Redox Biocatalysis in Lidocaine-Based Hydrophobic Deep Eutectic Solvents: Non-Conventional Media Outperform Aqueous Conditions.

Redox biocatalysis is an essential pillar of the chemical industry. Yet, the enzymes' nature restricts most reactions to aqueous conditions, where the limited substrate solubility leads to unsustainable diluted biotranformations. Non-aqueous media represent a strategic solution to conduct intensified biocatalytic routes. Deep eutectic solvents (DESs) are designable solvents that can be customized to meet specific application needs. Within the large design space of combining DES components (and ratios), hydrophobic DESs hold the potential to be both enzyme-compatible - keeping the enzymes' hydration -, and solubilizers for hydrophobic reactants. We explored two hydrophobic DESs, lidocaine/oleic acid, and lidocaine/decanoic acid, as reaction media for carbonyl reduction catalyzed by horse liver alcohol dehydrogenase, focusing on the effect of water contents and on maximizing substrate loadings. Enzymes remained highly active and stable in the DESs with 20 wt % buffer, whereas the reaction performance in DESs outperformed the pure buffer system with hydrophobic substrates (e. g., cinnamaldehyde to form the industrially relevant cinnamyl alcohol), with a 3-fold specific activity. Notably, the cinnamaldehyde reduction was for the first time performed at 800 mM (~100 g L^-1) with full conversion, which opens up new avenues to industrial applications of hydrophobic DESs for enzyme catalysis.