-covalent binding of redox proteins on appropriate electrode surfaces increases the probability of fast electron transfer, a prerequisite for unmediated biosensors. Although direct electron-transfer reactions based on small redox proteins and modified electrode surfaces are not extensively used in amperometric biosensors, the understanding of possible electron-transfer mechanisms is important for systems with proteins bearing catalytic activity. 

Direct unmediated electrical communication between enzyme redox centers and the electrode by molecular wiring has been attracting attention recently/ In this technique the natural ability of polyanionic oxidoreductases to form electrostatic complexes with redox proteins for effective electron transfer can be applied in biosensor technology. However, in place of the redox protein, a polycationic redox macromolecule is used. The result is a three-dimensional network of an electrostatic complex of the enzyme and the redox polymer. In this system membranes are not needed, nor is unique orientation of the enzymes necessary. This is... 

If the presence of a soluble mediator shuttling the electrons between the electrode and the enzyme is to be avoided, other modes of wiring [15] the enzyme to the electrode ought to be sought after. One may think of direct electron transfer between the electrode and the enzyme at the adsorbed state. It has indeed been shown that small redox proteins, such as cytochrome c or ferredoxins may show an unmediated reversible electrochemical response when the electrode surface is adequately prepared [16-19]. There have been few reports of direct electron transfer with redox enzymes [20,21]. With flavoen-zymes, the observed signals are likely to be those of the free flavin deriving from the denaturation of the enzyme [21]. The high probability of denaturation of the adsorbed enzyme prevents the viability of this mode of electron transport in most cases. 

Let us now consider the V 1 limit. In this case the expression for the modified electrode rate constant k E is given by Eqn. 53. Again this expression is rather complex. There are two terms in the numerator on the rhs of Eqn. 53. The first of these describes mediated electron transfer and the second, the kinetics for direct reaction at the electrode surface. The denominator on the rhs of Eqn. 53 describes the concentration polarization of 5 in the layer, where it may be consumed at the electrode surface by direct unmediated reaction represented by the heterogeneous rate constant ks, or in a homogeneous reaction layer of dimension Xq. Let us now assume that the direct unmediated process can be neglected. If this is true, then we simplify Eqn. 53 as follows ... 

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