Enzyme-photo-coupled catalysis in gas-sprayed microdroplets

Abstract

Enzyme-photo-coupled catalysis produces fine chemicals by combining the high selectivity of an enzyme with the green energy input of sunlight. Operating a large-scale system, however, remains challenging because of the significant loss of enzyme activity caused by continuous illumination and the difficulty in utilizing solar energy with high efficiency at large scale. We present a large-scale enzyme-photo-coupled catalysis system based on gas-sprayed microdroplets. By this means, we demonstrate a 43.6-71.5 times improvement of solar energy utilization over that using a traditional bulk processing system. Owing to the improved enzyme activity in microdroplets, we show that chiral alcohols can be produced with up to a 2.2-fold increase in the reaction rate and a 5.6-fold increase in final product concentration.

Fig. 1. Picture of gas-spray microdroplet reactor and scheme of enzyme-photo-coupled catalysis system in microdroplets with reagent confinement. NAD, nicotinamide adenine dinucleotide; TEOA, triethanolamine; GCN, graphite-C₃N₄; M, [Cp*Rh(bpy)(H₂O)]²⁺; SsCR, Sporobolomyces salmonicolor carbonyl reductase.

Fig. 2. Enhanced catalytic performance in gas-spray enzyme-photo-coupled (EPC) system. (a) Microdroplet diameter measured using a high-speed camera at different gas velocities. (b) Calculated light intensity distribution with different illumination methods in the gas-spray reactor and bulk solution system (shown as a heat map), first column, near-uniform illumination using LED beads, second column, sided illumination using Xe-lamp, third column, illumination using LED strip, scale bar, 5 cm. (c) Regeneration of reduced nicotinamide adenine dinucleotide in the gas-spray microdroplet reactor and bulk solution system. NAD⁺, 1 mM; M, 0.2 mM; triethanolamine (TEOA), 400 mM; PBS buffer, 0.1 M (pH 6.5); graphite carbon-nitride (GCN), 0.33 mg mL⁻¹. (d) Enhanced reaction rate brought by the gas-spray EPC catalysis system. NAD⁺, 1 mM; M, 0.2 mM; TEOA, 400 mM; PBS buffer, 0.1 M (pH 6.5); GCN, 0.33 g L⁻¹; Sporobolomyces salmonicolor carbonyl reductase (SsCR), 1 g L⁻¹; substrate (methyl phenylglyoxylate, predissolved in 2% v/v dimethyl sulfoxide), 20 mM.

Fig. 3. Mechanism analysis of activity enhancement in microdroplets. (a and b) Distribution of enzyme and graphite carbon-nitride (GCN) in microdroplets, respectively, which is observed by staining enzymes with rhodamine isothiocyanate and laser confocal scanning microscopy. (a) Fluorescence image of the enzyme in microdroplets. (b) Fluorescence image of GCN in microdroplets. (c) Schematic of fluorescence intensity of the enzyme (red) and GCN (green). (d) Residual enzyme activity with time under light illumination in bulk solution and gas-spray reactor. (e) Calculated concentration distributions after different reaction times in microdroplets compared with the bulk solution system. (f) Calculated microdroplet-induced reaction rate enhancement.

Fig. 4. Comparison of EPC-catalyzed chiral alcohol synthesis in bulk solution system and gas-spray system, and the continuous chiral alcohol synthesis using gas-spray system. (a) Final concentration of products 2a–2f. (b) Selectivity of products 2a–2f. GCN, graphite carbon-nitride; NAD, nicotinamide adenine dinucleotide; SsCR, Sporobolomyces salmonicolor carbonyl reductase; TEOA, triethanolamine. Note that according to the sequence rule of chiral compounds, there are differences in the stereo configuration of 2a–2f, which is in consistent with previous report. (c) Variation of product concentration and outlet flow rate with reflux ratio β. Product concentration (blue column) is plotted on the left axis and the outlet flow rate (pink dot) is plotted on the right axis. (d) Overall production rate under different β conditions. (e) Effects of chiral pharmaceutical intermediates continuous synthesis process.