Influence of Natural Solutes and Ionic Liquids on the Yield of Enzyme-Catalyzed Reactions: Measurements and Predictions

The maximum yield of enzyme-catalyzed reactions is often limited by thermodynamic equilibrium. The knowledge of influencing factors on limitations of reactions is essential for process optimization to increase yields and to reduce solvent and energy consumption. In this work the effect of solvents/cosolvents [e.g., ionic liquid (IL)] and natural solutes on thermodynamic yield limitations of two enzyme-catalyzed model reactions were investigated, namely, an alcohol dehydrogenase (ADH) reaction (acetophenone + 2-propanol ⇌ 1-phenylethanol + acetone) and an alanine aminotransferase reaction (l-alanine + 2-oxoglutarate ⇌ pyruvate + l-glutamate). Experimental results showed that the equilibrium position and the equilibrium product yield of both reactions in aqueous single-phase systems strongly depend on the type and molality of the present natural solute/IL that were present as additives in the reaction mixture. In addition, the ADH reaction was investigated in pure IL and in an IL/buffer two-phase system. Compared to the aqueous reaction mixtures, the reactant solubility could be increased significantly, but at the cost of a lower product yield. Finally, thermodynamic modeling by means of ePC-SAFT was used to predict the equilibrium product yield of both reactions at different reaction conditions (natural solute/IL type and molality) in the aqueous mixtures as well as in the IL. Experimental and predicted results were in good agreement, showing that ePC-SAFT is a promising tool for predicting yield limitations in different reaction media

Influence of pH Value and Ionic Liquids on the Solubility of l‑Alanine and l‑Glutamic Acid in Aqueous Solutions at 30 °C

The solubility of the amino acids l-alanine and l-glutamic acid and its sodium salt (sodium l-glutamate monohydrate) in aqueous solutions at 30 °C and atmospheric pressure was investigated in the pH range between 3 and 9 and in the presence of the ionic liquids (ILs) 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]­[OTf]) and choline dihydrogencitrate ([ch][dhcit]) at pH 7. The solubility of l-alanine and l-glutamic acid in the solutions without IL was measured by UV spectroscopy and with a gravimetrical method. In the presence of an IL HPLC-analysis was applied. The solid phases were characterized using Raman spectroscopy and powder X-ray diffraction to distinguish the amino acids from their salts. While the solubility of l-alanine did not depend on pH within the considered pH range, the solubility of l-glutamic acid strongly increased with increasing pH. Below pH 6.2 the solid phase was characterized to be l-glutamic acid, while sodium l-glutamate monohydrate was found to be the solid at pH higher than 6.2. It could be observed that the solubility of sodium l-glutamate monohydrate was comparatively high, and increased with increasing pH. Upon addition of the ILs under investigation ([bmim]­[OTf]) and [ch]­[dhcit]) the solubility of l-alanine and l-glutamic acid was decreased. Original PC-SAFT was applied to predict the solubility of l-alanine and l-glutamic acid (and its sodium salt) in water, with and without the ILs under consideration, at the experimental conditions with quantitative agreement to the experimental data