Enzymatic reactions in non-conventional media: Prediction of solvent watercontent for optimum water activity

Most enzymes provide their optimum performance at a given water activity (a ,), which is generally solvent independent. For a given organic liquid solv ent 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 predicti ng this water concentration thus reducing substantially the amount of exper imental 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 a(w) value. If, in a ddition, the enzyme water activity curve is available, this methodology pro vides 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 wate r activity values. It is shown that: (a) for esterification reactions takin g place in hydrophobic organic solvents, the produced water can lead to a s ubstantial change in water activity, but not for less hydrophobic solvents; (b) introduction of dry CO2 into a system, pre-equilibrated to a certain w ater activity at atmospheric pressure, can lead to a substantial decrease i n the water activity especially at temperatures just above the critical one of the solvent and pressures larger than that.