Kinetics and Engineering Studies of Lipase-Catalyzed Transesterification in Organic Solvent

Adlercreutz

2000

Biotechnol. Bioeng.

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. © 2000 John Wiley & Sons, Inc. Biotechnol Bioeng 67: 319–326, 2000.

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Section: Nucleophile Selectivitysupporting

confidence: 52%

Section: Introductionmentioning

confidence: 99%

Effect of mass-transfer limitations on the selectivity of immobilized α-chymotrypsin biocatalysts prepared for use in organic medium

Adlercreutz

2000

Biotechnol. Bioeng.

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“…The problem with (nonimmobilized) sus-pended enzymes has been studied from a theoretical point of view by Kamat et al (1992). Studies have been done on the operation of continuous reactors with immobilized lipases (Luck et al, 1988;Indlekofer et al, 1992;Ison et al, 1994), but these go seldom beyond the presentation of theoretical models, which are often not corroborated by experimental data. These models are then used to predict conditions under which the reactors can be operated in the absence of mass transfer limitations.…”

Section: Introductionmentioning

confidence: 99%

Mass transfer studies on immobilized α-chymotrypsin biocatalysts prepared by deposition for use in organic medium

Adlercreutz

1998

Biotechnol. Bioeng.

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. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 59:364–373, 1998.

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Physical Characterization Of Porous Materials And Correlation With The Activity Of Immobilized Enzyme In Organic Medium

Biocatalysis and Biotransformation

Garcı́a

et al. 1998