Molar‐Scale Phenolic Acid Decarboxylation Using Thermostable Biocatalysts and Enzyme‐Compatible Deep Eutectic Solvents

The synthesis of biogenic hydroxy styrenes by enzymatic decarboxylation of phenolic acids (e.g., ferulic, p-coumaric, caffeic, or sinapic acids) is a promising route involving biorefineries and biocatalysis, in which the low substrate solubility in water requires the quest for nonconventional media. This work explores the tunability of deep eutectic solvents (DES) to simultaneously create enzyme-compatible and solubilizing systems, in which thermostable phenolic acid decarboxylases (PAD) may operate at 50-70 °C, decreasing the inherent DES viscosity. Four DESs displaying high solubility of ferulic acid are assessed (with additions of up to 20 vol% of phosphate buffer): Choline chloride-glycerol (ChCl-Gly, 1:2), choline chloride-ethylene glycol (ChCl-EG, 1:2), choline acetate-glycerol (ChAc-Gly, 1:2), and betaine-glycerol (Bet-Gly, 1:2). Bet-Gly DES exerts promising properties, being a halide-free solvent, an enzyme-stabilizer at 50-70 °C (1.4 higher melting temperatures for the N31 ancestor Bacillus subtilis PAD), and 14-fold higher ferulic acid solubility compared to buffer. Intensified conditions of up to 1 M ferulic acid (≈200 g L^-1, in a slurry-like system) in Bet-Gly (1:2, 20 vol% buffer) lead to excellent conversions (90%) in less than 5 h, demonstrating that biocatalysis can be performed under industrially relevant conditions when DES media are tuned for the specific requirements of the application.