Immobilization rate-determining step

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

Immobilized enzyme systems can be differentiated according to mode of immobilization, carrier properties, and rate-determining step. 

Rate-determining step. In immobilized (bio)catalyst systems, especially in porous particles, the catalytic step is only one of several rate processes in sequence ... 

The conversion of substrate to product also requires immobilized water molecules within the active site and an appropriate charge environment to facilitate the transfer of electrons and hydrogens to pyruvate. After pyruvate is bound, its reduction to lactate involves addition of two electrons, one proton, and one hydride ion (H ). NADH provides the electrons and hydride ion the proton comes from the imidazole ring of His-193. The rate-determining step in the covalent chemistry taking place in the catalytic vacuole is that of hydride transfer, which occurs at a rate of approximately 750 s 1 at room temperature for bovine A4-LDH (Dunn et al., 1991). 

The critical film thickness for rupture is of the order of 50 A. If the interaction time of the drops is too short to reach the critical film thickness, the drops will not coalesce. The drainage of the film is the rate-determining step in coalescence of deformable drops in polymer blends. Various models have been proposed to describe the film drainage. One model assumes fully mobile interfaces, another model assumes immobile interfaces, and a third model assumes partially mobile interfaces. The mobility of the interfaces is strongly dependent on the presence of impurities, such as surfactants. Surfactants reduce the mobility of the interfaces due to interfacial tension gradients [315]. 

Medium-to-high conversion The diffusion mechanism of the propagating radicals becomes complex after the onset of the gel effect. Large chains become immobile however, the chain ends may move by reptation or reaction diffusion. Monomer and short species may still be highly mobile in the polymerizing system. Translational center-of-mass diffusion may become the rate-determining step for radical-radical termination. 

On the other hand, the second constant (k2) representing the rate of transition from the second active form of GOD to an inactivated form is hi er for the PAAc-GOD (1.71x10 min l) than for the PAAc-PEO-GOD (2.09x10" min"l). The results on the rate of thermoinactivation suggest that the second transition step is the rate determining step for the denaturation of the immobilized enzyme. 

Table 2 lists the two parameters n and Qx necessary to describe the model as determined with columns differing by the density of immobilized polyclonal antibody. As previously described, from the variation of the column capacity one can evaluate the contribution to the transport to the binding sites (I/nmt = 0.040) and calculate the effective adsorption rate constant ka. The results agree with those obtained from frontal analysis. The value of the apparent adsorption rate constant k is close to the value of Aa for experiments carried out both at high flow rates and with an immunoadsorbent column of low capacity 22). In this case, the rate-controlling step is the biospecific interaction. 

The general principles of metal uptake by biota from soil systems are treated in chapters 7 to 10 of the book. The same rules govern radionuclide uptake by biological systems. Uptake and assimilation depend on the chemical and biological properties of the element. The rate-limiting step for the uptake of many radionuclides, those strongly immobilized by soil constituents, is the movement to the interface between the soil solution and the biological membrane. If absorption is more rapid than the movement of the solute to the interface, uptake becomes diffusion limited. The adsorption properties of the soil are therefore determinant. 

The liberated peroxide or NADH species can be readily detected at relatively modest potentials (0.6-0.8 V against Ag/AgCl, depending upon the working electrode material). The overall reaction involves many steps and the actual response is determined by the rate limiting step in the overall reaction scheme. This involves diffusion of the substrate molecule towards the surface and its reaction with the immobilized enzyme, which in turn is regenerated into its native form by reaction with the (oxygen or NAD" ) cofactor. 

Solution. The decomposition of the primary complex ES to the free enzyme E and the product P is assumed to be the rate-determining (slow) step. The expression below is valid for both homogeneous (where the enzyme is used in the native or soluble form) and for immobilized enzyme reactions. The reaction rate v is given by ... 

X 10 g/mL (1 ppb). A key point for constructing high-performance OP sensors in bienzyme systems may be to suitably control the ratio of the catalytic activity of AChE and ChOx in the sensor. In the present case, the catalytic activity of ChOx should be higher than that of AChE fhe overall rate of the reactions is determined by the rate of the AChE-catalyzed reaction because OP compoimds disturb this step. Therefore, excess amoimts of ChOx usually are mixed with AChE, and the ChOx-AChE mixture is immobilized on the surface of the electrode to make the AChE-catalyzed reaction a rate-limiting step. In this situation, the overall reaction rate can be determined by the rate of the ChE-catalyzed reaction because the subsequent electrochemical oxidation of H2O2 is suf-ficienfly fast. On the other hand, in our sensors, ChE and ChOx were immobilized separately, layer by layer, for optimizafion of the amounts of enzymes. 

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