Lipase-catalyzed reaction in the packed-bed reactor with continuous extraction column to overcome a product inhibition

The resolution of rac-α-methyl-β-propiothiolactone (rac-MPTL) was performed in a packed-bed reactor (PBR) using Pseudomonas cepacia lipase (PCL) in organic media to produce enantiopure (R)-MPTL. By comparing enzyme stability of three enzyme forms, i.e. commercial PCL powder, Celite-immobilized PCL, and cross-linked enzyme crystals of PCL (CLECs-PCL), Celite-immobilized PCL was chosen for the construction of PBR because of its comparable stability to that of CLEC and easy handling. Owing to the severe product inhibition by 3-mercapto-α-methylpropionic acid (MMPA), the batch reaction system was inappropriate for the hydrolysis with PCL at high concentration of rac-MPTL. To overcome these problems, PBR with a continuous extraction column was used. The product inhibition was successfully overcome by incorporating an aqueous extraction unit. However, the yield of (R)-MPTL (enantiomeric excess, ee>99%) was only about 20% based upon the initial concentration of rac-MPTL, due to the concomitant partitioning of rac-MPTL to the aqueous phase in the extraction column as well as the subsequent auto-hydrolysis of rac-MPTL to rac-MMPA during extraction. To reduce the undesired partitioning of rac-MPTL to aqueous phase, various salts were screened to modulate the partitioning coefficient of the reaction components. Using 1 M ammonium sulfate solution as the aqueous phase of the extraction column, the yield of (R)-MPTL (ee>99%) was enhanced to 40%.