Enantioselectivity of Lipase Catalyzed Transesterification between Tributyrylglycerol and 2-Octanol in Organic Solvents

Abstract

Pseudomonas cepacia lipase (PCL) catalyzed transesterification between tributyrylglycerol (1) and 2octanol (2) to give (R)-1-methylheptyl butyrate (3) has been studied using eight kinds of organic solvents at 10-70 °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 ( ΔΔH‡ and ΔΔS‡) were determined using a linear correlation between ΔΔG‡ (= RT ln E) and T. The ΔΔH‡ values were negative for the reactions in the bulky solvents and positive in the others, and the ΔΔS‡ values in the former were larger than those in the latter.
From the a bove 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.