Kinetic Analysis and Simulation Studies for Lipase-Catalysed Resolution of Racemic 2-Methyl-1-Pentanol

Abstract

The lipase-catalysed optical resolution of a racemic mixture of 2-methyl-1-pentanol by transesterification using vinyl acetate as acyl donor has been studied experimentally. A mechanistic model has been developed for the double-substrate reaction sequence treating both enantiomers as competing substrates. The model is based upon a ping-pong mechanism with alternative substrates involving an acyl-enzyme intermediate. The kinetic constants of the model have been evaluated using initial rate experiments and nonlinear regression analysis. The model successfully predicts the evolution of the enantiomeric excess of substrate (eeR) and the degree of conversion with time for batch experiments with various initial concentrations of vinyl acetate and (R,S)-2-methyl-l-pentanol. Furthermore, the rate equations have been used to theoretically study the dynamic progression of a continuous enzyme-catalysed resolution process. The enantiomeric excess as a function of conversion for different process configurations is discussed. It is found, that the maximum attainable eeR is strongly dependent on the residence time distribution of the continuous reactor and is rather low for a continuous stirred tank reactor (CSTR) due to competitive inhibition effects.KEY WORDS Enzyme kinetics, mathematical modelling, optical resolution, lipase, 2-methyl-lpentanol. LIST OF SYMBOLS

Stereoselective bioconversions in continuously operated fixed bed reactors: Modeling and process optimization

“…In a previous report (Indlekofer et al, 1993), the transesterification of (R,S)-2-methyl-l-pentanol with vinylacetate ( Fig. 1) in dichloromethane as organic solvent was studied.…”

“…steady states were obtained by appropriately changing the mean residence time. Based on the kinetic parameters of lipase-catalyzed transesterification of (R,S)-2-methyl-1 -pentanol obtained in the preliminary kinetic analysis (Indlekofer et al, 1993) and the diffusional properties of the biocatalyst described above, attention was given to accurately load the carrier material such as to maximize the productivity of the continuous process and to fully exhibit the enzymes' native enantioselectivity. Assuming a number of 4s2 = 1 as a limiting constraint, a critical lipase loading of X , = 0.36 could be roughly estimated, and immobilized lipase biocatalysts in the range of 0.1 < X , < 0.4 were prepared.…”

“…It has been repeatedly pointed out (Indlekofer et al, 1993;Rakels et al, 1994;Straathof et al, 1992) that for kinetic resolution processes ideally mixed reactors are generally inferior. For low to moderate enantiomeric ratios, as is the case in the resolution of (R,S)-2-methyl-l-pentanol studied in this article, we previously demonstrated the necessity to use a plug flow type reactor in order to achieve reasonable enantiomeric purities when the reaction is to be transferred to a continuous process (Indlekofer et al, 1993). However, using packed bed reactor systems with the biocatalyst immobilized into a porous support matrix may lead to substantial decreases in biocatalyst performance due to diffusional limitations of substrates and products.…”

“…To evaluate previously mentioned effects like axial dispersion (backmixing), liquid-solid mass transfer, and internal pore diffusion determining the effective stereochemical outcome of the process, a detailed mathematical model for the heterogeneous biotransformation was developed. Kinetics of the reaction were taken into account as previously analyzed (Indlekofer et al, 1993), and further model parameters such as water adsorption, competitive hydrolytic side reaction, pore diffusion, and axial dispersion were independently determined. The aim of the model was to predict the steady-state performance of the continuous kinetic resolution process in terms of conversion and enantiomeric excess as a function of the mean residence time, and to assess and experimentally verify a critical internal biocatalyst activity, finally allowing optimal productivity of the reactor.…”

Adsorptive control of water in esterification with immobilized enzymes. Continuous operation in a periodic counter-current reactor

“…Water control by means of in situ adsorption onto a selective medium is of particular interest for continuous processing, since adsorbents can be easily integrated in packedbed reactors often used for enzymatic conversions (Indlekofer et al, 1993(Indlekofer et al, , 1996Balcao et al, 1996). A critical concern, in this case, is the selection of an appropriate regeneration scheme and the design of an efficient reactor system.…”