Synthesis of Esters Catalyzed by Lipases in Water- in-Oil Microemulsions

There are two basic advantages in using enzymes as catalysts in organic media instead of aqueous solutions. First, organic solvents favor the solu­ bility of hydrophobic substrates and, second, the presence of such solvents shifts the thermodynamic equilibrium of condensation/hydrolysis reactions in favor of the desired product. Different approaches have been proposed to facilitate the reversal of the normal hydrolytic action of enzymes. These include various macroheterogeneous biphasic systems such as liquid-liq­ uid systems composed of a water-immiscible organic solvent and water, nearly anhydrous systems in which the enzyme is usually suspended as a powder or in an immobilized form adsorbed onto a suitable carrier in organic solvents or gases in a supercritical state, and various homogeneous and microheterogeneous media such as mixtures of water-miscible organic solvent and water as well as different types of microemulsion system (re­ verse micelles). The subject of enzyme catalysis in media with low water content has been reviewed by several authors (1~6). Water-in-oil (w/o) microemulsions are thermodynamically stable, optically isotropic colloidal dispersions of water in oil (any apolar solvent) stabilized by surfactant molecules (7). The surfactant molecules are adsorbed spontaneously at interfaces and separate the nonpolar and aque­ ous phases, thus decreasing the interfacial tension down to very low values (10 s dyn/cm). The optical transparency of the system is the result of one liquid being finely dispersed into the other, forming microdroplets (reverse