Deep eutectic solvent tailoring the electrode-enzyme interface for phenylalanine dehydrogenase immobilization and efficient bioelectrosynthesis

Deep eutectic solvents (DESs) are a group of emerging solvents for biocatalysis with tailored performance. Herein, we report the usage of hierarchical zeolitic imidazole framework-8 (HZIF-8) and halogenated DES for the immobilization of phenylalanine dehydrogenase. Effects of three different halogenated DESs (I, Cl and Br) on the activity of immobilized enzymes for production of homophenylalanine were studied. The optimal activity towards with the model reaction of catalyzing ethyl 2-oxo-4-phenylbutyrate (EOPB) to L-homophenylalanine was 6.11 U/mg at 70 °C, 5.4-fold of that of the free enzyme, with 78.4% of its original activity retained after six recycles. The immobilized enzymes were further loaded on a poly(methylene blue) electrode with electrochemical NAD+ regeneration, leading to bioelectrosynthesis of phenylpyruvate. Using phenylalanine-ethanolamine (Phe-MEA) as the substrate, 13.85 mM phenylpyruvate was obtained after 8 h, with a total turnover number (TTN) value of 138.5 for NADH regeneration, while a TTN of 102.2 using L-Phe as the substrate.