Thermodynamics of enzyme-catalyzed esterifications: I. Succinic acid esterification with ethanol
Altuntepe, E. and Greinert, T. and Hartmann, F. and Reinhardt, A. and Sadowski, G. and Held, C.
APPLIED MICROBIOLOGY AND BIOTECHNOLOGY
Volume: 101 Pages: 5973-5984
Succinic acid (SA) was esterified with ethanol using Candida antarctica lipase B immobilized on acrylic resin at 40 and 50 °C. Enzyme activity in the reaction medium was assured prior to reaction experiments. Reaction-equilibrium experiments were performed for varying initial molalities of SA and water in the reaction mixtures. This allowed calculating the molality-based apparent equilibrium constant Km as function of concentration and temperature. Km was shown to depend strongly on the molality of water and SA as well as on temperature. It could be concluded that increasing the molality of SA shifted the reaction equilibrium towards the products. Water had a strong effect on the activity of the enzyme and on Km. The concentration dependence of Km values was explained by the activity coefficients of the reacting agents. These were predicted with the thermodynamic models Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT), NRTL, and Universal Quasichemical Functional Group Activity Coefficients (UNIFAC), yielding the ratio of activity coefficients of products and reactants Kγ. All model parameters were taken from literature. The models yielded Kγ values between 25 and 115. Thus, activity coefficients have a huge impact on the consistent determination of the thermodynamic equilibrium constants Kth. Combining Km and PC-SAFT-predicted Kγ allowed determining Kth and the standard Gibbs energy of reaction as function of temperature. This value was shown to be in very good agreement with results obtained from group contribution methods for Gibbs energy of formation. In contrast, inconsistencies were observed for Kth using Kγ values from the classical gE-models UNIFAC and NRTL. The importance of activity coefficients opens the door for an optimized reaction setup for enzymatic esterifications. © 2017, Springer-Verlag Berlin Heidelberg.