Nonaqueous solvents, biocatalytic

Biocatalytic reduction has been performed in nonaqueous solvents to improve the efficiency of the reaction. This section explains the use of organic solvent, supercritical fluids, and ionic liquid. 

In biocatalytic systems, catalase is mainly used in immobilized state. High activity of immobilized catalase was achieved on its sorption immobilization on cellulose [6], on silica gel modified with fatty acids or phospholipids [7] as well as on activated carbon fibres and brics/tissues/ [8]. Biocatalytic activity of catalase immobilized on cellulose was also studied in nonaqueous solvents [9,10]. In [9] it was found that unlike the enzyme dissolved in water-dimethylformamide medium, on the oxidation of o-dianisidine in the presence of dimethylfonnamide the immobilized catalase does not show any peroxidase activity. It was used [10] for working out an organic-phase amperometric biosensor by immobilizing the enzyme in a polymeric film on a glass-carbon surface. 

Over the past three decades, an increasing concern was put on application of nonaqueous solvents to facilitate biocatalytic reactions where several industrially attractive advantages are presented, such as increased solubility of nonpolar substrates, reversal of hydrolysis reactions, alternation of enzyme selectivity, and suppression of water-dependent side reactions. However, there are some inherent problems and technical challenges, including inactivation of biocatalysts, potentially reduced protein stability and lowered reaction rates due to mass-transfer limitations, and/or the increased rigidity of protein structure. 

Numerous studies have revealed that biocatalytic reductions can be performed well in the reaction media containing nonaqueous solvents, which vary from conventional organic solvents to greener solvents, such as ionic liquids and supercritical fluids. The use of nonaqueous solvents has not only enhanced the efficiency of bioreductions by allowing the reactions to be conducted at high substrate concentrations, but also altered enzymatic selectivity, including chemo-, regio-and enantioselectivities. 

Biocatalytic enzymes are usually best stored in the solid form. In favorable cases, they can even be employed in reactions in this form (typically in nonaqueous solvents since the solids normally dissolve in water). Enzyme solids are usually made by removing water from an initial aqueous solution by lyophilization, spray drying, or precipitation. Solid enzymes maximize the catalytic activity per unit weight, which can be important on very large scales. In addition, having enzymes in solid form avoids the pitfalls associated with cell growth and isolation, enzyme preparation, stabilities and transport of enzyme solutions. A large number of enzymes are available in powder form and this is a common form used by suppliers. 

The use of organic solvents as reaction media for biocatalytic reactions can not only overcome the substrate solubility issue, but also facilitate the recovery of products and biocatalysts as well. This technique has been widely employed in the case of lipases, but scarcely applied for biocatalytic reduction processes, due to the rapid inactivation and poor stability of redox enzymes in organic solvents. Furthermore, all the advantages for nonaqueous biocatalysis can take effect only if the problem of cofactor dependence is also solved. Thus, bioreductions in micro- or nonaqueous organic media are generally restricted to those with substrate-coupled cofactor regeneration. 

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