Enantioselectivity of lipase catalyzed transesterification between tributyrylglycerol and 2-octanol in organic solvents

Pseudomonas cepacia lipase (PCL) catalyzed transesterification between tributyrylglycerol (1) and 2-octanol (2) to give (R)-1-methylheptyl butyrate (3) has been studied using eight kinds of organic solvents at 10-70 degrees C. The enantioselectivity expressed as log E (where E is the enantiomeric ratio) was negatively correlated with the solvent hydrophobicity log P (where P is a partition coefficient of a given solvent between octanol and water) except bulky solvents, in which the log E values were smaller than those expected from log P. The log E against log P plot was varied by the reaction temperature (T). The enantioselectivity increased with T for the reaction in the bulky solvents, was independent of T in heptane, and decreased in the others. Activation parameters (Delta Delta H-double dagger and Delta Delta S-double dagger) were determined using a linear correlation between Delta Delta G(double dagger) (= -RT In E) and T. The Delta Delta H-double dagger values were negative for the reactions in the bulky solvents and positive in the others, and the Delta Delta S-double dagger values in the former were larger than those in the latter. From the above findings, it was concluded that the enantioselectivity was affected by a combination of the nature and structures of the solvent and the reaction temperature. A plausible active site model of PCL was proposed and discussed.