Dynamic Behavior of Complex Coacervates with Internal Lipid Vesicles under Nonequilibrium Conditions

During the evolution of life on earth, the emergence of lipid membrane-bounded compartments is one of the most enigmatic events. Endosymbiosis has been hypothesized as one of the solutions. In this work, using a coacervate droplet formed by single-stranded oligonucleotides (ss-oligo) and poly(l-lysine) (PLL) as the protocell model, we monitored the uptake of liposomes of different types and studied the dynamic behavior of the resulting composite droplet under the electric field. The coacervate droplet exhibits affinity for the liposomes of varying charges. However, the permeation of liposome is also controlled by electrostatic interactions. Dominated by electrostatic attraction, the positively charged liposome is retained inside the droplet as growing fibrous structures, while the negatively charged liposome is mainly coated on the droplet surface. Permeation and even distribution occur when the liposome and the droplet carry the same charges, or at least one of them is neutral. As an electric field is applied to trigger repetitive cycles of vacuolization in the ss-oligo/PLL droplet, the fibrous structure formed by the positively charged liposome is basically intact, while a new phase is generated together with uneven mass transport as the negatively charged liposome is internalized. Interestingly, the release of daughter droplets with similar components occurs on the droplet containing neutral liposomes. Our work not only provides a step toward creating protocells with hierarchical structures and biofunctions using a biogenetic material via simple mixing but also sheds light on the possible origin of the lipid structure inside a living organism.