Bioreduction in Biphasic Aqueous-Organic Systems

The use of biphasic aqueous-organic systems in biocatalytic reductions is of great interest because the enzyme and its cofactor are dissolved in the aqueous phase, where the reaction takes place, while the hydrophobic substrate and product are mostly located in organic solvent layer and partitioned into the aqueous phase. This distribution reduces the concentrations of toxic substrate and product around the enzyme in aqueous layer and relieves the enzyme from substrate and product inhibition. Other distinctive features of this biphasic system are simple separation, easy regeneration of the enzyme, and easy recovery of the products. However, in this system, the reaction rates are relatively low because of a low rate of mass-transfer across the interface. Although this hindrance can be eliminated by intensive agitation, the increased interface often results in faster denaturation and inactivation of the enzyme. 

The requirement of good biocompatibility and partitioning behavior of substrate and product are restrictive criteria for the selection of a suitable solvent. Many attempts have been made to correlate the toxicity of different solvents to their physicochemical properties. Laane et al. first addressed this issue in a comprehensive fashion and pointed out that the logarithm of the partition coefficient, logP, as an indicator of solvent hydrophobicity, correlates best with enzyme activity [14]. The product recovery capacity and selectivity are quantified by the partition coefficient and the separation factor whose accurate values can only be obtained experimentally, but preliminary screening can be done with established databases [15]. 

Studies indicated that alkane compounds (e.g., hexane and heptane), which will not distort the water coat of enzyme molecules, are also good candidates. For example, an aqueous-hexane (1 1) biphasic system was identified to have an 

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