Enzymatic synthesis in biphasic aqueous-organic systems. I. Chemical equilibrium shift

A theoretical analysis of the causes of chemical equilibrium shift (i.e., change in product yield), accompanying replacement of water as the reaction medium by biphasic system "water-water-immiscible organic solvent", is given. A model is described, which is based on equilibrium partition of reagents between the two phases and establishment of a chemical equilibrium. In terms of this model, the apparent equilibrium constant, Kbiphasic which is evolved from the equilibrium concentrations of the reagents referred to the total volume of the biphasic system, should depend on the ratio of the volume of the organic and aqueous phases and the partition coefficients. The theoretical dependences were verified experimentally. Firstly, it was shown that for oxidation of isobutanol into isobutyraldehyde, catalyzed by alcohol dehydrogenase, the equilibrium shift that takes place in a water-hexane biphasic system is determined by the partition coefficients of the reagents found in an independent experiment. Varying the composition of the organic phase (hexane or ethyl acetate and their mixtures), the equilibrium could be shifted (compared to the aqueous solutions) towards both the initial reagents and the end-products: thereby changing the apparent equilibrium constant by two orders of magnitude. Secondly, alpha-chymotrypsin-catalyzed synthesis of ethyl ester of N-benzoyl-L-phenylalanine (from the respective acid and alcohol) in a biphasic system containing chloroform, benzene, carbon tetrachloride or diethyl ether, has been studied. The apparent equilibrium constant has been experimentally demonstrated to depend on the ratio of the aqueous and organic phase volumes in an extreme fashion, as has been predicted by the theory. The yield of the product in the reaction optimum amounts to 100%, whereas in water it is as low as 0.01%.