ENANTIOSELECTIVITY OF LIPASE-CATALYZED TR ANSESTERIFICATION 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 b utyrate (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 hy drophobicity log P (where P is a partition coefficient of a given solv ent between octanol and water) except bulky solvents, in which the log E values were smaller than those expected from log P. The log E again st log P plot was varied by the reaction temperature (T). The enantios electivity increased with T for the reaction in the bulky solvents, wa s independent of T in heptane, and decreased in the others. Activation parameters (Delta Delta H-double dagger and Delta Delta S-double dagg er) were determined using a linear correlation between Delta Delta G(d ouble dagger) (= -RT In E) and T. The Delta Delta H-double dagger valu es were negative for the reactions in the bulky solvents and positive in the others, and the Delta Delta S-double dagger values in the forme r were larger than those in the latter. From the above findings, it wa s concluded that the enantioselectivity was affected by a combination of the nature and structures of the solvent and the reaction temperatu re. A plausible active site model of PCL was proposed and discussed.