Disperse soluble immobilized enzyme

The biologically active enzymes can be used as catalysts either in a soluble, dispersed form or in a carrier-bound form. Because of the need for the isolation step and losses of enzyme activity, enzyme processes are sparingly used at the present time. Immobilized enzymes show promise for minimizing these activity losses and for facilitating enzyme recovery (Pitcher, 1978). 

Because enzymes are insoluble in organic solvent, mass-transfer limitations apply as with any heterogeneous catalyst. Water-soluble enzymes (which represent the majority of enzymes currently used in biocatalysis) have hydrophilic surfaces and so tend to form aggregates or stick to reaction vessel walls rather than form the fine dispersions that are required for optimum efficiency. This can be overcome by enzyme immobilization, as discussed in Section 1.5. 

The prospect of using enzymes as heterogeneous catalysts in scC02 media has created significant interest. Their low viscosity and high diffusion rates offer the possibility of increasing the rate of mass-transfer controlled reactions. Also, because enzymes are not soluble in supercritical fluids, dispersion of the free enzymes potentially allows simple separations without the need for immobilization. 

Phthalocyanine (Pc) complexes of transition metals have received much attention in the scientific literature of the last two decades, not only as mild catalysts for selective oxidation reactions but also as functional models for enzymes. Unfortunately, their use is hampered by their reduced solubility in solvents and their tendency to form adducts even when used in solution. Provided such complexes can be immobilized individually on a catalyst carrier, it is expected that an enhanced dispersion of the complex will be achieved. The use of heterogenized Pc complexes will also no longer be restricted by the nature of the reaction medium or solvent. The issue has been reviewed as early as 1986 [3]. 

Because enzymes are not soluble in SCFs it should be possible to disperse free enzyme in the SCF and recover the enzyme without the need to immobilize it on a support. 

The conditions required to favor esterification can be obtained in different manners. It is possible to add a water-miscible solvent that will lower the water concentration and increase the solubility of organic substrates and products. It is also possible to work in a two-phase system with a non-water-miscible solvent, which will serve as a reservoir for the substrates and products. This can be achieved either with macroscopic phases or with highly dispersed systems such as reversed micelles. In the above-mentioned cases, the enzyme-catalyzed reaction takes place in the aqueous phase or at the phase interface. The enzyme can be dissolved in this phase or immobilized by covalent attachment to a solid carrier... 

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