Enzymatic resolution of racemates contaminated by racemic product

Enzyme-catalyzed kinetic resolution is sometimes performed starting with substrate already containing small amounts of the racemic product. Then the determination of the enantiomeric ratio may be seriously disturbed when this parameter is calculated from the degree of conversion and the enantiomeric excess of either the substrate or the product (Chen et al., 1982, 1987) or when it is calculated directly from the enantiomeric excess of substrate and product (Rakels et al., 1993). This paper presents modifications of these methods in order to correctly determine the enantiomeric ratio as well as the amount of racemic product in the substrate. The theoretical predictions were verified for the hydrolysis of racemic ethyl 2chloropropionate, catalyzed by carboxylesterase NP. Despite the presence of racemic product in the substrate, accurate and reliable values for the enantiomeric ratio were obtained by using the modified methods.