Mediated electron transfer diffusion

As an alternative to DET, small, artificial substrate/co-substrate electroactive molecules (mediators) can be used to shuttle electrons between the enzyme and the electrode (Figure 5.3b). This involves a process in which the enzyme takes part in the first redox reaction with the substrate and is re-oxidized or reduced by the mediator which in turn is regenerated, through a combination of physical diffusion and self-exchange, at the electrode surface. The mediator circulates continuously between the enzyme and the electrode, cycled between its oxidized and reduced forms, producing current. This process is known as mediated electron transfer (MET). 

Marcus theory deals with the first-order electron transfer rate constant for a donor-acceptor system presuming fixed separation. However, there are often systems that do not undergo Marcus behavior and have to be treated using the Rehm-Weller formalism.Here, the second-order diffusion-mediated electron transfer rates are calculated using Eq. 4, where AG =free energy of activation for... 

It was found that some small redox-active chemicals (termed electron shuttles) can diffuse across the cell membrane and serve as electron carriers to assist the electron transfer from the bacteria to the electrode. The electron shuttle mediated electron transfer process usually contains three steps, i.e., being reduced by cells (the electron shuttle is converted to a reductive state). 

Chronoabsorptometry has also been applied to studying reactions on surfaces rather than in solution. In a recent report, not only was the Dapp for electron diffusion during oxidation of a layer of [Os(bpy)2(bpy )] ", where bp/ = 4,4 -(C02H)bpy reported, but also the rate of the mediated electron transfer from [Ru(bpy)3] + in solution to the Os(II) on the surface [252]. 

Unlike Os(bpy)32+, substution-labile M(bpy)3 + complexes can be exchanged into zeolite Y. For example, the Co I complex, which is nearly identical in size to the Os complex, can dissociate and then re-assemble inside the 13 A diameter supercages. Consequently the maximum loading of Co(bpy)32+ is about 2 X 10 4 moles/gram in zeolite Y, whereas the maximum for the Os complex (on the external surface of the particles) is 7 x lO moles/gram. Figure 2 dso shows the electrochemistry of Os(bpy)32+/Co(bpy)32+/zeolite Y electrodes. In this case the surface Os couple appears to be unable to mediate electron transfer from the Co complex entrained in the zeolite. Since the size of the Co2+/3+ and Os2+/3+ waves are comparable in these experiments, it is likely that only the Co(bpy)33+/2+ molecules on the zeolite external surface can be oxidized and reduced, and that the contribution of bulk charge transport diffusion to the current, on this timescale, is minimal. 

Typically, the addition of redox mediators is used as a way to shuttle electrons from the active site to electrode surfaces [13]. This mechanism is referred to as mediated electron transfer (MET). Redox mediators are usually small, mobile molecules containing redox moieties that assist in transferring electrons between redox enzymes and electrodes by diffusing in and out of the enzyme active site. Despite helping overcome the tunnehng distance between the active site and electrode surface, the use of mediators also has certain disadvantages these include high costs, potential toxicity, and instability of the systems, since mediators can diffuse over time. 

When discussing the transfer of electrons from the enzyme active site to the electrode surface, thus generating catalytic current, there are two types of electron transfer mechanisms mediated electron transfer (MET) and direct electron transfer (DET) [13]. Most oxidoieductase enzymes that have been commonly used in BFC development are unable to promote the transfer of electrons themselves because of the long electron transfer distance between the enzyme active site and the electrode surface as a result, DET is slow. In such a case, a redox-active compound is incorporated to allow for MET. In this approach, a small molecule or redox-active polymer participates directly in the catalytic reaction by reacting with the enzyme or its cofactor to become oxidized or reduced and diffuses to the electrode surface, where rapid electron transfer takes place [14]. Frequently, this redox molecule is a diffusible coenzyme or cofactor for the enzyme. Characteristic requirements for mediator species include stability and selectivity of both the oxidized and reduced forms of the species. The redox chemistry for the chosen mediator is to be reversible and with minimal overpotential [15]. 

Thus, to realize the full potential of BECs, investigators are now eonsidering mediated electron transfer (MET) to inerease the eleetronie assoeiation between the total enzyme on the eleetrode and, henee, the power density. By using a redox mediator to shuttle eleetrons to and from eleetrode to enzyme, it is possible to exploit the entire three-dimensional eleetrode area. These mediators, whether diffusion-based or wired [30], therefore eapitafize on a catalyst layer that is much thicker (100 pm) than the monolayers in DET. Although MET has been shown to produce power densities several orders of magnitude higher than DET, the presence of an added electron transfer step, however, introduces another avenue for possible ineffieieney. 

The oxidation or reduction of a substrate suffering from sluggish electron transfer kinetics at the electrode surface is mediated by a redox system that can exchange electrons rapidly with the electrode and the substrate. The situation is clear when the half-wave potential of the mediator is equal to or more positive than that of the substrate (for oxidations, and vice versa for reductions). The mediated reaction path is favored over direct electrochemistry of the substrate at the electrode because, by the diffusion/reaction layer of the redox mediator, the electron transfer step takes place in a three-dimensional reaction zone rather than at the surface Mediation can also occur when the half-wave potential of the mediator is on the thermodynamically less favorable side, in cases where the redox equilibrium between mediator and substrate is disturbed by an irreversible follow-up reaction of the latter. The requirement of sufficiently fast electron transfer reactions of the mediator is usually fulfilled by such revemible redox couples PjQ in which bond and solvate... 

The large size of redox enzymes means that diffusion to an electrode surface will be prohibitively slow, and, for enzyme in solution, an electrochemical response is usually only observed if small, soluble electron transfer mediator molecules are added. In this chapter, discussion is limited to examples in which the enzyme of interest is attached to the electrode surface. Electrochemical experiments on enzymes can be very simple, involving direct adsorption of the protein onto a carbon or modified metal surface from dilute solution. Protein film voltammetry, a method in which a film of enzyme in direct... 

MnP is the most commonly widespread of the class II peroxidases [72, 73], It catalyzes a PLC -dependent oxidation of Mn2+ to Mn3+. The catalytic cycle is initiated by binding of H2O2 or an organic peroxide to the native ferric enzyme and formation of an iron-peroxide complex the Mn3+ ions finally produced after subsequent electron transfers are stabilized via chelation with organic acids like oxalate, malonate, malate, tartrate or lactate [74], The chelates of Mn3+ with carboxylic acids cause one-electron oxidation of various substrates thus, chelates and carboxylic acids can react with each other to form alkyl radicals, which after several reactions result in the production of other radicals. These final radicals are the source of autocataly tic ally produced peroxides and are used by MnP in the absence of H2O2. The versatile oxidative capacity of MnP is apparently due to the chelated Mn3+ ions, which act as diffusible redox-mediator and attacking, non-specifically, phenolic compounds such as biopolymers, milled wood, humic substances and several xenobiotics [72, 75, 76]. 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

An additional condition may be imposed, even when a cofactor-independent enzyme is used, if a mediator molecule is involved in the electron transfer process, as is often the case with oxidases. Laccases, for example, may employ small-molecule diffusible mediator compounds in their redox cycle to shuttle electrons between the redox center of the enzyme and the substrate or electrode (Scheme 3.1) [1, 2]. Similarly, certain dehydrogenases utiHze pyrroloquinoline quinone. In biocatalytic systems, mediators based on metal complexes are often used. 

Large molecules (e.g., biological macromolecules) diffuse slowly, so that the rate of direct electrolysis is low the electroactive center may be localized in the molecule in such a way that electron transfer from an electrode is difficult, and some biologically active molecules lose their activity on contact with an electrode. These difficulties may be overcome by using a mediator. 

These reconstitution experiments supported the model for electron transfer shown in figure 14.8. In this model the complexes do not bind to each other directly. Instead, movement of electrons from complexes I and II to complex III is mediated by diffusion of UQH2 from one complex to the other within the phospholipid bilayer. Similarly, electrons move from complex III to complex IV by the diffusion of reduced cytochrome c along the surface of the membrane. Remember that cytochrome c differs from the other cytochromes in being a water-soluble protein. It is attached loosely to the membrane surface by electrostatic interactions. 

DNA mediated photoelectron transfer reactions have been demonstrated60 . Binding to DNA assists the electron transfer between the metal-centered donor-acceptor pairs. The increase in rate in the presence of DNA illustrates that reactions at a macromolecular surface may be faster than those in bulk homogeneous phase. These systems can provide models for the diffusion of molecules bound on biological macromolecular surfaces, for protein diffusion along DNA helices, and in considering the effect of medium, orientation and diffusion on electron transfer on macromolecular surfaces. 

---