Kinetics of lipase-catalyzed esterification in organic media: Correct model and solvent effects on parameters

The Ping-Pong model (incl. alcohol inhibition) is not the correct model to describe the kinetics of a lipase-catalyzed esterification reaction. The first product, water, is always present at the start of the reaction. This leads to an equation with one extra parameter. This new equation fits our experimental data on the esterification of sulcatol and fatty acids in toluene, catalyzed by Candida rugosa lipase. The new model does not significantly improve the mean square of the fit; however, using a model which can be expected to be more correct, results in the conclusion that a larger part of the differences can be explained by substrate solvation. For comparison of the kinetic constants in different solvents, it is essential to make corrections for solvation. The deviation from the average corrected kinetic constant shows to what extent differences can be explained by substrate solvation and an effect on the enzyme. We have made corrections for solvation with the new model for the esterification in toluene, hexane, trichloroethane, and diisopropyl ether. This has resulted in kinetic constants that deviate less from the average value.