| 1. |
- Adlercreutz, Patrick, et al.
(författare)
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Aspects of biocatalyst stability in organic solvents
- 1987
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Ingår i: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422. ; 1:2, s. 99-108
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Forskningsöversikt (refereegranskat)abstract
- The stability of biocatalysis in systems containing organic solvents is reviewed. Among the examples presented are homogeneous mixtures of water and water-miscible organic solvents, aqueous/organic two-phase systems, solid biocatalysts suspended in organic solvents, enzymes in reverse micelles and modified enzymes soluble in water immiscible solvents. The stability of biocatalysts in organic solvents depends very much on the conditions. The hydrophobicity or the polarity of the solvent is clearly of great importance. More hydrophobic solvents (higher log P values) are less harmful to enzymes than less hydrophobic solvents. The water content of the system is a very important parameter. Some water is essential for enzymatic activity; however, the stability of enzymes decreases with increasing water content. Mechanisms of enzyme inactivation are discussed.
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| 2. |
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| 3. |
- Bloomer, S., et al.
(författare)
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Triglyceride interesterification by lipases : 2. Reaction parameters for the reduction of trisaturated impurities and diglycerides in batch reactions
- 1991
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Ingår i: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422. ; 5:2, s. 145-162
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Tidskriftsartikel (refereegranskat)abstract
- A model system consisting of pure triolein and palmitic acid and LipozymeTM, an immobilized lipase (E.C. 3.1.1.3.). has been used to determine the effects of various reaction parameters on the reaction rate and the formation of by-products in the interesterification reaction. The goal was to minimize the level of diglycerides and eliminate trisaturated triglycerides at an endpoint chosen so that the results could be applied to the production of cocoa butter substitutes. The levels of diglycerides, which are essential reaction intermediates, and trisaturated glycerides, which are believed to be formed as a result of spontaneous acyl migration of mono- and diglyceride intermediates, were determined at a defined endpoint. A lag period was observed in which no tripalmitate was formed. The content of Lipozyme used was the most powerful factor in eliminating tripalmitate formation and reducing diglycerides; by using large quantities of Lipozyme, the reaction reached the endpoint before the tripalmitate formation became measurable and low levels of diglycerides were formed. The effects of varying the ratio of palmitic acid to triolein were investigated. A complex relationship between the ratio of substrate components emerged in which the diglyceride content increased with increasing triolein concentration and the tripalmitate content was lowest at a molar ratio of palmitic acid to triolein of 3.5. The reaction was run at 70, 80, and 90°C; best results were obtained at 70° The water activity of the reaction was adjusted prior to catalysis and maintained during the reaction by equilibrating the reaction mixture and enzyme and running the reaction in an atmosphere of controlled water activity. A direct relationship between diglycerides and water activity was observed, and the level of tripalmitate formed corresponded to the time required to reach the endpoint. The reaction system was tested using ethyl palmitate instead of palmitic acid as acyl donor; the diglyceride content again increased with increasing water activity, but larger amounts of diglycerides were formed. Much shorter reaction times were required, with small quantities of tripalmitate formed.
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| 4. |
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| 5. |
- Larsson, Karin M., et al.
(författare)
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Three systems used for biocatalysis in organic solvents a comparative study
- 1990
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Ingår i: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422. ; 4:2-3, s. 163-175
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Tidskriftsartikel (refereegranskat)abstract
- The activity and operational stability of horse liver alcohol dehydrogenase (HLADH) and αchymotrypsin were investigated in three systems commonly used for biocatalysis in organic solvents: 1. enzyme adsorbed on a solid support (celite) and added to the organic solvent (isooctane) 2. enzyme powder directly added to the organic solvent (isooctane). 3. enzyme dissolved in a microemulsion (AOT/isooctane). The activity and the operational stability in all systems were strongly dependent on the water content. The initial reaction rate was high in both the microemulsion and the celite system, but was much lower when adding the enzymes directly to the organic solvent. HLADH was observed to be more stable when added directly to the organic solvent or dissolved in the microemulsion than when adsorbed on celite, whereas for αchymotrypsin stability was higher when adsorbed on celite or added directly to the organic solvent. For a hydrolytic reaction, a microemulsion was preferred due to the high water content. When adding the enzymes directly to the organic solvent both HLADH and chymotrypsin were adsorbed strongly to the glass walls of the reaction vessel. None of the systems were superior in all respects for the two enzymes studied.
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| 6. |
- Ljunger, Gudrun, et al.
(författare)
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Reactions catalyzed by peg-modified αchymotrypsin in organic solvents. Influence of water content and degree of modification
- 1993
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Ingår i: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422. ; 7:4, s. 279-288
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Tidskriftsartikel (refereegranskat)abstract
- αChymotrypsin was modified with cyanuric chloride activated monomethoxypolyethylene glycol (MPEG) with molecular weights 1900 and 5000. Using the higher molecular weight MPEG a product that was soluble in benzene at moderate levels of modification was obtained, whereas with MPEG 1900 almost all the enzyme's amino groups had to be modified for dissolving the conjugate. The catalytic activity decreased with increasing degree of substitution. Apparent Vmax was considerably higher for the less modified enzyme preparation than for the more modified one, while Km,app stayed almost constant. The modified enzyme was used for peptide synthesis. The reaction was dependent on the content of dissolved water. Both Vmax,app and Km,app increased with increasing water content. It was possible to achieve a process with complete conversion of substrate to dipeptide.
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| 7. |
- OHRNER, N, et al.
(författare)
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THIOETHYL-OCTANOATE, VINYL-OCTANOATE, ETHYL-OCTANOATE ESTERS AND OCTANOIC-ACID AS ACYL DONORS IN LIPASE-CATALYZED ACYL TRANSFER-REACTIONS
- 1994
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Ingår i: BIOCATALYSIS. - ROYAL INST TECHNOL,DEPT ORGAN CHEM,S-10044 STOCKHOLM 70,SWEDEN. : HARWOOD ACAD PUBL GMBH. - 0886-4454. ; 9:1-4, s. 105-114
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Tidskriftsartikel (refereegranskat)abstract
- S-Ethyl thiooctanoate, vinyl octanoate, ethyl octanoate and octanoic acid were studied as acyl donors in a lipase catalysed acyl transfer reaction with 2-octanol as acyl acceptor. The reaction conditions had a pronounced effect on the equilibrium displacement and the apparent enantioselectivity. The thioethyl and vinyl esters proved to be efficient acyl donors under atmospheric pressure and 39-degrees-C, affording a high apparent enantiomeric ratio. Under these reaction conditions the apparent enantioselectivity seemed to be enhanced by water, which was present in the reaction system and caused the production of octanoic acid, by hydrolysis of the acyl enzyme.
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| 8. |
- Otamiri, Marina, et al.
(författare)
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Complex formation between chymotrypsin and ethyl cellulose as a means to solubilize the enzyme in active form in toluene
- 1992
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Ingår i: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422. ; 6:4, s. 291-305
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Tidskriftsartikel (refereegranskat)abstract
- Chymotryspin and ethyl cellulose were mixed in an aqueous phosphate buffer solution and freeze dried. Due to complex formation between the substances it was possible to dissolve or at least finely disperse these preparations in toluene. The chymotrypsin-ethyl cellulose complexes were characterized by light scattering measurements. Complexes were also formed by mixing enzyme powder in toluene containing ethyl cellulose and buffer but this was a slow process. Experiments with radioactively labelled bovine serum albumin showed that this protein was also solubilized in toluene in the presence of ethyl cellulose and buffer salts. Chymotrypsin complexes were used to catalyze the esterification of N-acetyl-L-phenylalanine with ethanol in toluene. The presence of buffer salts greatly increased the initial reaction rate of the esterification reaction. The complexes were consideraly more active and stable than enzyme powder in toluene.
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| 9. |
- Pinheiro, H. M., et al.
(författare)
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Quinones as external electron acceptors in steroid dehydrogenation with entrapped cells in organic medium
- 1993
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Ingår i: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422. ; 7:2, s. 83-96
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Tidskriftsartikel (refereegranskat)abstract
- A series of quinone-based compounds were tested for their ability to act as external electron acceptors in the 1-dehydrogenation of-αmethyl-hydrocortisone-21-acetate, with polyurethane-entrapped Arthrobacter simplex cells in buffer-saturated n-decan-1-ol. This organic solvent was needed to solubilize the steroid substrate. In aqueous medium, the conversion with free cells virtually stopped after one hour, probably due to substrate limitation. All the tested quinones acted as external electron acceptors, increasing the bioconversion rate. The process kinetics were complex. However, when keeping the concentration of one of the substrates (steroid or quinone) constant and varying that of the other, Michaelis-Menten kinetics provided a reasonably good model for the initial reaction rates, and apparent kinetic constants were estimated. The most effective of the tested external electron acceptors were 2,6-dimethyl-p-benzoquinone and menadione. Mass transfer limitations seemed to appear after some hours of reaction, with low concentrations of the more efficient quinones, when the biocatalyst microenvironment was quinone- and possibly oxygen-depleted. Monosodium glutamate was included with the cells in the immobilisation foam, as an activity-stabilizing agent. It was observed that some of the quinones apparently formed complexes with this glutamate, thereby influencing the kinetics of the process. The catalytic half-life of the system depended on the quinone concentration and optimal values (60-80 h) were observed at 1 mM levels of 2,6-dimethyl-p-benzoquinone or menadione. Quinone toxicity, direct or through the formation of peroxides in the aerobic reoxidation process, may be at the origin of enzyme deactivation.
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| 10. |
- Reslow, Mats, et al.
(författare)
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Modification of the microenvironment of enzymes in organic solvents. Substitution of water by polar solvents
- 1992
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Ingår i: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422. ; 6:4, s. 307-318
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Tidskriftsartikel (refereegranskat)abstract
- Enzyme catalysis in water-immiscible organic solvents is strongly influenced by the amount of water present in the reaction mixture. Effects of substitution of part of the water by other polar solvents were studied. In an alcoholysis reaction catalyzed by chymotrypsin deposited on celite, it was possible to exchange half of the water by formamide, ethylene glycol or dimethyl sulfoxide with often increased initial reaction rate. Furthermore, these substitutions caused the suppression of the competing hydrolysis reaction. However, formamide caused enzyme inactivation, and ethylene glycol participated as a reactant in the alcoholysis to some extent, hence dimethyl sulfoxide was considered the best water substitute among the solvents tested. These effects were noted for chymotrypsin catalyzed alcoholysis in several water immiscible solvents and also for interesterification reactions catalyzed by Candida cylindracea lipase on celite. In the latter case a change in the stereoselectivity was observed. At a low water content a high stereoselectivity was observed; when the amount of polar solvent was increased, either by doubling the water content or adding an equal amount of DMSO, the stereoselectivity decreased.
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