Active Site Model of Lipases and Control of Stereoselectivity of Lipase-catalyzed Transesterifications

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Stereoselectivity of lipase-catalyzed transesterificationsEnzymes have become increasingly popular as chiral catalysts in organic synthesis and lipases are especially attractive for this purpose because they are relatively inexpensive can function in both organic and aqueous solutions, are simple to use and show high enantioselectivity for a broad range of substrates. We now examined stereochemistry of enantioselective transesterification of racemic alcohols in organic solvents by using lipase QL which is readily available and inexpensive. On the basis of the observed enantioselectivities, we proposed the active site model for lipase QL from Alcaligenes sp.as a rule of thumb to identify the primary and secondary alcohols which can be accommodated in the active site and their faster-reacting enantiomer in this acylation.Synthesis of optically active crown ethers and their enantiomer selectivities in the complexation with chiral aminesThe knowledge of the enantiomer selectivity in the complexation of optically active crown ethers with chiral amines serves as an important information for studing the chiral recognition in the formation of an enzyme-substrate complex.We now found the temperature dependent reversal of the enantiomer selectivity in the complexation of crown ethers with the amine and the enantiomer selectivities increased with increasing temperature above the isoenantioselective temperature. It is the generally accepted view that lower temperatures enhance the enantiomer selectivity in chiral processes but our results suggests that the view is not always correct.