ENZYME-ACTIVITY IN SUPERCRITICAL FLUIDS

Supercritical fluids are materials above their critical point that represent a unique class of nonaqueous media for biocatalysis and bioseparation. The inherent gas-like low viscosities and high diffusivities of supercritical fluids increase the rates of mass transfer of substrates to enzyme. Conversely, the liquid-like densities of supercritical fluids result in higher solubilizing power than those observed for gases. Unlike the behavior of gases and liquids, the physical properties of a supercritical fluid can be adjusted over a wide range by a relatively small change in pressure or temperature. In a supercritical fluid, the careful regulation of the density enables reactant and product solubility to be controlled, thus simplifying downstream separations. The extraction power of supercritical carbon dioxide has been used extensively in both the chemical and food industries. The use of supercritical fluids as a dispersent for biocatalysis was first described in 1985, and there is now a growing trend in using supercritical fluids as reaction media for enzymatic catalysis. The advantages of using enzymes in supercritical fluids include the following: 1. Synthesis reactions in which water is a product can be driven to completion. 2. The solubilities of hydrophobic materials are increased relative to those in water. 3. The thermostability of biomolecules in supercritical fluids is greater than in water. 4. The solvent can be readily recycled. 5. Biochemical reactions and separations can be integrated into a single step. Among potentially interesting solvents for enzymatic catalysis, carbon dioxide is the most widely used supercritical fluid. However, there is a growing interest in using other supercritical fluids (e.g., ethylene, fluoroform, ethane, sulfur hexafluoride, and near-critical propane). In this review, we focus on describing enzymatic catalysis in supercritical fluids performed to date, and we address the fundamental issues associated with supercritical fluid-based biocatalysis.