Application of a thermostable Baeyer-Villiger monooxygenase for the synthesis of branched polyester precursors

BACKGROUND
It is widely accepted that the poor thermostability of Baeyer‐Villiger monooxygenases limits their use as biocatalysts for applied biocatalysis in industrial applications. The goal of this study was to investigate the biocatalytic oxidation of 3,3,5‐trimethylcyclohexanone using a thermostable cyclohexanone monooxygenase from Thermocrispum municipale (TmCHMO) for the synthesis of branched ε‐caprolactone derivatives as building blocks for tuned polymeric backbones. In this multi‐enzymatic reaction, the thermostable cyclohexanone monooxygenase was fused to a phosphite dehydrogenase (PTDH) in order to ensure co‐factor regeneration.

RESULTS
Using reaction engineering, the reaction rate and product formation of the regio‐isomeric branched lactones were improved and the use of co‐solvents and the initial substrate load were investigated. Substrate inhibition and poor product solubility were overcome using continuous substrate feeding regimes, as well as a biphasic reaction system with toluene as water‐immiscible organic solvent. A maximum volumetric productivity, or space‐time‐yield, of 1.20 g L^‐1 h^‐1 was achieved with continuous feeding of substrate using methanol as co‐solvent, while a maximum product concentration of 11.6 g L^‐1 was achieved with toluene acting as a second phase and substrate reservoir.

CONCLUSION
These improvements in key process metrics therefore demonstrate progress towards the up‐scaled Baeyer‐Villiger monooxygenase‐biocatalyzed synthesis of the target building blocks for polymer application.