Water Effects on Equilibria

One of the most common reasons for using low-water media for enzymatic reactions is that one wants to use a hydrolase for catalyzing reactions other than hydrolysis. In low-water media these enzymes can be efficiently used to catalyze reversed hydrolytic reactions and various types of tranferase reactions. 

It is clear that the water activity is of crucial importance for the equilibrium yield in a reversed hydrolysis reaction. As expected, the equilibrium yield increases with decreasing water activity. This has been shown, for example, for the condensation of glucose and octanol [62], esterification of lysophospholipids with fatty acids [29, 63], and in normal lipase-catalyzed esterification reactions [64, 65]. The same situation is observed in ionic liquids [66]. 

In addition to its direct influence via the water activity in the system, the amount of water also influences the activity coefficients of the other components in the mixture, and therefore equiUbrium constants like K0 can vary with the water activity in the system (Table 1.5) [29, 63, 64]. This can be seen as a solvent effect on the equilibrium constant The tendency in esterification reactions is that increases with decreasing water activity, which means that it is favorable to use low water activity because of both the direct effect of water activity on the equilibrium and the influence of water on K0. 

It is worth pointing out that the equilibrium positions in many esterification reactions in hydrophobic solvents are quite favourable, so that high yields can be obtained even if the water activity is close to 1. In these cases, it is the effective solvation of the ester product in the medium that is a main driving force the reaction. This is further discussed below. 

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