Microemulsion biocatalysis

An auspicious new strategy, in order to perform biocatalysis with hydrophobic substrates in w/o-microemulsion, is the usage of whole cells instead of purified enzymes [3,124,141]. There exist only a few surfactant-oil systems, in which whole cells are stable and suitable for a segmentation. Mainly the biodegradable surfactant based on sorbitan (Tween and Span) seems to be well suited for the solubilisation of whole cells in organic reaction media [142,143]. 

Biocatalysis and Microemulsions in Near-critical and Supercritical Fluids... 

In some cases, substrates and enzymes are not soluble in the same solvent. To achieve efficient substrate conversion, a large interface between the immiscible fluids has to be established, by the formation of microemulsions or multiple-phase flow that can be conveniently obtained in microfluidic devices. Until now only a couple of examples are published in which a two-phase flow is used for biocatalysis. Goto and coworkers [431] were first to study an enzymatic reaction in a two-phase flow in a microfluidic device, in which the oxidation ofp-chlorophenol by the enzyme laccase (lignin peroxidase) was analyzed (Scheme 4.106). The surface-active enzyme was solubilized in a succinic acid aqueous buffer and the substrate (p-chlorophenol) was dissolved in isooctane. The transformation ofp-chlorophenol occurred mainly at... 

The main practical problem in large-scale use of microemulsions as a medium for biocatalysis is that of workup. Separating surfactant from product is not a trivial issue, since normal purification procedures, such as extraction and distillation, tend to be troublesome due to the well-known problems of emulsion formation and foaming caused by the surfactant. 

In particular, in pharmaceuticals, agrochemicals, foodstuffs, and in biocatalysis functional components are preferably finely dispersed in an oily or an aqueous environment. Microemulsions are eminently suitable to serve that purpose, provided that the droplets are sufficiently large to solvate the functional molecules. By way of example, the relation between the radius of a water in oil microemulsion droplet and the number of water molecules it contains is given in Table 11.3. It shows that the droplet should have a size of at least a few nanometers to be able to dissolve (hydrate) polar compounds in its interior. 

SCFs are an environmentally friendly alternative to organic solvents as media for biocatalysis. A key feature of biocatalysis in SCFs is the tunability of the medium [75]. Enzymatic activity in SCFs has been proven and well documented [76]. Limiting factors, which may affect enzymatic activity in supercritical solvent systems, have been identified and are well characterized. A major limitation to the broader use of SCFs is their inability to dissolve a wide range of hydrophilic and ionic compounds, which greatly impedes their ability to carry out biolransformation with polar substrates. The interest in water-in-SCF microemulsion as reaction media stems from the fact that in such systems high concentrations of both polar and apolar molecules can be dissolved within the dispersed aqueous and continuous SCF phases, respectively. 

IL-based microemulsion as a green solvent has been extensively investigated in the fields of material synthesis, polymerization, biocatalysis, organic synthesis, drug release, protein extraction, and capillary electrophoresis. 

Nonaqueous IL microemulsions were also used as catalysts to improve reaction efficiency. Gayet et al. established an IL-in-oil microemulsion system with benzylpyridinium bis(trifluoromethanesulfonyl)imide ([BnPyrJNTfj), TX-lOO, and toluene, in which the Matsuda-Heck reaction between methoxybenzene diazotate and 2,3-dihydrofuran took place [46]. The reaction yield in this IL-in-oil microemulsion was twice as high as that in neat ILs. The results provided a basis for designing a nonaqueous IL microemulsion microreactor and also showed that nonaqueous IL microemulsion might have good prospects of applications in biocatalysis and nanomaterial synthesis. 

Valis, T.P, Xenakis, A., Kolisis, F.N. 1992. Comparative studies of lipase from Rhizopus delemar in various microemulsion systems. Biocatalysis 6, 267-279. 

Stamatis, H., Xenakis, A. 1999. Biocatalysis using microemulsion-based polymer gels containing lipase. J. Mol. Catal. B Enzym. 6, 399—406. 

Jenta, T.R.-J., Batts, G., Rees, G.D., Robinson, B.H. 1997. Biocatalysis using gelatine microemulsion-based organogels containing immobilized Chromobacterium viscosum lipase. Biotech. Bioeng. 53, 121-131. 

Properties such as large interfacial area and an ability to solubilize both oil-soluble and water-soluble reactants in a single phase system makes microemulsions ideal as reaction media (Flanagan and Singh, 2006 Gaonkar and Bagwe, 2002). For example, Morgado and co-workers (1996) nsed a continnons reversed micellar system to synthesize lysophospholipids and free fatty acids from lecithin hydrolysis, with applications to the food, pharmaceutical and chemical industries. Hydrolysis was catalyzed by porcine pancreatic phospholipase A. Carvalho and Cabral (2000) reviewed the use of reversed micellar systems as reactors to carry out lipase-catalyzed esterification, biocatalysis, transesterificadon, and hydrolysis reactions. 

From the perspective of apphcations, the traditional micellar enzymology has found potential apphcations in the biosynthesis and bioresolution of chiral drugs and in the preparation of biodiesel (via transesterification reaction). Microemulsion-based gelation creates favorable conditions for the reuse of enzymes. As a solvent, room-temperature ILs have potential advantages over molecular organic solvents, so the use of I L-based microemulsions will bring new opportunities for and give fresh impetus to biocatalysis and biotransformation. 

Moniruzzaman, M., Kamiva, N., and Goto, M. (2009) Biocatalysis in water-in-ionic liquid microemulsions a case study with horseradish peroxidase. Langmiur, 25, 977-982. 

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