Catalysts enzymatic

Five-membered unsubstituted lactone, y-butyrolactone (y-BL), is not polymerized by conventional chemical catalysts. However, oligomer formation from y-BL was observed by using PPL or Pseudomonas sp. lipase as catalyst. Enzymatic polymerization of six-membered lactones, 8-VL and l,4-dioxan-2-one, was reported. 8-VL was polymerized by various lipases of different origins. The molecular weight of the enzymatically obtained polymer was relatively low (less than 2000). 

Esters are common components in cosmetics and skin-care products. They can be synthesized from fatty acids and alcohols using either chemical or enzymatic reactions. The chemical reactions are normally catalysed by acid catalysts. Enzymatic synthesis is carried out under milder conditions and therefore it provides products of very high purity. A range of esters such as isopropyl palmitate and isopropyl myristate are now produced industrially using enzymatic synthesis. The reactions are carried out in solvent-free systems using an immobilised lipase as catalyst. In order to get high yields in the reactions, water is removed continuously. 

In chemical synthesis, particularly for pharmaceutical applications, they give highly stereo- and regioselective syntheses that often are impossible using dassical catalysts. Enzymatic reactions combined with enantio-resolution can generate en-antiopure materials with high yields [150]. Enzymes can be extremely enantioselec-... 

Studies on enzyme-catalyzed polymerization ( enzymatic polymerization ) has been of increasing importance as a new trend in macromolecular science. Enzyme catalysis has provided a new synthetic strategy for useful polymers, most of which are difficult to produce by conventional chemical catalysts. Enzymatic polymerization also affords a great opportunity for use of nonpetrochemical renewable resources as starting substrates of functional polymeric materials (as shown in the industrial examples cited above). 

Detailed classification of heterogeneous catalysts covers solid catalyst, solid acid catalyst, solid base catalyst, enzymatic catalyst, and nonenzymatic heterogeneous catalyst. Solids include MgO, CaO, Al—Mg hydrotalcites, La203, ZnO, Ce02, CaCOa, sulfated zirconium oxide, sulfated tin oxide, etc. 

The first step in the reaction sequence is a regioselective and stereoselective franj-acetoxypalladation of one of the double bonds, thus forming a jr-allylpalladium(II) intermediate, which is then attacked by a second nucleophile. By variation of the concentration of chloride ions, reactions selective for either the franj-diacetate or the cw-diacetate (eq 9) can be accomplished. The use of other chloride salts resulted in poor selectivity. The selectivity for the trans product at chloride-free conditions is further enhanced if the reaction is carried out in the presence of a sulfoxide co-catalyst. Enzymatic hydrolysis of the cis-meso-diacetate yields cw-l-acetoxy-4-hydroxy-2-cyclohexene in more than 98% ee, thus giving access to a useful starting material for enantios-elective synthesis. ... 

Enzymatic Catalysis. Enzymes are biological catalysts. They increase the rate of a chemical reaction without undergoing permanent change and without affecting the reaction equiUbrium. The thermodynamic approach to the study of a chemical reaction calculates the equiUbrium concentrations using the thermodynamic properties of the substrates and products. This approach gives no information about the rate at which the equiUbrium is reached. The kinetic approach is concerned with the reaction rates and the factors that determine these, eg, pH, temperature, and presence of a catalyst. Therefore, the kinetic approach is essentially an experimental investigation. 

It is apparent that the use of enzymatic catalysis continues to grow Greater availabiUty of enzymes, development of new methodologies for thek utilization, investigation of enzymatic behavior in nonconventional environments, and the design and synthesis of new biocatalysts with altered selectivity and increased stabiUty are essential for the successhil development of this field. As more is learned about selectivity of enzymes toward unnatural substrates, the choice of an enzyme for a particular transformation will become easier to predict. It should simplify a search for an appropriate catalyst and help to estabhsh biocatalytic procedures as a usehil supplement to classical organic synthesis. 

Biological. Several recent patents have claimed the production of ethylene oxide from a wide variety of raw materials using enzymatic catalysts (221—224). However, no commercial production routes based on biological mechanisms have been proposed. 

Kinetics is the branch of science concerned with the rates of chemical reactions. The study of enzyme kinetics addresses the biological roles of enzymatic catalysts and how they accomplish their remarkable feats. In enzyme kinetics, we seek to determine the maximum reaction velocity that the enzyme can attain and its binding affinities for substrates and inhibitors. Coupled with studies on the structure and chemistry of the enzyme, analysis of the enzymatic rate under different reaction conditions yields insights regarding the enzyme s mechanism of catalytic action. Such information is essential to an overall understanding of metabolism. 

The immobilization of enzymes with the formation of insoluble forms is usually intended for the development of specific catalysts for technical purposes. Here, we consider another medico-biological problem of the preparation of insoluble enzymatic systems based on crosslinked polyelectrolytes, used in the replacement therapy for oral administration. 

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

The experimental evidences that medium engineering might represent an efficient method to modify or improve enzyme selectivity (alternative to protein engineering and to the time-consuming search for new catalysts) were immediately matched by the search for a sound rationale of this phenomenon. The different hypotheses formulated to try to rationalize the effects of the solvent on enzymatic enantioselectivity can be grouped into three different classes. The first hypothesis suggests that... 

For an efficient enzymatic DKR the following requirements must be fulfilled (i) the KR must be very selective ( > 20) (ii) the racemization must be fast (at least 10 times faster than the enzyme-catalyzed transformation of the slow reacting enantiomer, krac >10 kent-s) (hi) the racemization catalyst must not react with the product of the reaction (iv) the KR and the racemization must be compatible under the same reaction conditions. 

Very recently Page and coworkers have reported the D KR of sec-amines using a low catalyst loading of an Ir complex for the racemization, and Candida rugosa lipase for the enzymatic resolution [38]. 

Assuming that the enzymatic reaction is highly enantioselective, then even after only four cycles the enantiomeric excess will have reached 93.4% whereas after seven catalytic cycles the enantiomeric excess is >99% (Figure 5.3). This type of deracemization is really a stereoinversion process in that the reactive enantiomer undergoes stereoinversion during the process. One of the challenges of developing this type of process is to find conditions under which the enzyme catalyst and chemical reactant can coexist, particularly in the case of redox chemistry in which the coexistence of an oxidant and reductant in the same reaction vessel is difficult to achieve. For this... 

Although the aminolysis of esters to amides is auseful synthetic operation, usually it presents some disadvantages in terms of drastic reaction conditions, long reaction times or strong alkali metal as catalyst, which are usually not compatible with other functional groups in the molecule [6]. For this reason, enzymatic aminolysis of carboxylic acid derivatives offers a clean and ecological way for the preparation of different kind of amines and amides in a regio-, chemo-, and enantioselective manner. 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

The pentagon stabilization has been found in a biochemical phenomenon [80], The hydrogen on the thiazolium ring 9 (Scheme 7) is easily ionized to afford the corresponding carbene 10, a key catalyst in enzymatic reactions for which thiamine (vitamin B-1,11) pyrophosphate is the cofactor. The pentagon stability is expected to contribute to this unusual deprotonation. A lone pair generated on the carbon atom in 10 can similarly delocalize through the vicinal C-N and C-S a bonds in a cyclic manner. 

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