Direct electron transfer methods

Investigations of enzyme-catalyzed direct electron transfer introduce the basis for a future generation of electrocatalysts based on enzyme mimics. This avenue could offer new methods of synthesis for nonprecious metal electrocatalysts, based on nano-structured (for example, sol—gel-derived) molecular imprints from a biological catalyst (enzyme) with pronounced and, in some cases, unique electrocatalytic properties. Computational approaches to the study of transition state stabilization by biocatalysts has led to the concept of theozymes . " ... 

A similar but direct electron transfer from the metal to the monomer is operative when alkali and alkaline-earth metals (e.g., sodium) are used as the initiator.169,173,177 In this case, however, initiation is slow relative to propagation because of the low metal surface area available, and this method is used only for special purposes. 1,1-Dipehylethylene, for example, forms a dianion that, for steric reasons, is not capable for further head-to-tail addition of the monomer, but it can be used to initiate the polymerization of other monomers.178... 

Amperometric biosensors based on flavin-containing enzymes have been studied for nearly 30 years. These sensors typically undergo several chemical or electrochemical steps which produce a measurable current that is related to the substrate concentration. In the initial step, the substrate converts the oxidized flavin adenine dinucleotide (FAD) center of the enzyme into its reduced form (FADH2). Because these redox centers are essentially electrically insulated within the enzyme molecule, direct electron transfer to the surface of a conventional electrode does not occur to a substantial degree. The classical" methods (1-4) of indirectly measuring the amount of reduced enzyme, and hence the amount of substrate present, rely on the natural enzymatic reaction ... 

The oxidation potential of the substrate S in Figure 2 is beyond the range accessible by the electrochemical method so that direct electron transfer from S to the anode hardly occurs, and also the high oxidation potential necessary for the direct oxidation of S causes unexpected side reactions involving oxidation of the solvent or supporting electrolyte. However, when a compound Mrxi (a reduced form of M) which may be oxidized at a sufficiently lower potential than S is added to the reaction system, the oxidation of Mied to Mox (an oxidized form of M) will take place prior to the oxidation of S. Provided that Mox is able to oxidize S to product P, the oxi tion of S will be achieved at a potential lower than that necessary for its direct oxidation. Oxidation of S with Mox may be effected in two ways, namely by direct electron transfer (homogeneous electron transfer) from S to Mox in solution or by chemical oxidation of S with Mox. The former system is called a homomediatory system and the latter a heteromediatory (or chemomediatory) system. The compound M is called a mediator or an electron carrier, since M mediates electron transfer between S and the anode. When Mox oxidizes S in solution, Mox is reduced to Mrxi... 

Different charge-compensating cations in zeolite L have been tested for their promotional effect in n-hexane aromatization. Apparently, high basicity of the alkaline and alkaline earth promoter favors n-hexane aromatization. Basicity and selectivity both increase from Li and Cs 331) and from Mg to Ba (22,25). Bezouhanova et al. studied the FTIR bands of linearly adsorbed CO in the range of 2060-2075 cm . One band at 2075 cm", which is also found on unsupported Pt, is attributed to extrazeolite Pt particles, a second band shifts from 2060 cm" for Li to lower wavenumbers with K and Rb 331). Another criterion, used by Larsen and Haller, is the measured rate of competitive hydrogenation of benzene and toluene, which has been found to correlate with the zeolite basicity (25). As described in a previous section, this method had previously been used by Tri el al. to probe for the electron deficiency of Pt particles in acidic zeolites 332). The rate data are analyzed in terms of a Langmuir-Hinshelwood model and the ratio of the adsorption coefficients of toluene and benzene, A, /b, is determined. It was found to decrease from 8.6 for Pt/Si02, and 5.4 for Pt/MgL, to 4.4 for Pt/BaL. As direct electron transfer from the cations to neutral Pt particles is unlikely, an interaction of Pt with the zeolite framework or with... 

A different way of making electrolytic reduction is through electrocatalytic hydrogenation, which is a kind of indirect electrolysis. Protons are reduced and the key intermediate is Me(H) (with Me being platinum, palladium, rhodium, or nickel), and the potential determining step is the formation of this reactive intermediate. Selective reductions may be performed by this method, and the potential used for the formation of Me(H) is often less negative than that required for the direct electron transfer to the reducible substrate. Hence, electrolysis occurs with a lower energy consumption. The selectivity of the reaction may depend on the support for the catalyst. 

The direct electrochemistry of redox proteins has developed significantly in the past few years. Conditions now exist that permit the electrochemistry of all the proteins to be expressed at a range of electrodes, and important information about thermodynamic and kinetic properties of these proteins can be obtained. More recently, direct electron transfer between redox enzymes and electrodes has been achieved due to the more careful control of electrode surfaces. The need for biocompatible surfaces in bioelectrochemistry has stimulated the development of electrode surface engineering techniques, and protein electrochemistry has been reported at conducting polymer electrodes 82) and in membranes 83, 84). Furthermore, combination of direct protein electrochemistry with spectroscopic methods may offer 85) a novel way of investigating structure-function relationships in electron transport proteins. 

Layer-by-layer (LbL) assembly is a unique technique for the fabrication of composite films with precise thickness control at the nanometer scale [111, 112], The method is based on the alternate adsorption of oppositely charged species from their solutions. The attractive feature of this approach is its ability to assemble complex structures from modular components, and integrate them into self-assembling constructions for a wide range of applications. The LbL method has been successfully exploited in the construction of dendrimer biosensors [113,114], The LbL films provide a favorable environment for the intimate contact between the dendrimer and biomolecule (enzymes or proteins), promoting a direct electron transfer between them and the underlying electrodes. 

If the active site of the enzyme is located sufficiently close to the electrode surface electrons can be transferred directly from the enzyme to the electrode as depicted in Figure 5.3a. In the case of an anodic reaction, the electrode replaces the natural co-substrate (such as oxygen) as an electron acceptor. This process is known as direct electron transfer (DFT), often categorized as third-generation enzyme electrodes in the biosensor literature, and is the most elegant and simplest method of bioelectrocatalysis between an enzyme active site and an electrode. 

No direct electron transfer between melanin particles suspended in aqueous buffers and electrodes has been observed. This allowed the use of the polarographic method in monitoring the concentration changes of TF and Fe mediators reacting with d,L-dopa melanin (.210). Moreover,... 

Hemoproteins are a broad class of redox-proteins that act as cofactors, e.g. cytochrome c, or as biocatalysts, e.g. peroxidases. Direct ET between peroxidases such as horseradish peroxidase, lactoperoxidase," or chloropcroxidasc"" and electrode surfaces, mainly carbonaceous materials, were extensively studied. The mechanistic aspects related with the immobilized peroxidases on electrode surfaces and their utilization in developing biosensor devices were reviewed in detail. The direct electrical contact of peroxidases with electrodes was attributed to the location of the heme site at the exterior of the protein that yields close contact with the electrode surface even though the biocatalyst is randomly deposited on the electrode. For example, it was reported " that non-oriented randomly deposited horseradish peroxidase on a graphite electrode resulted in 40-50% of the adsorbed biocatalyst in an electrically contacted configuration. For other hemoproteins such as cytochrome c it was found that the surface modification of the electrodes with promoter units such as pyridine units induced the binding of the hemoproteins in an orientation that facilitated direct electron transfer. By this method, the promoter sites induce a binding-ET process-desorption mechanism at the modified electrode. Alternatively, the site-specific covalent attachment of hemoproteins such as cytochrome c resulted in the orientation of the protein on the electrode surfaces and direct ET communication. ... 

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