Efficient enzymatic depolymerization of polycaprolactone into 6-hydroxyhexanoic acid by optimizing reaction conditions and microbial conversion of 6-hydroxyhexanoic acid into adipic acid for eco-friendly upcycling of polycaprolactone

Polycaprolactone (PCL) is one of the most widely used biodegradable plastics. An enzymatic method that utilizes Candida antarctica lipase B (CalB) was used to successfully depolymerize PCL into 6-hydroxyhexanoic acid (6-HHA) and confirmed by high-performance liquid chromatography analysis. To improve the 6-HHA production titer, we optimized various depolymerization conditions, such as pH, reaction temperature, enzyme addition timing, and enzyme quantities. The optimized reaction conditions included a calcium carbonate-controlled pH, a reaction temperature of 50 °C, and the addition of 0.69 U CalB once every 24 h for 2 days. These variables produced a titer of 114.9 g/L of 6-HHA and productivity of 2.87 g/L/h from 100 g/L PCL. We also attempted to produce adipic acid (AA) using 6-HHA prepared from PCL depolymerization. Using a recombinant Escherichia coli strain with the 6-HHA dehydrogenase and 6-oxohexanoic acid dehydrogenase genes from Acinetobacter strain SE19, 6-HHA was successfully converted into AA. To best our knowledge, ours is the first report regarding an optimization study to improve the PCL depolymerization efficiency via an enzymatic reaction method and the microbial upcycling of 6-HHA into AA. Thus, this study sets the groundwork for the eco-friendly recycling of waste PCL into a value-added chemical using a biocatalytic method.