Catalytic power

Base catalysis is most effective with alkali metals dispersed on solid supports or, in the homogeneous form, as aldoxides, amides, and so on. Small amounts of promoters form organoalkali comnpounds that really contribute the catalytic power. Basic ion exchange resins also are usebil. Base-catalyzed processes include isomerization and oligomerization of olefins, reactions of olefins with aromatics, and hydrogenation of polynuclear aromatics. 

These rate constants are for the hydrolysis of cinnamic anhydride in carbonate buffer, pH 8.45, total buffer concentration 0.024 M, in the presence of the catalysts pyridine, A -methylimidazole (NMIM), or 4-dimethylaminopyridine (DMAP). In the absence of added catalyst, but the presence of buffer, the rate constant was 0.005 24 s . You may assume that only the conjugate base form of each catalyst is catalytically effective. Calculate the catalytic rate constant for the three catalysts. What is the catalytic power of NMIM and of DMAP relative to pyridine ... 

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. 

Enzymes are characterized by three distinctive features catalytic power, specificity, and regulation. 

Enzymes display enormous catalytic power, accelerating reaction rates as much as lO over uncatalyzed levels, which is far greater than any synthetic catalysts can achieve, and enzymes accomplish these astounding feats in dilute aqueous... 

This idea also helps to explain some of the mystery surrounding the enormous catalytic power of enzymes In enzyme catalysis, precise orientation of catalytic residues comprising the active site is necessary for the reaction to occur substrate binding induces this precise orientation by the changes it causes in the protein s conformation. 

In Chapter 16, we explore in greater detail the factors that contribute to the remarkable catalytic power of enzymes and examine specific examples of enzyme reaction mechanisms. Here we focus on another essential feature of enzymes the regulation of their aetimty. 

The specificity of enzyme reactions can be altered by varying the solvent system. For example, the addition of water-miscible organic co-solvents may improve the selectivity of hydrolase enzymes. Medium engineering is also important for synthetic reactions performed in pure organic solvents. In such cases, the selectivity of the reaction may depend on the organic solvent used. In non-aqueous solvents, hydrolytic enzymes catalyse the reverse reaction, ie the synthesis of esters and amides. The problem here is the low activity (catalytic power) of many hydrolases in organic solvents, and the unpredictable effects of the amount of water and type of solvent on the rate and selectivity. 

The previous chapters taught us how to ask questions about specific enzymatic reactions. In this chapter we will attempt to look for general trends in enzyme catalysis. In doing so we will examine various working hypotheses that attribute the catalytic power of enzymes to different factors. We will try to demonstrate that computer simulation approaches are extremely useful in such examinations, as they offer a way to dissect the total catalytic effect into its individual contributions. 

The reaction between Fe(IlI) and Sn(Il) in dilute perchloric acid in the presence of chloride ions is first-order in Fe(lll) concentration . The order is maintained when bromide or iodide is present. The kinetic data seem to point to a fourth-order dependence on chloride ion. A minimum of three Cl ions in the activated complex seems necessary for the reaction to proceed at a measurable rate. Bromide and iodide show third-order dependences. The reaction is retarded by Sn(II) (first-order dependence) due to removal of halide ions from solution by complex formation. Estimates are given for the formation constants of the monochloro and monobromo Sn(II) complexes. In terms of catalytic power 1 > Br > Cl and this is also the order of decreasing ease of oxidation of the halide ion by Fe(IlI). However, the state of complexing of Sn(ll)and Fe(III)is given by Cl > Br > I". Apparently, electrostatic effects are not effective in deciding the rate. For the case of chloride ions, the chief activated complex is likely to have the composition (FeSnC ). The kinetic data cannot resolve the way in which the Cl ions are distributed between Fe(IlI) and Sn(ll). 

The ease of autoxidation of Cu(I) is a source of the catalytic power of Cu(II) mentioned previously. In a hydrochloric acid medium the rate law determined... 

Enediynes 38 undergo [2+2+2] cycloaddition reactions to afford polycyclic cyclohexadienes 39 in presence of a cobalt catalyst (Scheme 5.11) [14]- In this system, the presence of a NHC ligand improved the catalytic power of cobalt when compared with phosphine ligands. In addition to increased yields, lower ligand... 

With the characterized mechanism, the next key question is the origin of its catalytic power. A prerequisite for this investigation is to reliably compute free energy barriers for both enzyme and solution reactions. By employing on-the-fly Born-Oppenheimer molecular dynamics simulations with the ab initio QM/MM approach and the umbrella sampling method, we have determined free energy profiles for the methyl-transfer reaction catalyzed by the histone lysine methyltransferase SET7/9... 

Selective cleavage of peptides and proteins is an important procedure in biochemistry and molecular biology. The half-life for the uncatalyzed hydrolysis of amide bonds is 350 500 years at room temperature and pH 4 8. Clearly, efficient methods of cleavage are needed. Despite their great catalytic power and selectivity to sequence, proteinases have some disadvantages. Peptides 420,423,424,426 an(j proteins428 429 can be hydrolytically cleaved near histidine and methionine residues with several palladium(II) aqua complexes, often with catalytic turnover. 

Naray-Szab6, G. 1983. Unusually Large Electrostatic Field Effect of the Buried Aspartate in Serine Proteinases Source of Catalytic Power. Int. J. Quant. Chem. 23, 723. 

Enzymes have several remarkable catalytic properties such as high catalytic power and high selectivities under mild reaction conditions, as compared with those of chemical catalysts. In the field of organic synthesis, enzymes have often been employed as catalyst functional organic compounds were synthesized by the enzymatic selective reactions [1-5]. 

Shindo H, Huang PM (1985a) The catalytic power of inorganic components in the abiotic synthesis of hydroquinone-derived humic polymers. Appl Clay Sci 1 71-81... 

Wang MC, Huang PM (1986) Humic macromolecular interlayering in nontronite through interaction with phenol monomers. Nature (London) 323 529-531 Wang MC, Huang PM (1988) Catalytic power of nontronite, kaolinite, and quartz and their reaction sites in the formation of hydroquinone-derived polymers. Appl Clay Sci 4 43-57... 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

Ghosh et al.32 have demonstrated another bis(oxazoline) derivative chiral ligand 86 for asymmetric Diels-Alder reaction and obtained excellent results. Reaction of an equimolar mixture of chiral ligand 86 and Cu(C104)2 6H20 produces the aqua complex 87 (w being water molecule), which shows excellent catalytic power in asymmetric Diels-Alder reactions. As depicted in Scheme 5 27, the reaction of 88 with cyclopentadiene gives product 89 with more than 80% yield, over 99 1 diastereoselectivity and up to 99% ee. 

Enzymes are remarkable molecular devices that determine the pattern of chemical transformations in biological systems. The most striking characteristics of enzymes are their catalytic power and specificity. As a class of macromolecules, they are highly effective in catalyzing diverse chemical reactions because of their ability to specifically bind to a substrate and their ability to accelerate reactions by several orders of magnitude. Applying enzymes or organisms in... 

Immobilized enzymes are attached to a solid support by adsorption or chemical binding or mechanical entrapment in the pores of a gel structure but retain their catalytic power. Their merit is ease of separation from the finished reaction product. 

Enhancement ratio, ER Quantified as k Jk Dcat, is used to express the catalytic power of a biocatalyst. It is a comparison between the catalysed reaction occurring at its optimal rate and the background rate. 

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