Abstract
Pseudomonas cepacia lipase (PCL) catalyzed transesterification between tributyrylglycerol (1) and 2-alkanol (2) was studied at 30°C using various organic solvents. Log E (E : the enantiomeric ratio) showed a rough negative correlation with the solvent hydrophobicity (log P) and inverse of the dielectric constant (D) of the solvent except in the case of bulky solvents. Log E vs. log P and 10/D plots varied according to the structure [carbon number (CN)] of (2). E was minimal at CN=8 and 10 for (2). The initial rate constant (k/s-1mg-1) was measured for (R) - and (S) -2-octanols [(R) - and (S) - (2b)]. Log k showed a rough positive correlation with log P, but was essentially independent of D except for bulky solvents. The slope of log k vs. log P for (R) - (2b) was less than for (S) - (2b).
Enantioselectivity (E value) is thus shown to be affected by solvent and substrate (2) structures and solvent features. A plausible active site mode of PCL catalyzed transesterification was proposed and discussed.
