Metal affinity immobilization of oxidoreductases for the enantioselective synthesis of (R)-citronellal from geraniol

(R)-Citronellal is a key chiral intermediate in the synthesis of the correct final enantiomer of (–)-menthol, one of the most commercialized flavours worldwide. Currently, its industrial production relies on three main chemical routes, often involving energy-intensive distillation steps. Enzymatic approaches represent a less energy-demanding alternative for its synthesis. Focusing on a recently developed bienzymatic cascade, we aimed to improve this process for potential industrial applications. Starting from inexpensive, commercially available geraniol, a copper radical oxidase (CgrAlcOx) followed by an ene reductase (OYE2) are used to obtain (R)-citronellal. Using metal affinity immobilization on the his-tagged enzymes, we aimed to enable enzyme reusability and increase solvent tolerance. After screening a panel of resins and water-immiscible co-solvents, we successfully obtained 95% (R)-citronellal with 96.9% ee in a concurrent cascade after 7 hours of reaction time, starting from 10 mM geraniol. This work demonstrates the potential of enzyme immobilization to achieve high enantiomeric excess in the production of (R)-citronellal, representing a first step towards making enzymatic processes competitive with well-established chemical methods. On the other hand, further research and optimization are required to fully prove the scalability of the process.