Abstract
Pseudomonas cepacia lipase (PCL), which catalyzes transesterification between 1,2,3-tripropanetriyl tributyrate (1) and aliphatic secondary alcohol 2 to give the corresponding (R)-butyrate 3, has been studied using twelve organic solvents and fifteen alcohols at 30 °C. Among systems studied, 4-nonanol in carbon tetrachloride was only the case where no reaction was observed. The plots of the enantioselectivity (E value) against carbon number (CN) of 2 showed the minima at CN = 8, 9 and 10 for 2-, 3- and 4-alkanols, respectively. The log E values showed a rough negative correlation with the solvent hydrophobicity (log P) except bulky solvents, in which the log E values were smaller than those expected from log P. Their correlations were different depending on the structure of 2. The initial rate constants (k) of enantiomers in racemic 2 were estimated. The k value of (S) isomer showed the maxima at CN = 8, 9 and 10 for 2-, 3- and 4-alkanols, respectively. The k values showed a rough positive correlation with log P except bulky solvents, in which these values were larger than those expected from log P. The slope of the log k vs. log P plot for (R) isomer was smaller than that for the counterpart, and their correlation was also varied with the structure of 2.
Based on the above results, we concluded that the enantioselectivity was affected by a combination of the structures of substrate and solvent and the solvent hydrophobicity. A plausible active site model involving a participation of the solvent for PCL catalyzed transesterification was proposed and discussed.
