Biocatalytic production of a Nylon 6 precursor from caprolactone in continuous flow.

6-aminocaproic acid (6ACA) is a key building block and an attractive precursor of caprolactam, which is used to synthesize nylon-6, one of the most common polymers manufactured nowadays. (Bio)-production of platform chemicals from renewable feedstocks is instrumental to tackle climate change and decrease fossil fuel dependence. Here, we report the cell-free biosynthesis of 6ACA from 6-hydroxycaproic acid using a co-immobilised multienzyme system based on horse liver alcohol dehydrogenase, Halomonas elongata transaminase, and Lactobacillus pentosus NADH oxidase for in-situ cofactor recycling, with >90% molar conversion (m. c.) The integration of a step to synthesize hydroxy-acid from lactone by immobilised Candida antarctica lipase B resulted in >80% m. c. of ε-caprolactone to 6ACA, >20% of δ-valerolactone to 5-aminovaleric acid, and 30% of γ-butyrolactone to gamma-aminobutyric acid, in one-pot batch reactions. Two serial packed-bed reactors were set-up using these biocatalysts and applied to the continuous flow synthesis of 6ACA from ε-caprolactone, achieving a space-time yield of up to 3.31 g 6ACA h -1 L -1 with a segmented liquid/air flow for constant oxygen supply.