One-pot biosynthesis of aromatic D-amino acids and neuroactive monoamines via enantioselective decarboxylation under in situ product removal using ion exchange resin

Living organisms have evolved a myriad number of enzymes, which inspires reaction engineers to reroute native biocatalytic utility into scalable synthetic purposes. One of such examples is aromatic L-amino acid decarboxylase (AADC), displaying several enzymatic properties desirable for synthetic applications, including a high turnover rate for C-C bond cleavage and strict enantioselectivity. However, exploitation of the catalytic potential has been limited to the preparation of monoamines and has not yet been proven to afford the synthesis of chiral compounds. Here we report AADC-catalyzed kinetic resolution of diverse aromatic amino acids to demonstrate one-pot production of enantiopure D-amino acids as well as neuroactive monoamines. To this end, we used AADC from Bacillus atrophaeus (AADC-BA) displaying broad substrate specificity and excellent enantioselectivity. Kinetic resolution using AADC-BA afforded > 99% ee of D-amino acid leftover at ≈ 50% conversion of racemic substrates such as phenylalanine, tryptophan, tyrosine, homophenylalanine, and substituted phenylalanine analogues. Product inhibition of AADC-BA led us to develop in situ product removal (ISPR) using cation exchange resin, demonstrating that the ISPR strategy facilitated the kinetic resolution owing to selective removal of the inhibitory monoamine. We also demonstrated that low solubility of racemic substrate could be overcome by fed-batch operation.