Membrane-immobilized transaminases for the synthesis of enantiopure amines†

Abstract

For the manufacture of enantiopure amines, greener synthesis processes are needed. Transaminases (TAs) are able to produce chiral amines with excellent enantioselectivity and in mild conditions, and can be immobilized to target stability, recoverability, and reusability. In the perspective of process intensification, we propose to study TA immobilization onto polymeric membranes. Two main immobilization strategies were investigated, requiring prior membrane surface functionalization. On the one hand, a polyacrylonitrile (PAN) membrane surface was partially hydrolyzed and coated with polyethyleneimine (PEI) to electrostatically trap TAs. On the second hand, a polypropylene (PP) membrane was coated with polydopamine (PDA), which was subsequently modified with glycerol diglycidyl ether (GDE) in order to covalently graft TAs. The successful membrane functionalization was confirmed by surface characterization techniques (infrared spectroscopy, X-ray photoelectron spectroscopy, contact angle measurements, and scanning electron microscopy). Enzyme leaching was observed from the functionalized PAN membrane, highlighting the need to post-treat the reversibly immobilized TAs to improve their anchoring. The covalent coupling of TAs with PEI using glutaraldehyde (GA) was found highly effective to avoid leaching and to increase the enzyme loading, without affecting the specific activity of the biocatalyst. Similarly, the covalent grafting of TA onto functionalized PP membranes yielded very efficient biocatalysts (retaining 85% specific activity with respect to soluble TA) displaying perfect recyclability throughout successive cycles. Immobilizing either the S-selective HeWT or the R-selective TsRTA resulted in robust heterogeneous biocatalysts with antagonist enantioselectivities. Thus, chiral amine synthesis can be performed effectively with biocatalytic membranes, which paves the way to intensified continuous flow synthesis processes.