| 1. |
- Adlercreutz, Dietlind, et al.
(author)
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An enzymatic method for the synthesis of mixed-acid phosphatidylcholine
- 2004
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In: Journal of the American Oil Chemists Society. - : Wiley. - 0003-021X. ; 81:6, s. 553-557
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Journal article (peer-reviewed)abstract
- The enzymatic synthesis of PC with decanoic acid in the sn-1 and hexanoic acid in the sn-2 position is described. The procedure comprises the following enzymatic steps: (i) treatment of egg yolk with phospholipase A(2) (PLA(2)) to hydrolyze egg yolk PC to 1-acyl lysophosphaticlylcholine (LPC) (ii) esterification of I-acyl LPC with hexanoic acid catalyzed by PLA(2) to yield PC with hexanoic acid in the sn-2 position; (iii) removal of the FA in the sn-1 position by lipase-catalyzed ethanolysis to yield 2-hexanoyl LPC; and finally (iv) introduction of decanoic acid in this position by lipase-catalyzed esterification of 2-hexanoyl LPC to yield 1-decanoyl-2-hexanoyl-PC. Two egg yolks with a weight of 16 g were required to obtain 160 mg of the desired product. The chemical purity of the PC product and the positional purity of the FA were around 99%. The method is applicable for the synthesis of other mixed-acid PC species as well.
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| 2. |
- Adlercreutz, Dietlind, et al.
(author)
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Synthesis of phosphatidylcholine with defined fatty acid in the sn-1 position by lipase-catalyzed esterification and transesterification reaction.
- 2002
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In: Biotechnology and Bioengineering. - : Wiley. - 1097-0290 .- 0006-3592. ; 78:4, s. 403-411
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Journal article (peer-reviewed)abstract
- The incorporation of caproic acid in the sn-1 position of phosphatidylcholine (PC) catalyzed by lipase from Rhizopus oryzae was investigated in a water activity-controlled organic medium. The reaction was carried out either as esterification or transesterification. A comparison between these two reaction modes was made with regard to product yield, product purity, reaction time, and byproduct formation as a consequence of acyl migration. The yield in the esterification and transesterification reaction was the same under identical conditions. The highest yield (78%) was obtained at a water activity (a(w)) of 0.11 and a caproic acid concentration of 0.8 M. The reaction time was shorter in the esterification reaction than in the transesterification reaction. The difference in reaction time was especially pronounced at low water activities and high fatty acid concentrations. The loss in yield due to acyl migration and consequent enzymatic side reactions was around 16% under a wide range of conditions. The incorporation of a fatty acid in the sn-1 position of PC proved to be thermodynamically much more favorable than the incorporation of a fatty acid in the sn-2 position.
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| 3. |
- Adlercreutz, Patrick, et al.
(author)
-
Enzymatic conversions of polar lipids. Principles, problems and solutions
- 2001
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In: Journal of Molecular Catalysis - B Enzymatic. - 1381-1177. ; 11:4-6, s. 173-178
-
Research review (peer-reviewed)abstract
- This text provides a brief overview of the principles of enzymatic lipid conversion and some recent advances in the enzymatic conversion of glycerophospholipids and galactolipids. Lipases and phospholipases are used to exchange fatty acids or the polar group in the lipids. The reactions can be carried out either as hydrolysis-esterification sequences or as one-step transferase reactions. The scope and limitations of the different methods are discussed.
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| 4. |
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| 5. |
- Barros, Raúl J., et al.
(author)
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Effect of mass-transfer limitations on the selectivity of immobilized α-chymotrypsin biocatalysts prepared for use in organic medium
- 2000
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In: Biotechnology and Bioengineering. - 0006-3592. ; 67:3, s. 319-326
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Journal article (peer-reviewed)abstract
- The selectivity of preparations of α-chymotrypsin immobilized on Celite or polyamide and carrying out syntheses of di- and tripeptides in acetonitrile medium were studied. The study concerns the effect of mass- transfer limitations on three different kinds of selectivity: acyl donor, stereo- and nucleophile selectivities, defined respectively as the ratio of initial rates with different acyl donors; the enantioselectivity factor (E); and the ratio of initial rates of peptide synthesis and hydrolysis of the acyl donor. Strong mass-transfer limitations caused by increased enzyme loading had a very strong effect on acyl donor selectivity, with reductions of up to 79%, and on stereoselectivity, with reductions of up to 77% in relation to optimum values, both on Celite. Nucleophile selectivity was not affected as strongly by mass-transfer limitations. Using a small molecule (AlaNH2) as nucleophile, the onset of these limitations caused only minor reductions in selectivity, while when using a larger nucleophilic species (AlaPheNH2) it was reduced by up to 60% when increasing enzyme loading on Celite from 2 to 100 mg/g. The different way these kinds of selectivity are affected by the onset of mass-transfer limitations can be explained by a combination of different aspects: the kinetic behavior of the enzyme toward nucleophile and acyl donor concentrations, the relative concentrations of reagents used in the reaction media, and their relative diffusion coefficients. In short, higher concentrations of nucleophile than acyl donor are generally used, and the nucleophile most often used in the experiments hereby described (AlaNH2) diffuses faster than the acyl donors employed. These factors combined are expected to give rise to concentration gradients inside porous biocatalyst particles higher for acyl donor than for nucleophile under conditions of mass-transfer limitations. This explains why acyl donor selectivity and stereoselectivity are much more influenced by mass transfer limitations than nucleophile selectivity.
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| 6. |
- Barros, Raúl J., et al.
(author)
-
Enhancement of immobilized protease catalyzed dipeptide synthesis by the presence of insoluble protonated nucleophile
- 1999
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In: Enzyme and Microbial Technology. - 0141-0229. ; 24:8-9, s. 480-488
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Journal article (peer-reviewed)abstract
- α-Chymotrypsin immobilized on celite catalyzing a kinetically controlled dipeptide synthesis reaction in acetonitrile medium showed an odd behavior in response to additions of triethylamine to the reaction mixture. This base is used to deprotonate the nucleophilic reagent, l-alaninamide hydrochloride, in order to increase its nucleophilicity and solubility. However, the enzyme performance is apparently enhanced by additions of triethylamine below one equivalent (in the range 15-20 mm) while the used concentration of nucleophilic reagent is 30 mm. Under these conditions, the initial rate is up to 2.5 times higher and the nucleophile specificity is approximately 30% better than when one equivalent is added. The activating effect on initial rates of dipeptide synthesis was not observed when polyamide was used as support. Unlike polyamide, celite is a material with quite low porosity. Improvement of nucleophile specificity was observed using both supports. It is shown that this activation arises due to the presence of a separate dense liquid phase of insoluble l-alaninamide hydrochloride that intimately contacts with the enzyme preparation, and does not depend on the addition of triethylamine itself. Additions of l-alaninamide hydrochloride improved initial rates of synthesis more than 2.5-fold, and nucleophile specificity more than threefold. The initial rate activation was also observed when using non-porous glass beads to immobilize the enzyme at a loading of 5 mg enzyme g-1 glass but not at 1 mg enzyme g-1 glass when no mass transfer limitations in the immobilized enzyme layer are expected to occur. The results suggest that the presence of the separate phase helps to relieve mass transfer limitations on the system caused by overloading at the supports. One possible mechanism for the initial rate activation might be that the enzyme is partially desorbed from the support particles into the separate phase of nucleophile, and the better nucleophile specificity observed is due to increased local concentrations of the nucleophile within this phase. Copyright (C) 1999 Elsevier Science Inc. All rights reserved.
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| 7. |
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| 8. |
- Barros, Raúl J., et al.
(author)
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Mass transfer studies on immobilized α-chymotrypsin biocatalysts prepared by deposition for use in organic medium
- 1998
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In: Biotechnology and Bioengineering. - 0006-3592. ; 59:3, s. 364-373
-
Journal article (peer-reviewed)abstract
- Mass transfer limitations were studied in enzyme preparations of α- chymotrypsin made by deposition on different porous support materials such as controlled pore glasses, Celite, and polyamides of different particle sizes. It is the onset of mass transfer limitations that determines the position of the activity optimum with respect to enzyme loading on each support. The evidence of various experiments indicates that internal diffusional limitations are the important mechanism for the observed mass transfer limitations. External diffusion was not found to play an important role under the conditions used, and it was also found that when immobilizing multilayers of enzyme the buried enzyme molecules are active to a large extent. An extreme situation is observed on Celite at very high loadings. Under these conditions, this support is expected to have its pores completely filled with packed enzyme molecules, and then it is the diffusion within the enzyme layer that determines the observed rate. As the enzyme loading increases, the area of contact between the deposited enzyme layers and the liquid solution inside the pores diminishes, causing a decrease on the observed rate of an intrinsically fast reaction which apparently is incongruous with the presence of more enzyme in the system. This work shows that mass transfer limitations can be an important factor when working with immobilized enzymes in organic media, and its study should be carried out in order to avoid undesired reduced enzyme activities and specificities.
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| 9. |
- Barros, Raúl J., et al.
(author)
-
Modeling the performance of immobilized α-chymotrypsin catalyzed peptide synthesis in acetonitrile medium
- 2001
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In: Journal of Molecular Catalysis - B Enzymatic. - 1381-1177. ; 11:4-6, s. 841-850
-
Journal article (peer-reviewed)abstract
- A model was developed which describes simultaneous reaction and internal diffusion for kinetically controlled, immobilized α-chymotrypsin-catalyzed, oligopeptide synthesis in acetonitrile medium. The model combines the equations that describe the intrinsic kinetics of four different reactions and the physical characteristics of three different support materials, as determined experimentally, to predict the apparent initial activity and nucleophile selectivity of the immobilized biocatalyst. The model is able to predict reasonably well the experimentally observed initial rate and nucleophile selectivity vs. enzyme loading profiles. The reduction in observed initial rate with enzyme loading when fast reactions are carried out with α-chymotrypsin immobilized on celite, and the larger influence of mass transfer limitations on the initial reaction rates than on nucleophile selectivities are correctly predicted by the numerical calculations. The model is general in terms of its application to other systems - enzymes, reactions, support materials and/or kinetic schemes - as long as the intrinsic kinetics and the characteristics of the enzyme and support material are known.
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| 10. |
- Barros, Raúl J., et al.
(author)
-
Physical characterization of porous materials and correlation with the activity of immobilized enzyme in organic medium
- 1998
-
In: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422 .- 1029-2446. ; 16:1, s. 67-85
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Journal article (peer-reviewed)abstract
- A series of commonly used porous supports was characterized by determination of particle size distribution, porosity, surface area (total and distributions with pore diameters) and skeletal density. The performance of immobilized α-chymotrypsin catalyzed dipeptide synthesis in an acetonitrile medium was correlated with these physical properties.At high enzyme loading, when internal mass transfer limitations are expected to occur, the activity can be correlated with the support characteristic parameter. This is a combination of physical properties such as particle size, porosity, and volumetric porosity, which influence the substrate diffusion rate. At low enzyme loading the important parameter is the accessible surface area, which will determine how the enzyme is distributed in the pores of the support. When assessing the effect of the support material on enzymatic activity, the geometric considerations studied here should always be contemplated before making any assumptions about direct effects of support material on enzymatic catalytic properties.
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| 11. |
- Björup, Peter, et al.
(author)
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Effects of acetonitrile-water mixtures on α-chymotrypsin catalyzed dipeptide synthesis
- 1996
-
In: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422 .- 1029-2446. ; 13:3, s. 189-200
-
Journal article (peer-reviewed)abstract
- α-Chymotrpysin (EC 3.4 21.1) was immobilized by deposition on celite and subsequent cross-linking with glutaraldehyde. The effects of different mixtures of aqueous buffer and acetonitrile on the immobilized preparation were evaluated using a dipeptide synthesis as model reaction. The initial reaction rate at 6-95% of water increased with increasing water content. The maximum yield of peptide had two maxima; the first one at 6% of water (92%) and the second one at 80% of water (39%). The presence of two maxima was due to severe enzyme inactivation at intermediate water contents (50-60%). The immobilisation procedure slowed the inactivation of α-chymotrypsin. Cross-linked enzyme was inactivated to a lesser extent than both free enzyme and enzyme that had been deposited on celite. The increased resistance to inactivation was, however, not sufficient to make peptide synthesis attractive at intermediate water contents (50-60%). In order to obtain good peptide yields, low water contents (below 10%) should be used.
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| 12. |
- Brocca, Stefania, et al.
(author)
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Mutants provide evidence of the importance of glycosydic chains in the activation of lipase 1 from Candida rugosa
- 2000
-
In: Protein Science. - : Wiley. - 0961-8368 .- 1469-896X. ; 9:5, s. 985-990
-
Journal article (peer-reviewed)abstract
- Sequence analysis of Candida rugosa lipase 1 (LIP1) predicts the presence of three N-linked glycosylation sites at asparagine 291, 314, 351. To investigate the relevance of sugar chains in the activation and stabilization of LIP1, we directed site mutagenesis to replace the above mentioned asparagine with glutamine residues. Comparison of the activity of mutants with that of the wild-type (wt) lipase indicates that both 314 and 351 Asn to Gln substitutions influence, although at a different extent, the enzyme activity both in hydrolysis and esterification reactions, but they do not alter the enzyme water activity profiles in organic solvents or temperature stability. Introduction of Gln to replace Asn35 is likely to disrupt a stabilizing interaction between the sugar chain and residues of the inner side of the lid in the enzyme active conformation. The effect of deglycosylation at position 314 is more difficult to explain and might suggest a more general role of the sugar moiety for the structural stability of lipase 1. Conversely, Asn291Gln substitution does not affect' the lipolytic or the esterase activity of the mutant that behaves essentially as the wt enzyme. This observation supports the hypothesis that changes in activity of Asn314Gln and Asn351Gln mutants are specifically due to deglycosylation.
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| 13. |
- Costes, David, et al.
(author)
-
Cross-linked crystals of hydroxynitrile lyase as catalyst for the synthesis of optically active cyanohydrins
- 2001
-
In: Journal of Molecular Catalysis - B Enzymatic. - 1381-1177. ; 11:4-6, s. 607-612
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Journal article (peer-reviewed)abstract
- Purified hydroxynitrile lyase (HNL) from Manihot esculenta was crystallized by the sitting-drop vapour-diffusion method. The bipyramidal crystals formed (10-20 μm) were cross-linked with different amounts of glutaraldehyde and used as biocatalyst for the synthesis of optically active cyanohydrins. The cross-linked crystals were more stable than Celite-immobilized enzymes when incubated in organic solvents, especially in polar solvents. After six consecutive batch reactions in dibutylether, the remaining activity of the cross-linked crystals was more than 70 times higher than for the immobilized enzymes. Nevertheless, the specific activity of the cross-linked crystals (per milligram protein) was reduced compared to the activity of immobilized enzymes. The product enantiopurity was independent of the type of enzyme preparation used.
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| 14. |
- Costes, David, et al.
(author)
-
Hydroxynitrile lyase-catalyzed synthesis of cyanohydrins in organic solvents Parameters influencing activity and enantiospecificity
- 1999
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In: Enzyme and Microbial Technology. - 0141-0229. ; 25:3-5, s. 384-391
-
Journal article (peer-reviewed)abstract
- (S)-Hydroxynitrile lyases from Hevea brasiliensis, Manihot esculenta, and Sorghum bicolor and (R)-hydroxynitrile lyase from Prunus amygdalus have been used as biocatalyst for the enantiospecific addition of hydrogen cyanide to aldehydes in organic solvents. The effects of the reaction parameters on the enzymatic reaction rate and product enantiomeric excess (e.e.) are presented. The reaction rate increased with the solvent hydrophobicity but highly hydrophobic solvents were not adapted to high hydrogen cyanide concentrations and provoked loss of activity and product e.e. In the synthesis of 3-phenylpropionaldehyde cyanohydrin catalyzed by (S)-hydroxynitrile lyases from H. brasiliensis, an e.e. value of 88 ± 1% was obtained under optimized reaction conditions. Lower enantiomeric excess values were obtained under conditions where the enzyme was inactivated: high hydrogen cyanide concentration, high solvent log P, low enzyme loading. At lower temperature (down to -5°C) the e.e. was increased for all four enzymes used. Enzymes from different sources used under identical optimized reaction conditions were found to yield cyanohydrins with very different enantiopurities. This intrinsic enantiospecificity is not an effect of spontaneous reactions independent of the enzyme. Copyright (C) 1999 Elsevier Science Inc.
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| 15. |
- Costes, David, et al.
(author)
-
Stability and stabilization of hydroxynitrile lyase in organic solvents
- 2001
-
In: Biocatalysis and Biotransformation. - : Informa UK Limited. - 1024-2422 .- 1029-2446. ; 19:2, s. 119-130
-
Journal article (peer-reviewed)abstract
- The stability of hydroxynitrile lyase from Hevea brasiliensis has been studied in organic solvents. In dry solvents, the enzyme had half-lives in the range 1400-2500 hours. The enzyme half-life was one order of magnitude lower if the medium was water saturated. The substrates, aldehyde and hydrogen cyanide, were found to promote enzyme deactivation. The deactivation increased with substrate concentration, but was reduced in hydrophilic solvent. At high substrate concentration (2 M) in tert-butyl methyl ether, the enzyme half-life was 1.7 h when incubated with hydrogen cyanide while it was 1.0 h with 3-phenylpropionaldehyde. The addition of polyethylenimine, 125 mg per g of enzyme preparation, increased the enzyme half-life to 110 h when incubated with hydrogen cyanide and to 3.2 h with 3-phenylpropanaldehyde in tert-butyl methyl ether. Albumin and poly(ethylene glycol) gave similar stabilization effect.
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| 16. |
- Egger, Dietlind, et al.
(author)
-
Characterization and optimization of phospholipase A2 catalyzed synthesis of phosphatidylcholine
- 1997
-
In: BBA - Protein Structure and Molecular Enzymology. - 0167-4838. ; 1343:1, s. 76-84
-
Journal article (peer-reviewed)abstract
- The phospholipase A2 (PLA2) catalyzed synthesis and hydrolysis of phosphatidylcholine (PC) was studied in a water activity controlled organic medium. The aim of the study was to find the conditions most favorable for the synthetic reaction. To do this, the impact of various parameters such as water activity, substrate concentration and temperature on enzyme activity and equilibrium yield was determined. The PC to lysophosphatidylcholine (LPC) ratio at equilibrium increases with decreasing water activity and increasing fatty acid concentration, as can be expected from the law of mass action of an esterification reaction. The enzyme activity on the other hand decreases under conditions that favor the esterification. The best yield in the synthetic reaction is 60% at a water activity of 0.11 and an oleic acid concentration of 1.8 M. That is to our knowledge the highest yield ever reported in this reaction. Both the hydrolysis and synthesis reaction follow Michaelis-Menten kinetics, the apparent K(m) values are the same for PC and LPC, namely 4.9 mM. V(max) is 82.5 and 10.4 nmol h-1 mg-1 for the hydrolysis and synthesis reaction, respectively. Studies on PLA2 at water activity controlled conditions resulted in a more complete understanding of the enzymatic reaction and allowed to find the conditions most favorable for the synthetic reaction.
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| 17. |
- Hansson, Therese, et al.
(author)
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Influence of water activity on the competition between β-glycosidase-catalysed transglycosylation and hydrolysis in aqueous hexanol
- 2001
-
In: Enzyme and Microbial Technology. - 0141-0229. ; 29:8-9, s. 527-534
-
Journal article (peer-reviewed)abstract
- Five different β-glycosidases (Pyrococcus furiosus β-glucosidase, Sulfolobus solfataricus β-galactosidase, Caldocellum saccharolyticum β-glucosidase, almond β-glucosidase and Escherichia coli β-galactosidase) were evaluated as transglycosylation catalysts in hexanol containing various amounts of water. All enzymes catalysed both hydrolysis and transglycosylation of the glycosidic substrates (pentyl- and p-nitrophenyl-β-glucoside and p-nitrophenyl-β-galactoside). From the concentration ratio (alcohol/water) it was expected that the transglycosylation/hydrolysis ratio would decrease with increasing water activity in the hexanol. However, for all enzymes tested the selectivity for the alcohol increased with increasing water activity. This counteracted the effect of higher water concentration and in most cases the transglycosylation/hydrolysis ratio increased with increasing water activity. On the other hand, in hexanol/water two-phase systems, hydrolysis was by far the dominating reaction even though the total activity increased for all enzymes. The selectivity values were used to predict the maximal reaction yields in the kinetically controlled reactions. However, deviations were found in cases when the reactions became thermodynamically controlled: at high water contents secondary hydrolysis reduced the transglycosylation yields while higher transglycosylation yields than predicted were obtained at low water activity in some cases using enzymes poorly selective for the alcohol.
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| 18. |
- Jönsson, Åsa, et al.
(author)
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Low reaction temperature increases the selectivity in an enzymatic reaction due to substrate solvation effects
- 1997
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In: Biotechnology Letters. - 0141-5492. ; 19:1, s. 85-88
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Journal article (peer-reviewed)abstract
- Immobilized α-chymotrypsin was used as catalyst for studying temperature effects on acyl transfer reactions (acyl-donor: Bz-TyrOEt) in a water-immiscible organic solvent. The solubility of the two nucleophiles, Phe-NH2 and water, decreased with decreasing temperature. The relative decrease for the amide was larger (2.4-fold) than for water. Therefore the thermodynamic activity (estimated by the relative saturation) increased more for this substrate and hence the selectivity in the reaction was increased.
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| 19. |
- Jönsson, Åsa, et al.
(author)
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Temperature effects on protease catalyzed acyl transfer reactions in organic media
- 1996
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In: Journal of Molecular Catalysis B: Enzymatic. - : Elsevier BV. - 1381-1177. ; 2:1, s. 43-51
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Journal article (peer-reviewed)abstract
- The influence of reaction temperature on synthesis activity, product yield and nucleophile specificity for α-chymotrypsin and subtilisin Carlsberg were studied. The enzymes were immobilized on Celite and used in acetonitrile with a water content of 10%. Acyl-transfer reactions with Ac-PheOEt as acyl donor and 11 different amino acid amides and 3 dipeptides as nucleophiles were studied. The decrease in temperature from 25 to -1°C had a positive effect on the peptide yield in all reactions studied. The most efficient nucleophiles for the two enzymes α-chymotrypsin and subtilisin Carlsberg is arginine amide and glycine amide, respectively. When decreasing the reaction temperature the yield for α-chymotrypsin increased from 86 to 94% with arginine amide as nucleophile and for subtilisin the yield increased from 75 to 84% for glycine amide. The nucleophile specificity was determined as the p value, which describes the competition between nucleophile and water for the acyl enzyme. α-Chymotrypsin showed preference for both small and positively charged amino acid residues and subtilisin preferred small uncharged nucleophiles. The temperature did not affect the specificity order but all nucleophiles became more effective in the competition with water at low temperature. In addition, the results indicate that the temperature effect is more pronounced for the smaller nucleophiles.
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| 20. |
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| 21. |
- Jönsson, Åsa, et al.
(author)
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The influence of water activity on the enantioselectivity in the enzyme- catalyzed reduction of 2-pentanone
- 1998
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In: Journal of Molecular Catalysis - B Enzymatic. - 1381-1177. ; 5:1-4, s. 273-276
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Journal article (peer-reviewed)abstract
- The stereoselective reduction of 2-pentanone by alcohol dehydrogenase from Thermoanaerobium brockii was studied at controlled water activity and at different reaction temperatures. The reaction rate increased when water activity was increased from 0.32 to 0.96 and when raising the temperature from 5°C to 40°C. The enantioselectivity, E, reached a plateau value at high water activities. The enantioselectivity increased with decreasing reaction temperature.
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| 22. |
- Jönsson, Åsa, et al.
(author)
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Thermodynamic and kinetic aspects on water vs. organic solvent as reaction media in the enzyme-catalysed reduction of ketones
- 1999
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In: BBA - Protein Structure and Molecular Enzymology. - 0167-4838. ; 1430:2, s. 313-322
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Journal article (peer-reviewed)abstract
- The stereoselective reduction of ketones catalysed by alcohol dehydrogenase from Thermoanaerobium brockii was studied in different reaction media, hexane at controlled water activities, hexane with 2.5% water (biphasic) and pure water. The reactions were studied in the temperature range from -1 to 50°C. Increasing the water activity from 0.53 to 0.97 increased the reaction rate 16-fold. The rate was further enhanced in hexane when exceeding the water solubility and in pure water the rates were even higher. This was general for all ketones studied. At controlled water activity the entropy of activation (ΔS(≠)) was the dominating factor. Large negative ΔS(≠) values caused low reaction rates at low a(w). When increasing the carbon chain length of the substrate, for reactions in hexane, the decrease of reaction rate was mainly due to a decrease in ΔS(≠). In the comparison between hexane and pure water, ΔG(≠) values were higher in hexane due to higher ΔH(≠) values. The enantioselectivity (E value) increased from 2.6 at water activity 0.53 to 4.6 at water activity 0.97. Changing media from hexane (2.5%, v/v water) to pure water was not affecting the enantioselectivity or the specificity for different ketones. Copyright (C) 1999.
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| 23. |
- Kaur, Jasmedh, et al.
(author)
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Water transfer kinetics in a water activity control system designed for biocatalysis in organic media
- 1997
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In: Enzyme and Microbial Technology. - 0141-0229. ; 21:7, s. 496-501
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Journal article (peer-reviewed)abstract
- The performance of a water activity-controlling system in organic solvents using saturated salt solutions circulating in silicone tubing submerged in the solvent was studied. The mass transfer could be regarded as a flux through a cylindrical geometry. Integration over the barrier gave diffusion coefficients of water (D(aw)) which were similar for the different tubing sizes used. The driving force for the transfer was shown to be the difference in water activity and not the water concentration across the membrane. Hydrophilic solvents (ethyl acetate) gave higher transfer rates than more hydrophobic ones (diisopropyl ether). The D(aw) obtained in different solvents was influenced by the swelling behaviour of the tubing and the solubility of water. The water transfer was studied in a water 'producing' system consisting of 2.5 m silicone tubing submerged into a 250-ml tank reactor with a constant influx of water-saturated solvent. Different steady-state levels were obtained at different flow rates and the corresponding D(aw) values were calculated. The data obtained can be used to predict the required amount of tubing necessary to achieve a desired water transfer in a new application.
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| 24. |
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| 25. |
- Persson, Mattias, et al.
(author)
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Effects of solvent, water activity and temperature on lipase and hydroxynitrile lyase enantioselectivity
- 2002
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In: Enzyme and Microbial Technology. - 0141-0229. ; 30:7, s. 916-923
-
Journal article (peer-reviewed)abstract
- The influence of the reaction conditions on the enantioselectivity of reactions catalysed by lipases or hydroxynitrile leases (HNLs) in organic solvents was investigated. The lipases catalysed kinetic resolution of chiral secondary alcohol, or chiral carboxylic acids and the HNLs catalysed asymmetric addition of hydrogen cyanide to aldehydes. The temperature effects on enantioselectivity, were studied in detail. From measurements of the enantiomeric ratio (C) at different temperatures the activation parameters DeltaDeltaH(#) and DeltaDeltaS(#) were determined. In the lipase-catalysed reactions the enthalpic and entropic effects on E always counteracted, while in a few of the HNL-catalysed reactions, DeltaDeltaH(#) and DeltaDeltaS(#) had opposite sign, and therefore the effects cooperated to give high E values (-RTInE = DeltaDeltaG(#) = DeltaDeltaH(#) - TDeltaDeltaS(#)). In all the HNL-catalysed reactions and most of the lipase-catalysed ones, the enantioselectivity increased with decreasing reaction temperature. However, in one of the lipase-catalysed reactions, the enantioselectivity decreased with decreasing temperature. The theoretical background of these observations wars discussed. In the HNL-catalysed reactions, the enantioselectivity increased with increasing water content up to water saturation, while in the lipase-catalysed reactions the opposite trend was found in one case and in the others no significant effect was observed. Solvent mixtures of diisopropylether and hexane were used to obtain solvents with different log P values. The log P value of the solvent did not influence the enantioselectivity in the HNL-catalysed reactions. while the enantioselectivity increased with increasing log P value in two of the lipase-catalysed reactions. The reaction temperature was shown to be a very useful way to influence enzyme selectivity and the effects obtained could be rationalised. The influence of the reaction medium (solvent and water activity) is much more difficult to rationalise and predict. (C) 2002 Elsevier Science Inc. All rights reserved.
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