Microdroplet-promoted Amide Bond Formation between Substances on the Interface of Two Immiscible Liquid Phases

Effectively constructing amide bonds between substances on the interface of two immiscible solvents plays a key role in the covalent-conjugation of the two substances with totally different hydrophilicity. In this study, a coaxial dual-channel spraying device was used to explore the advantages of microdroplet techniques in promoting the aminolysis between substances at the interface of two immiscible solvents for amide bond formation. The effects of sample flow rate, sheath pressure, electric field strength and organic solvent on the coupling efficiency of amide bonds in two immiscible microdroplets were studied, using Z-glycine-4-nitrophenyl ester (E_a) dissolved in an organic solvent and lysozyme protein (P) dissolved in an aqueous buffer as model substrates. When the sheath pressure was 0.8 MPa, as the flow rate increased from 5 μL/min to 25 μL/min, and the conversion increased first and then decreased from 92% to 86%. This may be due to the fact that under the conditions of low flow rate (5 μL/min) and high pressure of sheath gas (0.8 MPa), solids tend to accumulate at the nozzle tip, affecting the flight of microdroplets and suppressing mass transfer between two immiscible phases. When the flow rate was too high, the initial droplet was too large, leading to a reducation in the liquid-liquid specific surface area and a decrease in relative conversion (R.C.). When the flow rate was 10 μL/min, pressure of the sheath increased ranging from 0.4 MPa to 0.8 MPa or the voltage changing from 0 kV to ±4 kV, and all R. C. were ≥85%. It was proposed that the high-pressure gas flow (≥0.4 MPa) plays a major role by exerting strong shear stress on the initial droplet, promoting the conversion of initial large droplets to smaller ones to increase the surface-to-volume ratio of the liquid-liquid interface and enhance the mass exchange efficiency between the two phases. When several hydrophobic organic solvents (such as toluene, ethyl acetate, hexanone, dichloromethane, n-butanol) were used in the two-phased aminolysis reaction, the conversion rates in the microdroplets system are 80%-91%, and the highest conversion rate is 91% in toluene. However, the ammonolysis reaction in the bulk solution shows obvious lower conversion rate (≤21%). Finally, when the activated esters were E_a, E_b (N-benzoxycarbonyl 4-amino-acetic acid p-nitrophenol ester), E_c (N-benzoxycarbonyl 6-amino-acetic acid p-nitrophenol ester), E_d (N-fluorenyl methoxycarbonyl 2-amino-acetic acid p-nitrophenol ester), E_e (N-fluorenyl methoxycarbonyl 4-amino-acetic acid p-nitrophenol ester), or E_f (N-fluorenyl methoxycarbonyl 6-amino-acetic acid p-nitrophenol ester) with large hydrophobic groups, the R.C. of the aminolysis reaction in the two immiscible droplet systems is 65%-91%, while corresponding R.C. in the bulk solution is less than 11%, which is much lower than that in the microdroplets systems, indicating that microdroplet technique has applicable to a wide range of substrates for promoting the amide bonds construction between substances at the interfaces of two liquid phases. This strategy opens an avenue of microdroplets technique in the covalent coupling of functional groups with different hydrophilicity.