Values Of Apparent Kinetic Parameters Research Articles

Comparative kinetics of the alkali-catalyzed sunflower oil methanolysis with co-solvent under conventional and microwave heating with controlled cooling

Abstract The kinetics of the alkali-catalyzed transesterification of sunflower oil with methanol in the presence of co-solvent (TSMPC) were investigated. The kinetics curves of the alkali-catalyzed TSMPC, in the temperature range of 26°C–55°C, were measured for conventional heating (CH) and microwave heating with controlled cooling. The results showed that for both heating modes, the kinetics of the alkali-catalyzed TSMPC reaction can be described with the kinetic model of the pseudo first-order reaction with respect to the concentration of the triglycerides. The values of apparent reaction rate constants, activation energies, and pre-exponential factors are also calculated. The existence of a linear correlation (compensation effect) between the values of apparent kinetic parameters determined for CH and microwave heating with controlled cooling conditions is established. The results confirmed that the increase in the transesterification rate in the microwave heating with controlled cooling conditions is not caused by overheating nor by the existence of hotspots. The model of mechanism of the impact of microwave heating on the kinetics of transesterification is hereby proposed.

Non-linear kinetic modelling of reversible bioconversions: Application to the transaminase catalyzed synthesis of chiral amino-alcohols

This work describes the establishment of a full kinetic model, including values of apparent kinetic parameters, for the whole cell E. coli mediated synthesis of the chiral amino-alcohol (2S,3R)-2-amino-1,3,4-butanetriol (ABT), using (S)-(−)-α-methylbenzylamine (MBA) as amino donor. The whole cell biocatalyst expressed the CV2025 ω-transaminase from Chromobacterium violaceum. Establishment of the most suitable reaction mechanism and determination of the complete forward and reverse kinetic parameter values for the reversible bioconversion where obtained using a hybrid methodology. This combined traditional initial rate experiments to identify a solution in the vicinity of the global minimum, with nonlinear regression methods to determine the exact location of the solution. The systematic procedure included selection and statistical evaluation of different kinetic models that best described the measured reaction rates and which ultimately provided new insights into the reaction mechanism; in particular the possible formation of a dead end complex between the amino donor and the cofactor enzyme complex. The hybrid methodology was combined with a microscale experimental platform, to significantly reduce both the number of experiments required as well as the time and material required for full kinetic parameter estimation. The equilibrium constant was determined to be 849, and the forward and reverse rate constants were found to be 97 and 13min−1, respectively, which greatly favoured the asymmetric synthesis of chiral ABT. Using the established kinetic model, the asymmetric synthesis of ABT was simulated, and excellent agreement was found between the experimental and predicted data over a range of reaction conditions. A sensitivity analysis combined with various simulations suggested the crucial bottleneck of the reaction was the second half reaction of the ping pong bi–bi mechanism, in part due to the low Michaelis constant of substrate l-erythrulose (ERY). The toxicity of MBA towards the transaminase was identified as another major bottleneck. The kinetic model was useful to give early insights into the most appropriate bioconversion conditions, which can improve the rate and yield of ABT formation, as well as minimizing the toxicity and inhibition effects of the substrates and products. The systematic methodology developed here is considered to be generic and useful in regard to speeding up bioconversion process design and optimization.

Kinetic study of 2-butanol O-acylation and sec-butylamine N-acylation catalyzed by Candida antarctica lipase B

The aim of this work was to study the differential behavior shown by Candida antarctica lipase B during the O-acylation and N-acylation of monofunctional alcohols and monofunctional amines. To achieve this, 2-butanol and sec-butylamine were used as model molecules. Yields, kinetics and enantioselectivity were studied for both reactions. Although a steady-state ordered ternary complex bi–bi mechanism was obtained for the O-acylation of 2-butanol, a ping–pong bi–bi mechanism was obtained for the N-acylation in case of low sec-butylamine concentrations. The values of apparent kinetic parameters were calculated: the enantiomeric ratios (E) were evaluated and confirmed the preference of C. antarctica lipase B for the (R)-enantiomer, which was consistent with the literature. The enantioselectivity was calculated for the alcohol (E ≈ 3.17) and for the amine (E ≈ 1.34). Concerning the O-acylation, the yields were found to be very similar for both enantiomers R and S. However, both initial rates and yields of the (R)-enantiomer N-acylation were higher than those of the (S)-enantiomer. In the last part of our study, the chemoselectivity of C. antarctica lipase B was evaluated, showing that C. antarctica lipase B was a chemoselective enzyme that preferentially catalyzed the O-acylation to the detriment of the N-acylation (C ≈ 92, for the selective acylation of (R)-enantiomers). These results provide new insights for the synthesis of products issued from the selective acylation of multifunctional substrates such as amino-alcohols.

A highly-accurate numerical method for calculating apparent kinetic parameters of immobilized enzyme reactions: 2. Accuracies of calculated values

To establish a highly-reliable numerical method for calculation of the values of apparent kinetic parameters in various types of immobilized enzyme reactions, the performance of the shooting method combined with a variable-order, variable-step Taylor-series method has been discussed in a series of two papers. In the first paper, every formulas are given and the calculation technique is described. In this second paper, these formulas are used to calculate the values of apparent kinetic parameters for the Thiele modulus varying from 1 to 700. As a result, the calculated values are found to have extremely-high accuracies that are comparable with the machine accuracy of the personal computer used, and the execution time is always within 1 s. On the other hand, the accuracies of the values calculated by the orthogonal collocation method are shown to reduce remarkably with increasing magnitude of the Thiele modulus.

A highly accurate numerical method for calculating apparent kinetic parameters of immobilized enzyme reactions: 1. Theory

To establish a highly reliable numerical method for calculation of the values of apparent kinetic parameters in various types of immobilized enzyme reactions, the performance of the shooting method combined with a variable-order, variable-step Taylor-series method has been discussed in a series of two papers. In this first paper, every formulae that are necessary to execute the numerical calculation are given and the procedure and technique for the calculation are described. As an example, the effectiveness factor is calculated for the Thiele modulus from 1 to 700 and the calculated values are found to have accuracies that are almost equal to the machine accuracy of the personal computer used. In the Taylor-series method, the step sizes are efficiently estimated from the ratio of the appropriate two of the first few terms in each Taylor-series solution to the relevant simultaneous differential equations, which contributes greatly to reduction in both the loss-of-significance error and the execution time.