Dual-site catalysis

This equation gives (0) = 0, a maximum at =. /Km/K2, and (oo) = 0. The assumed mechanism involves a first-order surface reaction with inhibition of the reaction if a second substrate molecule is adsorbed. A similar functional form for (s) can be obtained by assuming a second-order, dual-site model. As in the case of gas-solid heterogeneous catalysis, it is not possible to verify reaction mechanisms simply by steady-state rate measurements. 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

Finally, researchers have also noted that the selection of non-aqueous solvents can affect enzyme activity in more subtle ways. Enzymes that bind with a dual pocket binding site (secondary alcohol dehydrogenases) possess enantiomeric conformations capable of binding (Cowan, 1997). It was observed that increasing the dielectric constant increased protein flexibility, which increased binding and catalysis. These conditions may also allow increased binding of the usually less favored enantiomer. 

An interesting aspect of the elucidation of the structure and mechanisms of TPQ biogenesis and function is that for these amine oxidases certain amino acid residues have multiple roles during cofactor biogenesis and catalysis. This is clearly true for the tyrosine residue, which is converted into TPQ, It has also been demonstrated by site-directed mutagenesis that a second strictly conserved tyrosine residue, which is present in the active site is also required for TPQ biogenesis and influences catalysis. Furthermore, the bound copper serves dual functions in cofactor biosynthesis, as shown in Scheme 4 as well as during catalysis. The latter function appears to be in stabilization of transient intermediates in the oxidative half reaction. ... 

Pai CH, Chiang BY, Ko TP, Chou CC, Chong CM, Yen FJ, Chen S, Coward JK, Wang AH, Lin CH (2006) Dual binding sites for translocation catalysis by Escherichia coli glutathionylspermi-dine synthetase. EMBO J 25 5970-5982... 

The above studies [8-10] strongly indicate the participation of silanol groups as acid sites in the amine-catalyzed reactions. The synergistic catalysis of immobilized organic bases and silanol groups has been highlighted by the cyclic carbonate synthesis from carbon dioxide and epoxide. Acid-base dual activation mechanism of the cyclic carbonate synthesis is widely accepted. Sakakura and coworkers [11] reported silica-supported phosphonium salts as highly active catalysts for the cyclic carbonate synthesis. The results for propylene carbonate synthesis are summarized... 

Park and coworkers proposed that mesoporous silica having L-proline could enhance the chiral enhancement in asymmetric catalysis. The use of r-proline as active site would be useful for the asymmetric diethyl malonate addition reaction and asymmetric epoxidation reaction of a,p-unsaturated aldehydes. In addition, the plugged mesoporous sihca as a support certainly allowed the formation of dual mesoporosities. In particular, they can provide confinement effect for better enantiomeric excess in the asymmetric catalysis [63]. 

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