Substrate transport intrinsic rate

The intrinsic catalytic properties of enzymes are modified either during immobilization or after they were immobilized [25-27], In heterogeneous catalysis such as is carried out by immobilized enzymes, the rate of reaction is determined not simply by pH, temperature and substrate solution, but by the rates of proton, heat and substrate transport, through the support matrix to the immobilized enzyme. In order to estimate this last phenomenon, we have studied the internal mass transfer limitation both in hexane and in SC C02, with different enzymatic support sizes. 

In summary, ATP hydrolysis by P-gp correlates well with the intrinsic rate of substrate transport. A complete characterization of the interaction of a compound with P-gp is obtained by measuring the ATPase activity as a function of concentration. The rate of intrinsic substrate transport first increases with increasing concentration, reaches a maximum, and decreases again at high concentrations. The rate of intrinsic transport by P-gp depends not only on the substrate concentration but also on its affinity to the transporter substrates with high affinities for P-gp are transported more slowly than those with low affinities. 

In the absence of substrate transport limitations, the substrate concentration is Independent of radial position in the polymer. The rate measured under these circumstances would be an intrinsic reaction rate. The substrate material balance is greatly simplified because Equation 8 no longer needs to be solved. A specific example of this can be found in the literature (16). 

The mathematical model for substrate transport with reaction can be used to devise a general approach to predict reaction rates in catalyst particles. The observed rate is defined in terms of the intrinsic rate and an effectiveness factor... 

There are a number of factors which may influence the activity or selectivity of a polymer-immobilized catalyst. Substrate diffusion is but one. This article has reviewed the mathematical formalism for interpreting reaction rate data. The same approach that has been employed extensively in heterogeneous systems is applicable to polymer-immobilized systems. The formalism requires an understanding of the extent of substrate partitioning, the appropriate intrinsic kinetic expression and a value for the substrate s diffusion coefficient. A simple method for estimating diffusion coefficients was discussed as were general criteria for establishing the presence of substrate transport limitations. Application of these principles should permit one to identify experimental conditions which will result in the intrinsic reaction rate data needed to probe the catalytic properties of immobilized catalysts. 

For a triphasic reaction to work, reactants from a solid phase and two immiscible liquid phases must come together. The rates of reactions conducted under triphasic conditions are therefore very sensitive to mass transport effects. Fast mixing reduces the thickness of the thin, slow moving liquid layer at the surface of the solid (known as the quiet film or Nemst layer), so there is little difference in the concentration between the bulk liquid and the catalyst surface. When the intrinsic reaction rate is so high (or diffusion so slow) that the reaction is mass transport limited, the reaction will occur only at the catalyst surface, and the rate of diffusion into the polymeric matrix becomes irrelevant. Figure 5.17 shows schematic representations of the effect of mixing on the substrate concentration. 

Intrinsic kinetic data can only be measured provided that the overall reaction rate is not limited by mass transport. Only then reahstic parameters can be calculated concerning the influence of catalyst and substrate concentrations (reaction order) as well as the temperature dependency (activation... 

The calculated mass-transfer coefficients used in the program show that the transport resistance for hydrogen is higher than that for the substrate. Therefore, in the following simulation calculations, a high value and a much lower, but nevertheless realistic value of the global mass-transfer coefficient were introduced into Equation 13-26. The aim was to find out whether the reaction rate is mainly determined by mass transfer or by the surface reaction. As can be seen from Figures 13-24 and 13-25, the process is essentially determined by the intrinsic kinetics. 

Catalytic voltammetry reports on the intrinsic properties of the enzyme when the response is free from hmitation by (a) relatively slow rate of interfacial electron transfer and (b) mass transport of substrate to the adsorbed enzyme from the bulk of the electrolyte solution. The latter is... 

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