Measurement and prediction of solvent effect on enzymatic esterification reactions

Abstract

The solvent effect on the equilibrium position of the esterification reaction of tyrosol with (R)-(+)-α-lipoic acid (RLA) catalyzed by Novozym 435® (Candida antarctica lipase B immobilized on macroporous acrylic resin), in organic solvents and ionic liquids of various polarities has been experimentally examined. The solubilities of tyrosol in these organic solvents and ionic liquids have been also measured. It is shown that there is a correlation between the solubility of tyrosol in the reaction medium and the conversion yield of the reaction. Actually, high substrate solubility leads to a low conversion yield, with the higher conversion yields observed for the hydrophobic ionic liquids where tyrosol solubility is lower. Furthermore, tyrosol solubilites in the organic solvents and ionic liquids used for the enzymatic reactions of tyrosol with RLA have been measured, while the application of COSMO-RS model resulted to satisfactory predictions. Finally, the prediction of the solvent effect on the equilibrium position of esterification reactions has been investigated. The COSMO-RS model was used to calculate the activity coefficients of the chemical species involved in various reactions, carried out in different solvents. When applicable, the UNIFAC model has been also employed for comparison purposes. It is shown that the COSMO-RS method can be successfully applied for solvent selection with general applicability both for organic solvents and ionic liquids as well as in any kind of substrates.