In cell system, dynamic behaviors of nanometer- and micrometer-sized living tissues are induced through various chemical reaction networks accompanied with molecular conversions of key molecules. However, such hierarchy between macroscopic dynamics of cells and molecular conversions is very complicated. Here, as a tool for understanding the essence of the dynamic behavior found in cell system, two chemical systems for droplets exhibiting cell- mimicry behaviors have been described: transformation of molecular aggregates and locomotion mode of oil droplets.
In cell system, dynamic behaviors of nanometer- and micrometer-sized living tissues are induced through various chemical reaction networks accompanied with molecular conversions of key molecules. However, such hierarchy between macroscopic dynamics of cells and molecular conversions is very complicated. To understand the essence of the dynamic behavior found in cell system, more simple artificial system, where cell-mimicry behaviors can be observed, has been recently constructed by using self- propelled droplets . The mechanism of the self-propelled motion in aqueous solution is based on the difference in interfacial tension around the droplet and mass transfer as a result of the Marangoni effect . Some self-propelled droplets have exhibited the controlled behavior in response to gradient field of chemical compounds  and light irradiation . Our group has investigated the cell-mimicry locomotion of oil droplets observed in an oil-in-water emulsion containing surfactants through the molecular designs of system components . Among them, two cell-mimicry behaviors, which are induced by chemical conversions of key molecules, are here described.
The dynamics in emulsion system where oil and surfactant components were generated from an imine-containing oil component was observed (Figure 1). When the imine-containing oil was dispersed in an aqueous cationic surfactant solution containing hydrochloric acid, oil droplets started their motion as they transformed from flocculate components to spherical oil droplets, and their motion was sustained for around 30 min . Thereafter, micrometer-sized giant vesicles spontaneously generated around the stationary droplet.
Without hydrochloric acid at a certain concentration. Hydrolysis of the imine-containing oil compound produces ‘easy-to-move’ heptyloxybenzaldehyde (HBA) as an oil component and a fatty amine, and the latter is neutralized to generate a surfactant component in the presence of hydrochloric acid. Due to the hydrochloric acid concentration-dependent generation of oil and surfactant components, when the hydrochloric acid concentration is too low or too high, a series of deformation, self-propelled motion of spherical oil droplets, and generation of giant vesicles was not observed. Namely, it is thought that such dynamics were induced along with the compositional change of the constituent components of the system in response to a specific environment. Considering that the relevance of the imine bond to the origins of life was proposed , it is also interesting to relate that to a chemical system that embodies self-organized movement and transformation of molecular aggregates.
Dynamics of HBA droplets was observed in aqueous solution composed of gemini surfactants having an azobenzene group in the linker moiety (Azo) and non-reactive cationic surfactants (Figure 2). The moving direction of the oil droplets was changed so as to escape by UV light (λ = 365 nm) irradiation from the side, i.e. negative phototaxis . It was found that the self-propelling oil droplets respond sensitively (within 0.5 s) to UV light irradiation. Under the subsequent UV irradiation, oil droplets ceased, and after 130 s the stationary droplets then moved toward the light source (i.e. positive phototaxis).
Thus the locomotion mode of the oil droplets transferred from negative to positive against the UV irradiation. The negative phototaxis is due to the slight but instantaneous increase in the interfacial tension of the oil droplet surface caused by the photoisomerization of Azo, which sensitively responds to UV irradiation. Since the HBA oil droplet does not transmit UV light, the interfacial tension of the light-irradiated area on the droplet changes along with photoisomerization. Then, the Marangoni convection occurs from a region of low interfacial tension to that of high interfacial tension, inducing the negative phototaxis of self- propelled oil droplets. In addition, under UV irradiation, HBA was slowly but steadily oxidized at the minute timescale. Even though the precise mechanism is not completely understood, the positive phototaxis is thought to be associated with HBA- derived oxidized products mainly at the droplet side facing the UV irradiation. The molecular conversion induces lowering the interfacial tension around the droplets, thus gradually forming the flow fields that cause the positive phototaxis of the droplets. The transformation of locomotion mode may provide living-organism-mimetic properties, such as the adaptation to an external stimulus, in this case irradiation.
Chemical systems, where transformations of molecular aggregates and locomotion mode of oil droplets occur, have been demonstrated by the synthetic approach. Because these systems are similar to the hierarchical cell system from molecular interactions and chemical conversions at the nanometer scale to macroscopic behaviors at the micrometer scale, our synthetic approach could be a powerful tool for understanding the mechanism of the complicated cell system.
This work was supported by a Grant-in-Aid for Scientific Research (No. 16K17504 for T.B and No.
16H04032 for T.T.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.