Microbial secretion of lactate-enriched oligomers for efficient conversion into lactide: A biological shortcut to polylactide.

Recently, we have succeeded in establishing the microbial platform for the secretion of lactate (LA)-based oligomers (D-LAOs), which consist of D-LA and d-3-hydroxybutyrate (d-3HB). The secretory production of D-LAOs was substantially enhanced by the supplementation of diethylene glycol (DEG), which resulted in the generation of DEG-capped oligomers at the carboxyl terminal (referred as D-LAOs-DEG). The microbial D-LAOs should be key compounds for the synthesis of lactide, an important intermediate for polylactides (PLAs) production, eliminating the costly chemo-oligomerization step in the PLA production process. Therefore, in order to demonstrate a proof-of-concept, here, we attempted to convert the D-LAOs-DEG into lactide via metal-catalyzed thermal depolymerization. As a result, D-LAOs-DEG containing 68 mol% LA were successfully converted into lactide, revealing that the DEG bound to D-LAOs-DEG does not inhibit the conversion into lactide. However, the lactide yield (4%) was considerably lower than that of synthetic LA homo-oligomers (33%). We presumed that 3HB units in the polymer chain blocked the lactide formation, and therefore, we investigated the LA enrichment in the oligomers. As the results, the combination of an LA-overproducing Escherichia coli mutant (Δdld and ΔpflA) with the use of xylose as a carbon source exhibited synergistic effect to increase LA fraction in the oligomers up to 89 mol%. The LA-enriched D-LAOs-DEG were converted into lactide with greater yield (18%). These results demonstrated that a greener shortcut route for PLA production can be created by using the microbial D-LAOs secretion system.