Active site formation

Many enzymes (see Chapters 14 to 16) derive at least some of their catalytic power from oligomeric associations of monomer subunits. This can happen in several ways. The monomer may not constitute a complete enzyme active site. Formation of the oligomer may bring ail the necessary catalytic groups together to form an active enzyme. For example, the active sites of bacterial glutamine synthetase are formed from pairs of adjacent subunits. The dissociated monomers are inactive. 

Note that in some cases, consecutive and parallel reactions may proceed without the formation of an intermediate product of the first kind, when interaction of the initial molecules causes formation of final products excluding the stage of active site formation (these reactions... 

The relationship between catalytic activity and TMA content in MAO is shown in Figure 17.5. Residual TMA in the MAO decreases the catalytic activity. The reactivity of TMA is too strong to control the active site formation and decomposes the active sites for polymerization. 

Chen P, Hochstrasser M. Autocatalytic subunit processing couples active site formation in the 20S proteasome to completion of assembly. Cell 1996 86 961-972. 

For the rate to pass through a maximum with increase in [M] it follows that Kfji [M] > 1, in accord with published values of for propene, since strong coordination of monomer to the sites would give rise to independence of rate on [M]. A fall in rate, however, implies that monomer is competing with active site formation it is difficult to see the mechzmism by which this could occur as exclusion of organometal from the catalyst surface by monomer would appear to be unlikely. 

P. E. Sinclair, C. R. A. Catlow, Quantum chemical study of the mechanism of partial oxidation reactivity on titanosilicate catalysts Active site formation, oxygen transfer, and catalyst deactivation, J. Phys. Chem. B 103 (1999) 1084. 

When the rate of active site formation, B sites min , is constant and the rate of ethene dimerization on an active site is constant, R mol min site", then the amount of butene formed, n mol, at a time of t min, is expressed by n=j R< t-r)BdT = (1/2 )RBt. This relation well explains the change of the amount of butenes formed on Nb205/PVG during the irradiation(displayed in Fig.4). Therefore, the Nb(IV) species must be closely related to the intermediate of the reaction. 

Berezin, M. Y., Ignatov, V. M., Belov, P. S., Elev, I. V., Shelimov, B.N., Kazansky, V. B. (1991) Mechanism of Olefin Metathesis and Active Site Formation on Photoreduced Molybdenum Catalysts. 5. Metathesis of Unsaturated Fatty Acid Esters, Kinet. Ratal. 32, 379-389. 

Noticeably, active site formation requires enzyme protein having a certain spacial conformation, but it is necessary that amino acid groups beyond the active site of enzyme molecule could stabilize the special conformation. Thus, it has great importance to the catalytic activity of enzyme, correspondingly. For instance, in these amino acid residue groups, some are related with the enzyme activity modulating, some are correlated to correct spacial structure formation of enzyme molecule and some are related with the immunogenicity or other characteristics of enzyme molecule. 

Comparative analysis of kinetic data and the properties of pol3miers formed by the action of homogeneous and immobilized metal complexes shows the similar nature of the active centers in these systems. The active role of polymer supports consists of regulating the reactions of active-site formation and deactivation and sometimes promoting their regeneration. Under optimal conditions we can clarify the mechanism of each polymerization stage and study intermediates. 

Molecular masses increase under carrying out of stage of active sites formation of isoprene polymerization in the presence of Ti-Al catalytic system in turbulent regime may be explained after consideration of distribution of sites of macromolecules propagation by their kinetic activity (see 1.4.1). It is obvious from Figures 5.14 and 5.15 that hydrodynamic effect on catalytic system in... 

At the stage of formation of sites of macromolecules propagation under ethylene with propylene copolymerization initiated by V-Al catalytic systems (separation of fast stage of active sites formation and initiation and slow stage of copolymerization itself) in novel technological scheme of SKEPT production tubular turbulent apparatus is installed (Fig. 6.2, position 6, Fig. 6.4). Calculation of geometry of zone of catalytic system components mixing is also carried out on the base of quantitative dependences obtained in Chapter 3. 

Results of carrying out of stage of V-Al catalytic system active sites formation in turbulent regime are presented in Table 6.2. 

The results of active site formation of the V-Al catalytic system, in turbulent mode, for the copolymerisation of ethylene and propylene are presented in Table 5.3. 

The preparation and introduction of catalytic complex components to the polymeriser, using a tubular turbulent prereactor, enables the synthesis of copolymers (styrene-propylene (SSP) and styrene-ethylene-propylene (SSEPT)) which demonstrate high stability and durability. According to the laboratory results, obtained from the commercial production of ethylene and propylene copolymerisation, during separation of the active site formation and initiation of isoprene polymerisation on a Ti-Al catalytic system, an increase in copolymer yield and its molecular weight, as well as a decrease in catalyst rate were observed. 

So far, it has been assumed that active site formation was instantaneous. This step could have been taken into consideration by adding the elementary step described in the last row of Table 2.8. In this case, another generation term needs to be added to Equation 2.8,... 

In summary, Transition Metal Macrocylic Complexes have shown comparable activity to traditional Pt-based catalysts for an ORR. However, the poor stability and complicacy of preparation are major obstacles before wide application. Further, the active site structures are still a subject of controversy, which hinders efforts to control active site formation. 

Fig. 9.2 Schematic illustration of the micropore filling technique and active site formation during NPMC synthesis, (a) Two adjacent graphitic crystallites hosting a slit micropore in the BP carbon support, (b) cross section view of the empty micropore, (c) micropore after being filled with 1,10-phenanthroline and iron acetate precursors, and (d) active site formation and nitrogen-doped graphitic carbon deposition after subsequent heat treatments in argon and ammonia (from [28] with permission from AAAS)...

Two other activators have been investigated for sPS catalyst systems. Tetraphenyltin, when added to the titanium and MAO catalyst system, has been reported to significantly improve the yield of sPS and cause an increase in the sPS molecular weight (82). It was proposed that mixed aluminum-tin sites may form in the MAO structure, which could affect the active site formation. The influence of diphenylzinc on the metallocene MAO catalyst system for sPS polymerization has been reported for titaniiun and zirconium (83). 

A model for the fomiatitHi of iron sites in this preparation route was deduced (see Fig. 9 in Ref. [31]). On the basis of this, it was possible to explain the iron content ORR activity behavior observed by Jaouen et al. [42]. At higher iron concentrations, the formation of iron nitride particles was enhanced instead of active site formation. The reason for this was found in the occupation of the iron acetate molecules on the carbon support in order to enable the active site formation, only less than a monolayer of iron acetate should be present on the carbon surface. In this case, the formation of active sites is faster than the agglomeration of iron atoms. For higher occupations, however, the agglomeration of iron atoms becomes dominant, yielding higher concentrations of iron nitride instead of active sites [31]. 

Weaver and co-workers explained the particle size effect in terms of an ensemble effect related to surface morphology [230]. For Pt nanoparticles of diameter < 4 mn it was experimentally determined [231] that the fraction of flat terrace sites diminished considerably as compared to edge sites. Therefore, the probability of ensemble of active site formation situated on the terraces and needed for methanol dehydrogenation (especially for the removal of the first three hydrogen atoms) is lower for particles with diameters below 4 nm. It was estimated that the 2.5 nm diameter particle has an approximately five times lower availability of adjacent Pt atoms compared to the 8.8 mn diameter particle [230]. In the case of formic acid oxidation, on the other hand, it was proposed that Pt ensembles are not required for catalysis. Interestingly, the COad surface coverage decreased with particle size for both CFI3OFI and FICOOFI. 

Figure 3.44 Proposed mechanism for active site formation. Reprinted from [50] with permission from Nature Publishing Group.ft...

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