In general, amines are organic compounds comprising a functional group that contains a basic nitrogen atom with a lone pair, which can bind a proton to form an ammonium ion. Amines and, particularly chiral amines, are ubiquitous in biology and industry. Amines, particularly chiral amines, are key building blocks in many pharmaceutical, agrochemical and chemical applications. Chiral amines are particularly important in the production of physiologically active compounds. Moreover, in many applications of chiral amines, only one particular optically active form, either the (R) or the (S) enantiomer has the desired physiological activity.
(Chiral) amines can be produced both by chemical and biocatalytic synthesis routes. Chemical synthesis of optically active chiral amines via a one-step procedure requires high chemo-, regio-, diastereo-, and enantiocontrol. One of the biocatalytic synthesis routes to obtain chiral amines uses the enzyme transaminase (EC 2.6.1.X; also known as aminotransferases). Transaminases are pyridoxal phosphate dependent enzymes and catalyze the transfer of an amine (—NH2) group from an amine donor, for instance an amino acid or a simple amine such as 2-propylamine, to a pro-chiral acceptor ketone, yielding a chiral amine as well as a co-product ketone or alpha-keto acid, in the presence of the cofactor pyridoxal phosphate (PLP) which is continuously regenerated during the reaction (FIG. 1). Transaminases, particularly (R)- and (S)-selective transaminases, have received much attention as suitable catalysts for producing chiral amines, as they allow the direct asymmetric synthesis of (optically active) chiral amines from pro-chiral ketones.
Although the transaminase catalysed synthesis of chiral amines presents a high enantio- and regioselectivity, the transamination reaction is a reversible reaction, often with an unfavourable thermodynamic equilibrium which limits obtaining high chiral amine yields. Accordingly, an amine mixture is obtained comprising the chiral amine and the amine donor, requiring further purification of the chiral amine. In addition, the transaminase catalysed chiral amine synthesis is prone to substrate and product inhibition. In addition, substrate solubility issues may hinder the reaction as well.
Börner et al. (Organic Process Research & Development, 2015, 19(7), 793-799) describes an enzymatic chiral amine synthesis process setup combined with a selective solvent extraction of the product chiral amine, comprising a so-called reaction phase and a so-called stripping phase at different pH, separated by a supported liquid membrane, in casu a membrane with the pores loaded/impregnated with undecane. In the reaction phase (at pH≥9), the transaminase reaction takes place with isopropyl amine (IPA) or alanine (ALA) as amine donor. Although all uncharged substrate and product compounds can be (selectively) extracted by the organic solvent of the liquid membrane and can pass through the liquid membrane to the stripping phase (at pH<3), back extraction from the stripping phase into the organic membrane phase is prevented for the charged compounds (in casu the non-aminoacid amine donor and amine product in the stripping phase). This setup results in a selective in situ product removal, which steers the reaction equilibrium towards chiral amine synthesis. However, the liquid-membrane setup used in this study is not stable due to the liquid in the membrane pores being prone to leaking out during operation, requiring regularly regeneration of the liquid membrane, and is sensitive to transmembrane pressure differences. This setup is thus less suitable for long-term, large scale chiral amine synthesis and separation.
WO01/09042 discloses a process for recovering an aromatic amine dissolved in an aqueous fluid comprising transferring the undissociated aromatic amine from the aqueous fluid to an acidic stripping solution across a membrane, wherein the membrane is a non-porous, elastomeric (polymeric) selectively permeable membrane. In the process of WO01/09042, the uncharged/undissociated aromatic amines pass through the membrane by diffusion to the acidic stripping phase, which can be considered as the dissolution of the undissociated aromatic amines within the polymeric membrane. Similar as in Börner et al (Organic Process Research & Development, 2015, 19(7), 793-799), back extraction from the acidic stripping phase is prevented for the charged compounds. However, the performance and stability of an elastomeric polymeric membrane of WO01/09042 is often negatively affected when using particular solutions or liquids in the separation process or during membrane cleaning.
There remains a need in the art for the improved separation of organic compounds such as amines or organic acids from a solution, particularly from a solution comprising a chiral amine and/or multiple amines, that overcomes the limitations of the prior art. In this context, there is a particular need for improved enzymatic processes and systems to produce chiral amines that overcome at least some of the above indicated limitations of the transamination reaction and at the same time allow for long term application, which can be adapted to specific substrate donor amines/ketones and/or product amines.