Tri-enzyme fusion of tryptophan halogenase achieves a concise strategy for coenzyme self-sufficiency and the continuous halogenation of L-tryptophan

The halogenase-based catalysis is one of the most environmentally friendly methods for the synthesis of halogenated products, among which flavin-dependent halogenases (FDHs) have attracted great interest as one of the most promising biocatalysts due to the remarkable site-selectivity and wide substrate range. However, the complexity of constructing the NAD+-NADH-FAD-FADH2 bicoenzyme cycle system has affected the engineering applications of FDHs. In this work, a coenzyme self-sufficient tri-enzyme fusion was constructed and successfully applied to the continuous halogenation of L-tryptophan. SpFDH was firstly identified derived from Streptomyces pratensis, a highly selective halogenase capable of generating 6-chloro-tryptophan from tryptophan. Then, using gene fusion technology, SpFDH was fused with glucose dehydrogenase (GDH) and flavin reductase (FR) to form a tri-enzyme fusion, which increased the yield by 1.46-fold and making the coenzymes self-sufficient. For more efficient halogenation of L-tryptophan, a continuous halogenation bioprocess of L-tryptophan was developed by immobilizing the tri-enzyme fusion and attaching it to a continuous catalytic device, which resulted in a reaction yield of 97.6% after 12 h reaction. An FDH from S. pratensis was successfully applied in the halogenation and our study provides a concise strategy for the preparation of halogenated tryptophan mediated by multienzyme cascade catalysis.

Electrostatic Interaction-Controlled Formation of Pickering Emulsion for Continuous Flow Catalysis

Although noble metal or non-noble metal-catalyzed reactions are widely used, it is still difficult to apply these reactions in the large-scale synthesis of chemicals because most of the reactions are carried out by the inefficient batch reaction strategy. Herein, Pickering emulsion-based continuous flow catalysis was utilized to address this problem. Cellulose nanofibers with aldehyde groups (ACNF) were generated through oxidizing C2 and C3 hydroxyl groups of cellulose nanofibers into aldehyde groups by NaIO₄, followed by in situ depositing Ag nanoparticles on ACNF to produce Ag-decorated ACNF (ACNF@Ag) via a facile aldehyde-induced reduction method. ACNF@Ag with ∼2 wt % Ag (ACNF@Ag2) has been used to prepare the Pickering emulsion by controlling the electrostatic interaction between ACNF@Ag2 and the oil-water interface via adjusting the pH. It was found that the Pickering emulsion could be generated at a pH around 3.29 and was determined to be the oil-in-water emulsion. The reduction of organic molecules (4-nitrophenol (4-NP), methylene blue (MB), and methyl orange (MO)) was selected as a model reaction to test the reliability of the Pickering emulsion in continuous flow catalysis, which demonstrated very high conversion rates for 4-NP (>98%, 50 h), MB (>99%, 30 h), and MO (>96%, 40 h).

Ligand-Free Nano-Au Catalysts on Nitrogen-Doped Graphene Filter for Continuous Flow Catalysis

In this study, the authors rationally designed a high-performance catalytic filter for continuous flow catalysis. The catalytic filter consisted of ligand-free nanoscale gold (nano-Au) catalysts and nitrogen-doped graphene (N-rGO). The Au catalyst was fabricated in situ onto a pre-formed N-rGO support by the NaBH₄ reduction of the Au precursor, and the size of the nano-Au was fine-tuned. A hydrothermal pretreatment of graphene oxide enriched nitrogen-containing species on the surface of two-dimensional graphene supports and enhanced the affinity of Au precursors onto the support via electrocatalytic attraction. The nano-Au catalysts acted as high-performance catalysts, and the N-rGO acted as ideal filter materials to anchor the catalysts. The catalytic activity of the as-designed catalytic filter was evaluated using 4-nitrophenol (4-NP) hydrogenation as a model catalytic reaction. The catalytic filters demonstrated superior catalytic activity and excellent stability, where a complete 4-nitrophenol conversion was readily achieved via a single pass through the catalytic filter. The as-fabricated catalytic filter outperformed the conventional batch reactors due to evidently improved mass transport. Some key operational parameters impacting the catalytic performance were identified and optimized. A similar catalytic performance was also observed for three 4-nitrophenol spiked real water samples (e.g., surface water, tap water, and industrial dyeing wastewater). The excellent catalytic activity of the nano-Au catalysts combined with the two-dimensional and mechanically stable graphene allowed for the rational design of various continuous flow catalytic membranes for potential industrial applications.