Slow kinetic process

When sample components having ionizable groups are chromatographed the use of a background electrolyte and control of the eluent pH with an m>propridte buffer are mandatory. It is advisable to maintain a fairly high concentration of buffer in the medium in order to rapidly reestablish protonic equilibria and to thereby avoid peak sjditting or asymmetrical peaks due to slow kinetic processes. Acetic acid, phosphoric acid, and perchloric acid and their salts have been used for the control of pH. 

The kinetics of the reactions of phthalic and maleic anhydrides with Z-substituted phenols (Z = H, m-Me, p-Me, m-Cl, p-C 1, and -CN) (Scheme 8) were studied in aqueous solution at pH 8.5. Two kinetic processes well separated in time were observed. The fast process was attributed to the formation of the aryl ester in equilibrium with the anhydride and allowed the determination of the rate of nucleophilic attack of the phenol on the anhydride. From the slow kinetic process, the equilibrium constant for this reaction was determined. The Brpnsted-type plots for the nucleophilic attack of substituted phenols on the anhydrides were linear with slopes /SNuc of 0.45 and 0.56 for phthalic and maleic anhydride, respectively. The results are consistent with a mechanism involving rate-determining nucleophilic attack and also with a concerted mechanism.27... 

Stirred tank reactors are very useful when the reagents contain multiple components that could exist in separate phases. In addition, it is a convenient way to achieve long reaction times for the study of slow kinetic processes. The attainable reaction time depends on the size of the reactor (e.g., 100 to 2000 mL). Sample schematics of STR and PFR systems are given in Figs. 7 and 8, respectively. 

While it is clear that many proteins may be freeze-dried and reconstituted with little or no loss in activity, it is usually not obvious as to whether the freeze-dried protein is basically native in conformation or whether the solid-state conformation is distinctly nonnative, with the native and active conformation quickly forming during rehydration. The observation that lysozyme regains enzymatic activity in the solid state above about 20% water [56] demonstrates that a protein need not be in a predominantly aqueous system to maintain activity and presumably possess native structure. However, this observation does not necessarily imply that the structure is nonnative at lower water contents where the enzymatic activity disappears. The loss of activity at lower water contents [56] could simply be a consequence of greatly slowed kinetic processes (i.e., greatly restricted molecular mobility as the system passes into the glassy state). 

Reductive removal of these oxygen layers is a slow kinetic process, commencing at potentials well below the characteristic potential for the layer formation on each metal. Thus, adsorption results based on the commonly used triangular potential sweep method can depend on the anodic potential excursions, the frequency of potential cycling and the number of cycles, that is, the catalyst surface history. Similarly, kinetic studies of oxygen reduction can be influenced by the dependence of the oxygen layer formation... 

Acetonitrile is a toxic material, the lowest published toxic dose for humans being 570 mg/kg. A slow kinetic process may eventually convert the acetonitrile to acetic acid. 

In Figure 5 is represented the normal sorption hysteresis of the hazardous coal sample. It is an open hysteresis function measured by Sartorius 4112 Type micro balance applicable to 12 MPa pressure fl.3]. The open hysteresis indicates that at very low equilibrium pressure range a great amount of methane remains in the inside structure of coal. This phenomenon cannot only be explained by very slow kinetic process of desorption because the remained amount of methane is independent of time required to formation of desorption equilibrium pressure. The remained amount of methane can better be emphasized by data shown in Figure 6. 

Fig. 5. Calorimetric thermogram for the titration of 23 iM wt GSTPl-1 with 5 gL injections (1 qL first injection) of 2.1 mM EA in 20 mM sodium phosphate, 5 mM NaCl and 0.1 mM EDTA at pH 7.0 and 25°C. Inset plot Comparison between a peak from a calorimetric thermogram for the titration of wt enzyme with EA, reflecting the slow kinetic process caused by covalent modification (solid line) and a representative calorimetric trace of a typical binding peak in the absence of a kinetic process (dashed line).

Chai et al. (46) developed a novel automated gas chromatographic technique to study slow kinetic processes. The technique uses multiple headspace extraction and can be applied to reactions involving volatile formation or adsorp-tion/desorption phenomena. 

However, sometimes, even working at very low concentrations of the probe molecule and following the aforementioned recommendations, peaks with a large broadening are obtained. In these cases, it can be assumed that this asymmetry is not due to instrumental problems or experimental conditions. This effect is characteristic of the so-called slow kinetic process [7], Fig. 16.3, associated with markedly energetically heterogeneous surfaces containing preferential sites were desorption takes place in a slower way. Slow kinetic process depends on the concentration of the... 

It is impossible to reach any definitive conclusion concerning the cis-trans isomerization model for the moment. More data are needed on different proteins to clarify the situation. Furthermore, one cannot be convinced that it is the unique direction to search for an explanation for the slow kinetic process. 

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