Enantioselectivity of some lipases: Control and prediction

Different strategies for the resolution of racemates in lipase catalyzed hydrolyses, esterifications, and transesterifications are presented. The importance of the equilibrium conversion for the optimization of the enantiomeric excess is emphasized. Investigations have been carried out on esters of 1-phenylethanol and 2methylalkanoic acids. Results are presented, which provide general information about important parameters for the enhancement of the enantiomeric excess and the reaction rates. The application of a 'matched strategy' stereochemical control for increasing the enantiomeric excess is presented, i.e. the use of a chiral acid for the resolution of chiral alcohols. Molecular modelling of lipase from Rhizomucor mieliei as well as the use of a simple active site model of lipase from Candida cylindrucea to obtain information for the prediction and modification of enzyme specificity is presented. The use of lipases in organic synthesis has been the object of several investigations. Many lipases are commercially available, they accept a wide range of substrates, and they can be used for synthetic transformations in organic solvents (for ref. see 1). In this short account we shall report some recent results and general observations on parameters which enhance the enantiomeric excess in lipase catalyzed reactions. Our studies have been performed with some simple model substrates, such as esters of 1phenylethanol, and also with esters of 2-methylalkanoic acids, which are useful building blocks for the synthesis of various biologically active natural products, e.g. pheromone constituents of pine saw-flies (for ref. see 2). The use of molecular modelling and a simple active site model as tools for predictions and modifications of enzyme action will also be discussed. Structural data (ref. 3 6 ) confirm that lipases belong to the serine hydrolases, and that their action are most probably similar to those of other enzymes belonging to this group of hydrolytic enzymes, e.g. chymotrypsin. The kinetics of lipase-catalyzed transformations is complex, since lipases usually operate at hydrophobic interfaces and seem to require conformational changes for action. These complications have to be considered when the kinetics of lipase catalyzed reactions are studied. However, some general conclusions can be derived assuming homogeneous conditions. Calculations of the enantiomeric excess of a reaction product as a function of the conversion show that the enantiomeric excess of the product is not very much affected by a moderate change of the equilibrium conversion (Fig. la), The enantiomeric excess of the remaining substrate, however, is more dependent on the equilibrium conversion (Fig. lb). Thus, altering the reaction conditions so as to favour a high equilibrium conversion offers an opportunity to enhance the enantiomeric excess of the remaining substrate (ref. 7).