Enzymatic Reactions in Non-Conventional Media: Prediction of Solvent Water Content for Optimum Water Activity

Abstract

Most enzymes provide their optimum performance at a given water activity (aw), which is generally solvent independent. For a given organic liquid solvent at a specific temperature or for a supercritical solvent at a specific temperature and pressure this corresponds to a water concentration in which water has the desired activity. We present here a methodology for predicting this water concentration thus reducing substantially the amount of experimental work needed to find the optimum solvent with respect to equilibrium conversion.If the enzyme optimum water activity is known, the methodology predicts the required water content in the solvent to achieve this aw value. If, in addition, the enzyme water activity curve is available, this methodology provides the total water that must be added to the system (enzyme plus solvent) so that a specific water activity can be obtained.The same methodology can also be applied to predict the effect of the total water content of the system (initial or initial plus produced) on the water activity values. It is shown that: (a) for esterification reactions taking place in hydrophobic organic solvents, the produced water can lead to a substantial change in water activity, but not for less hydrophobic solvents; (b) introduction of dry CO2 into a system, pre-equilibrated to a certain water activity at atmospheric pressure, can lead to a substantial decrease in the water activity especially at temperatures just above the critical one of the solvent and pressures larger than that.