Lipase differentiation from esterase

Enantioface-differentiating conjugate additions have also been performed using modified hydrolytic enzymes14. Thus, the addition of diethylamine to ethyl ( )-4,4,4-trifluoro-2-butenoatc in an organic solvent is catalyzed by a lipase from Candida cylindracea, a-chy-motrypsin or pig liver esterase to afford ethyl (-)-3-diethylamino-4,4,4-trifluorobutanoate in moderate yield (36-47%) and low optical purity (8-37% ee, as determined by l9F NMR using a chiral shift reagent)14. 

The epidermis is an epithelium consisting of inner viable epidermis, a living hydrophilic layer, and outer nonviable epidermis, a hydrophobic layer made from dead cells. It is differentiated into stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale In the direction of dermis (Fig. 16.2). The viable epidermal layer has a thickness of about 0.02 to 0.2 mm. It is composed of many layers of keratinocytes, a widespread distribution of melanocytes, Langerhans cells, dendritic T cells, epidermotropic lymphocytes and Merkel cells, and a number of catabolic enzymes such as esterases, proteases, phosphatases, nucleotidases and lipases [Walters and Roberts, 2002 Barry, 2001). The outer and nonviable epidermis, namely, stratum corneum, is about 10 to 20 pm thick [Gregor and Ulrich, 2010). The stratum corneum Is deemed to be the major obstacle of drug permeation. 

Solubilisation of the lipases from both oat and wheat flours also leads to the release of esterase activity. Some partial separation of activity could be achieved in the case of wheat flour by differential solubilisation with varying concentrations of Tween 20. This technique also leads to some enrichment of lipase activity with respect to protein. 

This chapter illustrates the application of lipases and esterases as user-friendly biocatalysts in (i) desymmetrization of prochiral or meso-diols and diacetates, (ii) kinetic resolution of racemic alcohols, and (iii) preparation of enantiopure intermediate(s) from a mixture of stereoisomers by enzymatic differentiation. All the examples were taken from our own works in natural products synthesis. 

Although cyanohydrins are accessible with high enantiomeric purity via the above described oxynitrilase-catalyzed reactions, the resulting products are rather unstable with a tendency to racemization via their equilibrium with HCN and corresponding aldehydes. In contrast, enantiomerically pure cyanohydrin derivatives such as acetates are chemically quite stable also toward racemization. Therefore, bacterial ester hydrolases (esterases and lipases), which are well known for then capability of enantiomer differentiation in racemic esters and in particular commercially available lipases from Pseudomonas sp. (Amano, Japan), were employed for the preparation of enantiomerically pure cyanohydrin esters. 

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