Catalytic Currents types

Reduction currents, most frequently those of hydrogen ions, that are observed at more positive potentials than currents observed in the absence of the catalyst. The facilitation of the reduction is caused by the catalyst - usually a basic electroin-active compound - which is often adsorbed at the electrode surface. For both types of catalytic currents most information has been obtained for electrolyses using the dropping mercury electrode. 

Catalytic currents of the first type sometimes called regeneration currents show an increase with increasing concentration of the oxidant. At low concentration of the oxidant, increase of the limiting current as a function of concentration of the oxidant, is first nonlinear, but becomes linear above a certain concentration of the oxidant. As oxidizing agents hydrogen peroxide, chlorate, UO, and hydroxylamine were used. 

The kinetic behaviour of electrochemical biosensors is most commonly characterized using the dependence of the steady-state amperometric current on the substrate concentration. This type of analysis has some limitations because it does not allow for a decoupling of the enzyme-mediator and enzyme-substrate reaction rates. The additional information required to complete the kinetic analysis can be extracted either from the potential dependence of the steady-state catalytic current or from the shift of the halfwave potential with substrate concentration [154]. Saveant and co-workers [155] have presented the theoretical analysis of an electrocatalytic system... 

Flavocytochrome c3 (EC 1.3.99.1) isolated from Shewanella frigidimarina is a unique fumarate reductase of 63.8 kDa MW in a single subunit composed of two domains. The active site is located in the flavin domain. The heme domain contains four c-type hemes, each with a bis-His axial ligation. It has been proposed that this domain is similar to cytochrome c3 from Desulfovibrio desulfuricans. On pyrolytic graphite (edge) electrodes in the presence of polymyxin the single redox centres were examined. Fumarate addition is followed by a catalytic current [108]. 

In some cases the kinetic currents are small (sometimes 10% or even less) even at their maximum height when compared with waves of the equimolar solutions that are diffusion-controlled. This type of behaviour has been observed in particular in systems in which the waves of are obscured by the supporting electrolyte. However, not all kinetic currents are so small and whilst abnormally small currents may indicate kinetic currents, currents of the normal height do not allow us to exclude the role of chemical reactions. (Some catalytic currents are abnormally high.)... 

Sulfur trioxide is probably a worse pollutant than sulfur dioxide, because SO3 is the acid anhydride of strong, corrosive sulfuric acid. Sulfur trioxide reacts with water vapor in the air, as well as in auto exhausts, to form sulfuric acid droplets. This problem must be overcome if the current type of catalytic converter is to see continued use. These same catalysts also suffer from the problem of being poisoned —that is, made inactive—by lead. Leaded fuels contain tetraethyl lead, Pb(C2H5)4, and tetramethyl lead, Pb(CH3)4. Such fuels are not suitable for automohiles equipped with catalytic converters and are excluded hy U. S. law from use in such cars. 

Figure 4-6. (A) A close-up view of the active site of yeast cytochrome c peroxidase showing the residues in the distal pocket at which hydrogen peroxide is reduced to water. Overlaid on the structure of the wild type enzyme are the positions of residues in the W51F mutant (tryptophan is replaced by phenylalanine). (B) Voltammograms of a film of wild type CcP on a PGE electrode, obtained in the absence and presence of H2O2 at ice temperature, pH 5.0. The electrode is rotating at 200 rpm, but the catalytic current in this case continues to increase as the rotation rate is increased therefore under these conditions the electrocatalysis is diffusion controlled and few facts are revealed about the enzyme s chemistry. For the W51F mutant, the signal due to the reversible two-electron couple and the catalytic wave are both shifted >100 mV more positive in potential compared to the wild-type enzyme. Reproduced from ref. 46 and 47 with permission.

Substitutions at the active site specifically R392A, M153A and Q384N (Figure 5.26B-D) give rise to voltammetry that is distinctly different from the wild-type enzyme (Figure 5.26A) both in waveform and in nitrate concentration dependence. The Michaelis constants determined from solution assays vary considerably (from 400 pM for the WT enzyme to 66 mM for the R392A variant) and this is reflected in the nitrate concentration dependence of the voltammetry. Another outcome of this study was the connection made between the potential dependence of the catalytic current and the observation that the rate of nitrate reduction in solution assays has been found to increase as the concentration of reductant decreases. 

Fig. 2 Cyclic voltammetric analysis of enzymatic catalytic currents obtained with a reaction scheme of the type shown in Sch. 1. Variation of the normalized peak or plateau current (peak current when there is a peak, plateau current, when the peak has vanished into a plateau) with the two kinetic parameters A and a (full lines). Dotted lines asymptotic behavior for A —> 00 (the wave is then plateau-shaped).

A number of other features follow from the relatively large size of the diffusion zone when compared to the electrode radius. One of these is quite significant to biosensors based on enzyme systems. The large diffusion zone radidly dilutes the products of the electrode reaction, thus catalytic mechanisms of the type shown in Table 8.2, mechanisms 3-5, are not perceived at microelectrodes, unless the rate constant for the reaction is fast. It can be shown that for a reversible electrode reaction, the ratio of the catalytic current (i kc) to the faradaic current obtained in the absence of the homogeneous reaction is given by... 

This immunosensor-type assay uses ferrocene methanol as co-substrate (redox mediator) of glucose oxidase (GOD). The various steps of the assay, shown in Fig. 8.20, are as follows (1) Covalent immobilization of protein A on graphite-polystyrene screen-printed electrodes (SPEs) (2) Addition of the rabbit IgG to be quantified which is captured specifically by protein A (3) Addition of a biotinylated goat anti-rabbit antibody (4) Addition of avidin-GOD conjugate (5) Addition of glucose and ferrocene methanol (6) Measurement of catalytic current by flow injection immunoassay. The voltammetric current corresponds to the one-electron oxidation of the ferrocenyl group to ferricinium. The electrode can subsequently be regenerated up to 30 times. The feasibility of the assay has been demonstrated for the case of monoclonal mouse anti-human prolactin (PL) with a detection limit of 0.02 pg mL [90]. 

Catalytic Currents. For analytical studies, catalytic currents, of which two types are known, are of great interest. In the first case the product of the cell reaction is returned, by a chemical reaction, to the initial state of the analyte. As a result, the analyte concentration at the electrode surface is always high, which results in a considerable increase of the limiting current. 

Principally, two types of polarographic currents are described as catalytic currents. In the first type the limiting current of an electroactive substance is increased in the presence of a catalyst. 

Conversely the presence of such a catalytic wave was claimed for some substances containing no sulphur. From the study of the influence of the composition of the buffer solution, of the ionic strength and of the concentration of cobalt as well as of the concentration of the catalytically active substance, it has recently been possible to distinguish at least three different types of catalytic currents. Thus it is possible that the catalytic effects e.g. for uric acid, ) glycerinaldehyde, or dihydroxyacetone, ) have characteristics other than the catalytic waves due to thio-compounds. 

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]. 

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