Reaction rate with suspended lipase catalyst shows similar dependence on water activity in different organic solvents

We have studied the effect of thermodynamic water activity (a W) on the initial rate of esterification catalysed by an immobilised lipase (Lipozyme) suspended in an organic reaction mixture. The catalyst and the organic phase were separately pre-equilibrated to the same aw value. The rate shows similar dependence on aw in reaction mixtures based on five different organic solvents ranging in polarity from pentan-3-one to hexane, and in a liquid reactant mixture. There is a maximum at aw about 0.5, with a decline to 30-70% at aw of either 0.9 or less than 0.01. When the rates are presented in terms of water concentration in the organic phase (or total water content of the system), the maxima for the various solvents come at very different positions, reflecting the widely varying solubilities of water in the organic phase.