For reaction catalyzed

In recent years ionic liquids have also been employed as media for reactions catalyzed both by isolated enzymes and by whole cells, and excellent reviews on this topic are already available [47]. Biocatalysis has been mainly conducted in those room-temperature ionic liquids that are composed of a 1,3-dialkylimidazolium or N-alkylpyridinium cation and a noncoordinating anion [47aj. 

They observed abrupt changes in the slope of Arrhenius plots for reactions catalyzed by NADH oxidase and p-lactate oxidase that correlate well with phase transitions detected by the ESR spectra of the nitroxide spin labels bound covalently to the enzymes (Table 5.4). 

Scheme 12. Proposed mechanism for reactions catalyzed by MeReO(mtp) 2.

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

Fig. 39. H202 conversion profiles (a) for reaction catalyzed by Ti-SBA-15 (sample 1, 30 mg) and (b) by Ti-MMM (33 mg). Reaction conditions H202, 1.29 mmol Ti, 0.013 mmol MeCN, 3 ml T = 353 K [from Trukhan et al. (234)].

Fig. 2. Relationship between catalyst temperature and reaction time for reaction catalyzed by Ni/Al203(—) and Ni-MgO solid solutions (—) temperature (K) of the gas phase (a) 1019 (b) 899 (c) 809 (d) 625. The reaction was carried out in a fixed-bed reactor (a quartz tube of 2 mm inside diameter) at atmospheric pressure. Before reaction, the feed gas was allowed to flow through the catalyst undergoing heating of the reactor from room temperature to 1073 K at a rate of 25 K min-1 to ignite the reaction, and then the reactant gas temperature was decreased to the selected value. Reaction conditions pressure, 1 atm catalyst mass, 0.04 g feed gas molar ratio, CH4/O2 = 2/1 GHSV, 90,000 mL (g catalyst)-1 h-1) (25).

Reactions (377) and (378) are usually not ranked among catalytic processes, although they are, as a matter of fact, autocatalytic since they occur on the surface of carbon, one of the participants of the reactions. True, it is more usual to speak of autocatalysis for reactions catalyzed by products rather than by reactants, but it is not essential in our case. Equations (377) and (378) may be read from right to left as the reactions are reversible. 

Several studies using these tools have examined the effects of the silane ester structure on the hydrolysis rates for reactions catalyzed by hydronium ion, hydroxide anion, and general bases [35, 36, 38, 41-43]. In the following sections, we will discuss how the structure of the alkoxysilane impacted on the hydrolysis rate for each type of catalysis and, using these data, speculate about the possible reaction mechanisms. 

Cytochrome b5 affects the kinetics of drug metabolism by certain CYP enzymes hence, coexpression of this microsomal hemoprotein (together with NADPH-CYP reductase) can affect the catalytic efficiency of certain recombinant CYP enzymes (76,109). For example, the presence of cytochrome b5 tends to increase Fmax for reactions catalyzed by CYP3 A4, whereas it tends to decrease Km for reactions catalyzed by CYP2E1. In both cases, cytochrome b5 increases Vmax/Km, which is a measure of in vitro intrinsic clearance. The fact that some commercially available recombinant CYP enzymes are expressed with cytochrome b5 while others are not complicates the interpretation of results of studies performed with recombinant human CYP enzymes. 

Fig. 1. Turnover frequencies for isobutylene production at 773 K and 0.016 atm isobutane pressure (a) and 0.099 atm hydrogen pressure (b) for reaction catalyzed by Pt/Sn/SiC>2 (O), Pt/Sn/K/SiC>2 ( ), and Pt/Sn/K-L (O). Adapted from (40). Solid lines represent the fit for each set of data using the Horiuti-Polanyi mechanism.

In 1913, Michaelis and Men ten presented a general theory for enzyme kinetics, extended later by Briggs and Haldane, which accounts for the velocity curve shown in Figure 5.5. This theory for reactions catalyzed by enzymes having a single substrate assumes that the substrate S binds to the active site of the enzyme E to form the enzyme-substrate complex ES, which yields the product P and the free enzyme E ... 

MEIJER, A.H., CARDOSO, M.I.L., VOSKUILEN, J.T., DE WAAL, A., VERPOORTE, R., HOGE, J.H.C., Isolation and characterization of a cDNA clone from Catharanthus roseus encoding NADPH cytochrome P-450 reductase, an enzyme essential for reactions catalyzed by cytochrome P-450 mono-oxygenases in plants. Plant J., 1993,4,47-60. 

Though the values of the Michaels complex (Km 10 4 - 10 5 M) for reactions catalyzed by natural enzymes and catalytic antibodies were found to be of the same order of magnitude, the catalytic constants (kc,) for CAs commonly 104 fold lower than that for correspondent enzymes. Experimental ratio values (kcat/lWat) for CAs ranges within 102-106, while these values for similar enzymatic reactions can reach 1017... 

As previously outlined, values of kjK for reactions of FKBP increase with increasing hydrophobicity of the Pj residue. When AG values for these reactions are dissected into A//t and ASl values (see Table IV), we see that these reactions proceed with large values of AH and ASi. In contrast, kJK values for reactions catalyzed by CyP have no significant dependence on the Pj residue and these reactions proceed with more negative values of A//t and A5t. 

The general rule seems to be that the B-value changes during resolution and that the e.e.-value depends on the degree of conversion during asymmetization. Principally this should not be restricted to enzymatic catalysis, but also be valid for reactions catalyzed by non-enzymatic catalysts. 

Figure 19.3 DHAP-aldolase catalyzed aldol addition of DHAP to N-Cbz-aminoaldehydes. Reaction conversion to aldol adduct in gel emulsion (black bars) and dimethylformamide/ water 1 4 system (gray bars) for reactions catalyzed by (a) RAMA, (b) RhuA and (c) FucA.

In considering the rate law, the most serious alternative to (2) appears to be (10). Barton and Wright point out that this law depends on the assumption that (9) is effectively at equilibrium and that the experiments show some scatter. They therefore reject all alternatives. Mechanisms of reaction involving the iodyl cation, IO2, cannot be excluded in consideration of the oxygen exchange rates, but do not lead to simple interpretation of catalysis by nucleophiles. They have been proposed several times, e.g. in refs. . For reaction catalyzed by a nucleophile, B , the mechanism may involve the reactions... 

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