Substrate sorption into the polymer matrix of Novozym 435 and its effect on the enantiomeric ratio determination

In the enantioselective esterification of 4-methyloctanoic acid with ethanol by immobilised Candida antarctica lipase B (Novozym 435®), the enantiomeric excesses determined during the course of the reaction deviated strongly from the theoretical values, leading to unacceptably large confidence intervals for the enantiomeric ratio (E value). This observation was in contrast to our previous findings for transesterification and hydrolysis reactions with this enzyme. Herein, the three reactions are compared; the anomalous results in the esterification reaction appear to be caused by adsorption of 4-methyloctanoic acid inside the enzyme beads. We found that on average 1.19 g of 4-methyloctanoic acid was incorporated per g of Novozym 435®. If the concentration of this substrate was adjusted accordingly in the calculations, the resulting E values showed acceptable confidence intervals. In previous research on transesterification reactions in excess apolar solvent (comparable affinity for the beads), sorption does not play an important role because only small amounts of substrate were lost. For hydrolysis reactions, sorption takes place but the acid is released from the beads upon titration and no effect on the E value is found. However, for esterification reactions, sorption should not be neglected since there is no driving force to release the acid from the beads