Understanding Vesicle Fission and Fusion via Constrained Self-consistent Field Theory

Despite the wide existence of vesicles in living cells as well as their important applications like drug-delivery vehicles, the underlying mechanism of vesicle fission and fusion remains under debate. Classical models cannot fully explain the results observed in experiments and simulations. Here, we use vesicles formed by polyelectrolytes as a model system and apply the constrained self-consistent field theory to track the energy and morphology along the fission/fusion pathway. The theory can capture the coupling between the position-dependent interaction and the morphological evolution of the two vesicles. We show that there are two discontinuous transitions accompanied by topological changes between three different states: single-cavity vesicle, hemifission/hemifusion, and two separated vesicles. This is consistent with the predictions of the classical stalk model for fusion and the necking model for fission. Besides, we predict the possibility of the direct transition between the state of a single-cavity vesicle and the state of two separated vesicles without the hemifission/hemifusion intermediate if the repulsion between the two vesicles is strong enough, in agreement with simulation results of vesicles formed by amphiphilic molecules. Moreover, we study the micromechanics of different types of PE associates, including pearl-necklace (PN), sphere, and vesicle. In particular, the vesicle behaves like highly plastic materials, characterized by the long force plateau and constant necking thickness, while the PN and sphere are found similar to brittle and common ductile materials with a much smaller ultimate extension ratio. Although our results are based on a model system of PE vesicles, the theory and conclusions can be generalized to vesicles formed by amphiphilic molecules such as surfactants and block copolymers due to the essential topological feature.