In situ removal of inhibitory products with ion exchange resins for enhanced synthesis of chiral amines using ω-transaminase

Abstract

ω-Transaminase (ω-TA) serves as an attractive biocatalyst for the asymmetric synthesis of chiral amines from prochiral ketones. However, the ω-TA reactions suffer from exceedingly unfavorable equilibrium and severe product inhibitions. Most reaction engineering strategies to overcome such limitations have focused on coproduct removal, which fails to mitigate enzyme inhibition by the desired amine product. Herein, we demonstrated in situ removal of both inhibitory products using ion exchange resin (IER). l-Alanine was chosen as a cosubstrate for S-selective ω-TA by considering a zwitterionic character, affording selective adsorption of the resulting products, i.e. (S)-amine and pyruvate on cation and anion exchange resin, respectively. The addition of both IERs to the reaction mixture containing 10 mM acetophenone and 200 mM l-alanine led to a 65 % yield of (S)-α-methylbenzylamine, whereas the same reaction without IER permitted only 3 % yield close to a thermodynamic limit. Computational prediction of the reaction yield was carried out using a Langmuir analysis, showing good agreements with the experimental results. The in situ product removal (ISPR) enabled amination of 2-octanone to reach even 84 % reaction yield. Preparative-scale synthesis of (S)-2-aminohexane was carried out using a batch recirculation reactor implemented with consecutive IER columns. The ISPR strategy could afford virtually inhibition-free reaction conditions and facile recovery of both valuable products.