Direct electron transfer limitations

Oxidation—Reduction. Redox or oxidation—reduction reactions are often governed by the hard—soft base rule. For example, a metal in a low oxidation state (relatively soft) can be oxidized more easily if surrounded by hard ligands or a hard solvent. Metals tend toward hard-acid behavior on oxidation. Redox rates are often limited by substitution rates of the reactant so that direct electron transfer can occur (16). If substitution is very slow, an outer sphere or tunneling reaction may occur. One-electron transfers are normally favored over multielectron processes, especially when three or more species must aggregate prior to reaction. However, oxidative addition... 

The electron-transfer rate between large redox protein and electrode surface is usually prohibitively slow, which is the major barricade of the electrochemical system. The way to achieve efficient electrical communication between redox protein and electrode has been among the most challenging objects in the field of bioelectrochemistry. In summary, two ways have been proposed. One is based on the so-called electrochemical mediators, both natural enzyme substrates and products, and artificial redox mediators, mostly dye molecules and conducted polymers. The other approach is based on the direct electron transfer of protein. With its inherited simplicity in either theoretical calculations or practical applications, the latter has received far greater interest despite its limited applications at the present stage. 

First examples of the amperometric detection of H202 accomplished in such a range were based on the use of an enzyme, namely horseradish peroxidase (HRP), a prototypical hemeprotein peroxidase, which catalyses the reduction of H202 and due to its peculiar structure, allows direct electron transfer between its active site and the electrode surface at low applied potential [14 17]. This approach, although it shows good sensitivity and accuracy, suffers from some important shortcomings such as low stability and the limited binding of HRP to solid surfaces. 

The redox chemistry of dioxygen and its reduction products is heavily dependent on mechanistic pathway, substrate, and solution acidity. For those circumstances that are limited by direct electron transfer, the redox mechanisms... 

A potentially promising route to learn more about the complex redox chemistry of hydrogenase is to look at the direct electron transfer between the enzyme and an electrode. A study of this phenomenon may also be relevant to a possible coupling of solar cells with hydrogenasecatalyzed H2 production. Results from this line of research have thus far been very limited. A response was obtained in differential-pulse polarography on the dropping mercury electrode modified with polylysine (cf. van Dijk et al., 1985). Attempts to use... 

On the basis of the above-mentioned experimental evidence, the Cu /Cu redox exehange in thiacrown complexes can be considered as an example of gated or directional electron transfer [64], as the rate of the eonformational change can limit the reaetion in one direction only. 

Quenching of Ru(blpy)3 by Tl in aqueous solutions also occurs by direct electron transfer (242). Photolysis of Ru(bipy)3 in solutions containing Tl " " produced Ru(bipy)33 with the limiting quantum yield = 2.0 0.4. Stern-Volmer constants for the quenching of the emission from Ru (blpy) 3 and for the production of Ru(bipy)3 " were the same within... 

By analyzing the dynamic behaviour of GOD ring-disk electrodes, Albery and Bartlett (1985) have shown that the sensor response to low glucose concentration is limited by substrate diffusion through the covering membrane. In contrast to Kulys (1986) they postulated a direct electron transfer between enzyme and electrode surface on the grounds that (i) TCNQ is not soluble enough, and (ii) the reoxidation of GOD is too slow to explain the measured current as a result of dissolved TCNQ-. 

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