WATER ACTIVITY FAILS TO PREDICT CRITICAL HYDRATION LEVEL FOR ENZYME-ACTIVITY IN POLAR ORGANIC-SOLVENTS - INTERCONVERSION OF WATER CONCENTRATIONS AND ACTIVITIES

When studying biocatalysts in organic media, it is useful to be able t o convert between water concentration and thermodynamic water activity (a(w)). We have obtained relationships for this purpose based on publ ished vapor-liquid equilibrium data for a range of commonly used polar solvents. Their use is illustrated by the reanalysis of literature da ta for the activity of an immobilized laccase. Kinetic constants measu red as a function of water concentration can thus be related to a(w). We show that the critical water content for significant V-m values is characterized by a similar a(w) (about 0.4) in five different alcohols ; however; the critical a(w) becomes progressively higher in other wat er-miscible solvents in the order: dioxane, acetonitrile, tetrahydrofu ran, acetone (critical a(w) about 0.8). This indicates that such polar solvents probably have important direct effects on the enzyme. (C) 19 97 by Elsevier Science Inc.