Enzyme catalysis glycols

Ester copolymers were synthesized by lipase-catalyzed copol5mierization of lactones, divinyl esters, and glycols (177). The nmr analysis showed that the resulting product was not a mixture of homopolymers, but a copolymer derived from the monomers, indicating that two different modes of pol5mierization, ringopening polymerization and polycondensation, simultaneously take place through enzyme catalysis in one pot to produce ester copolymers. 

Chapters are also included on yeast-mediated stereoselective biocatalysis, stereoselective synthesis of steroids, chemo-enzymatic synthesis of enantiopure arylpropionic acids, supercritical carbon dioxide as a solvent in enzyme catalysis, state-of-the-art techniques in enzyme immobilization, biocatalysis by polyethylene glycol-modified enzymes, and enzymatic deprotection techniques in organic synthesis. 

Isobe, K. and Nishise, H. (1995) A new enzymic method for glycolaldehyde production from ethylene glycol. Journal of Molecular Catalysis B-Enzymatic, 1 (1), 37—43. 

Lipase has been used in organic solvents to produce useful compounds. For example, Zark and Klibanov (8) reported wide applications of enzymes to esterification in preparing optically active alcohols and acids. Inada et al (9) synthesized polyethylene glycol-modified lipase, which was soluble in organic solvent and active for ester formation. These data reveal that lipases are very useful enzymes for the catalysis different types of reactions with rather wide substrate specificities. In this study, it was found that moditied lipase could also synthesize esters and various lipids in organic solvents. Chemically moditied lipases can help to solve today s problems in esteritication and hopefully make broader use of enzymatic reactions that are attractive to the industry. 

When a polymer is treated with enzymes for surface modification, some of the undesired protein tends to adsorb on the polymer surface, which subsequently creates problems in the surface analysis and causes a slow down in the rate of catalysis. Adsorbed proteins can be removed from the surfaces by washing with large volumes of 1.5% Na2C03 and water (Eischer-Colbrie et al., 2006) as part of a preparation for surface analysis. Protein-resistant molecules such as polyethylene glycol can be used to prevent the nonspecific protein adsorption. Surfaces can be precoated with an inert protein such as bovine serum albumin (Salisbury et al., 2002) for increasing the rate of catalysis. 

It can be noted that the way in which the enzyme is prepared in the dry form for catalysis in organic solvent is responsible for striking differences (up to two orders of magnitude) in the enzyme-specific activity. Furthermore, it is worth mentioning that the transesterification activity of lipase from B. cepacia entrapped in sol gel (sol gel-AK-lipase BC) was 83% of the activity in water measured using tributyrin as a substrate [6]. Analogously, in the case of CALB lyophilized with methoxypoly(ethylene glycol) (CALB -i- PEG) the activity was 51% of the activity in water in the hydrolysis of vinyl acetate [7]. It is important to note that, for both... 

In order to generate terminal hydroxyl groups an excess of glycol is currently used. The reaction takes place in uncatalysed reaction conditions (self catalysis by the acidic carboxyl groups) but the best perfomances (low reaction time, low final acidity) are obtained in the presence of specific catalysts, such as p-toluene sulfonic acid, tin compounds (stannous octoate), antimony, titanium (tetrabutyltitanate), zinc (zinc acetate), manganese (manganese acetate) or lead compounds and more recently enzymic catalysts (lipases) [1, 25]. 

The role of the metal ion in ester hydrolysis catalysed by CPA has been examined with both Zn +- and Co +-substituted enzymes. When the terminal carboxyl of the substrate is electrostatically linked to argenine-145 and the aromatic side-chain lies in a hydrophobic pocket, the only residues close enough to the substrate to enter catalysis are glutamate-270, tyrosine-248, the metal ion, and its associated water. Low-temperature studies aid the elucidation of the mechanism. Between - 25 and - 45 °C in ethylene glycol-water mixtures two kinetically discrete processes are detected, the slower of which corresponds to the catalytic rate constant. The faster reaction is interpreted as deacylation of a mixed anhydride acyl-enzyme intermediate formed by nucleophilic attack by glutamate-270 on the substrate (Scheme 6). Differences in the acidity dependences of the catalytic rate constant with the metal ions Zn + (p STa 6.1) and Co +-(pATa 4.9) suggest that ionization of the metal-bound water molecule occurs and is involved in the decay of the anhydride. The catalytic rate constant shows an isotope effect in DgO. 

Many enzymes form covalent bonds with their substrates during catalysis, such as the acyl-enzyme intermediate in carboxyl ester hydrolysis (Scheme 2.1) or the glycol monoester intermediate in epoxide hydrolysis (Scheme 2.85). Despite the covalent enzyme-substrate bond, such species are metastable and should be regarded as activated intermediates . Some enzymes utilize cofactors, such as... 

Borderline-SN2-Type Mechanism. Some enzymes, such as limonene-1,2-epoxide hydrolase, have been shown to operate via a single-step push-pull mechanism [573]. General acid catalysis by a protonated aspartic acid weakens the oxirane to facilitate a simultaneous nucleophilic attack of hydroxyl ion, which is provided by deprotonation of H2O via an aspartate anion. Due to the borderline-SN2-character of this mechanism, the nucleophile preferentially attacks the higher substituted carbon atom bearing the more stabilized 5 -charge. After liberation of the glycol, proton-transfer between both Asp-residues closes the cycle. 

Several improvements of the process, such as the reaction with acyl chlorides or the application of two-phase reaction systems with propylene glycol and an emulsifier in order to build a microemulsion, have been described in the literature and patents [21]. Another approach for the synthesis of sugar esters is the use of enzymes. Enzymatic catalysis in the field of carbohydrate chemistry has been actively explored over years in laboratory... 

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