Chiral epoxide production using mycobacterium solubilized in a water-in-oil microemulsion

Microemulsion Microstructure(s): A Tutorial Review

Microemulsions are thermodynamically stable, transparent, isotropic single-phase mixtures of two immiscible liquids stabilized by surfactants (and possibly other compounds). The assortment of very different microstructures behind such a univocal macroscopic definition is presented together with the experimental approaches to their determination. This tutorial review includes a necessary overview of the microemulsion phase behavior including the effect of temperature and salinity and of the features of living polymerlike micelles and living networks. Once these key learning points have been acquired, the different theoretical models proposed to rationalize the microemulsion microstructures are reviewed. The focus is on the use of these models as a rationale for the formulation of microemulsions with suitable features. Finally, current achievements and challenges of the use of microemulsions are reviewed.

Microemulsion systems for catalytic reactions and processes

This mini-review shows the diversity of microemulsion systems for catalytic reactions with the potential for process development.

The potential of cloud point system as a novel two-phase partitioning system for biotransformation

Although the extractive biotransformation in two-phase partitioning systems have been studied extensively, such as the water-organic solvent two-phase system, the aqueous two-phase system, the reverse micelle system, and the room temperature ionic liquid, etc., this has not yet resulted in a widespread industrial application. Based on the discussion of the main obstacles, an exploitation of a cloud point system, which has already been applied in a separation field known as a cloud point extraction, as a novel two-phase partitioning system for biotransformation, is reviewed by analysis of some topical examples. At the end of the review, the process control and downstream processing in the application of the novel two-phase partitioning system for biotransformation are also briefly discussed.

Novel interface-binding chloroperoxidase for interfacial epoxidation of styrene

A unique interface-binding chloroperoxidase (CPO) was developed and examined for interfacial biocatalysis. Native CPO was conjugated with polystyrene (PS) to form a surfactant-like structure that self assembled at oil-water interfaces. While enantioselectivity of the enzyme was maintained, the interfacial assembly of the enzyme improved its overall catalytic efficiency as compared to that observed with the enzyme contained in the bulk aqueous phase. The PS conjugated CPO (PS-CPO) showed a 2.5-fold enhancement of enzyme productivity versus native CPO under batch reaction conditions for the epoxidation of styrene, whereas a 25-fold improvement was realized in a continuous feeding reaction to reach a productivity of 10 micromol h-1 mg protein-1. The interface-binding enzyme also demonstrated several other advantages such as suppressing unwanted side reactions including the hydrolysis of styrene epoxide products, stabilizing the enzyme by limiting its exposure to both the oxidant H2O2 and epoxide products, and alleviating the deactivating effect of interfacial stress on enzymes by functioning as a surfactant.

Saccharides as efficacious solubilisers for highly lipophilic compounds in aqueous media

The bioavailability of lipophilic substrates is critical for biotransformations with isolated enzymes as well as with whole cells. With the example of a series of lipophilic ketones the suitability of saccharides as potent solubilisers for highly lipophilic substrates was demonstrated. Best results were obtained for d-glucose, which increased substrate solubility up to 50 times. In whole-cell biocatalysis the sugar acts both as solubiliser and as carbon source for which reason this procedure does not impair cell physiology and is unique in being environmentally benign. The capability of saccharides to solubilise lipophilic compounds in aqueous media sources from their ability to form hydrophilic and lipophilic domains at hydrophobic interfaces, thus forming cyclodextrin-like structures around the lipophilic substrate.

Oxidative biotransformations using oxygenases

Considerable progress has been made in manipulating oxidative biotransformations using oxygenases. Substrate acceptance, catalytic activity, regioselectivity and stereoselectivity have been improved significantly by substrate engineering, enzyme engineering or biocatalyst screening. Preparative biotransformations have been carried out to synthesize useful pharmaceutical intermediates or chiral synthons on the gram to several-hundred-gram scale, by use of whole cells of wild type or recombinant strains. The synthetic application of oxygenases in vitro has been shown to be possible by enzymatic or electrochemical regeneration of NADH or NADPH.

Chiral epoxide production using mycobacterium solubilized in a water-in-oil microemulsion

The application of many biotransformation processes is limited because the substrates/products are poorly water soluble, can be further metabolized, or are inhibitory. Hence non-aqueous media (e.g. two-phase systems, low water environments) are being examined to determine whether they can be used to overcome these problems. One novel approach is to encapsulate whole cells in water-in-oil (w/o) microemulsions (reverse micelles). In this study we have investigated the influence of key system parameters on system stability and epoxidation activity of Mycobacterium M156 cells in reverse micelles comprised of a mixture of Tween 85 and Span 80 (10-20 w%, with an hydrophilic/lipophilic balance [HLB] of 10 and a weight ratio of Tween 85 to Span 80 = 5.7) in n-hexadecane. It was found that the minimum allyl phenyl ether (APE) concentration required in the bulk hexadecane solvent phase for epoxidation to occur was 15 mM, whereas the minimum molar ratio of water to surfactant (W(0)) was 35. The optimum epoxidation rate achieved was 3.8 nmol/mg dwt-min with an APE concentration of 50 mM, and a W(0) of 50, with an enantiomeric excess (ee) of 86%. However, epoxidation was found to terminate approximately 3 h after initiation, and the causes for this were postulated to be either: the deleterious effect of the solvent on the Mycobacteria; inactivation of the energy generating system; an insufficient energy supply, or; the instability of the monooxygenase enzyme. It was concluded that on balance emulsion systems are not an economically viable system for producing phenyl glycidyl ether (PGE).