Kinetics of a Multilamellar Lipid Vesicle Ripening: Simulation and Theory

Lipid vesicle ripening via unimolecular diffusion and exchange greatly influences the evolution of complex vesicle structure. However, this behavior is difficult to capture using conventional experimental technology and molecular simulation. In the present work, the ripening of a multilamellar lipid vesicle (MLV) is effectively explored using a mesoscale coarse-grained molecular model. The simulation reveals that a small MLV evolves into a unilamellar vesicle over a very long time period. In this process, only the outermost bilayer inflates, and the inner bilayers shrink. With increasing MLV size, the ripening process becomes complex and depends on competition between a series of adjacent bilayers in the MLV. To understand the diffusion behavior of the unimolecule, the potentials of mean force (PMFs) of a single lipid molecule across unilamellar vesicles with different sizes are calculated. It is found that the PMF of lipid dissociation from the inner layer is different than that of the outer layer, and the dissociation energy barrier sensitively depends on the curvature of the bilayer. A kinetics theoretical model of MLV ripening that considers the lipid dissociation energy for curved bilayers is proposed. The model successfully interprets the MLV ripening process with various numbers of bilayers and shows potential to predict the ripening kinetics of complex lipid vesicles.