Morphological Evolution Of Lipid Droplets In Self-assembly Process

Amphiphilic molecules, which have unique self-assembly characteristics and rich self-assembly shapes, have been widely used in many fields, such as drug delivery systems, biosensors, micro-controlled reactor, and have potential application in optics, electrics and electromagnetic. A majority of functions of membrane proteins and biological activity substances, in a real biological environment, have closely related with compositions and structures of membranes. Currently, computer simulation of multi-component bilayer lipid membranes has been a hotspot in biophysics. In this thesis, we prepared two-component lipid droplet in which lipid molecules have different hydrophobic chains for the initial states, with different ratios, to study the self-assembly process of the lipid molecules in water. After that, we discussed two vesicle shapes formed in the self-assembly processes of one-component lipid droplets.We applied the coarse-grained molecular dynamics (CGMD) method, selected DPPC which has two hydrophobic tails and DPC only one tail as research objects, and simulated the self-assembling behavior of two-component lipid droplets in water. With the proportion of DPCs increasing (from 10% to 90%), four kinds of morphologies were observed: vesicle, disc micelle, porous cage-like structure and bi-microporous structure with a figure of eight. The reasons for the formation of various shapes were obtained by analyzing the dynamic evolution processes, RDF and energy. With the fixed ratio of two components, the competition between boundary energy minimization and elastic energy is the principal driving force of the system in the early stages of self-assembly process. When the energy of system tended to be constant, the uniform distribution of two different components determined the final structures. The self-assembly processes are easier to form vesicles with adding a certain amount of DPCs in DPPCs. Moreover, if the proportion of DPCs added to DPPCs is suitable, the shapes of vesicles will be controlled. Using special initial state and tuning the proportion of composition can be an effective strategy to achieve the novel micelles. Used the same methods, one component lipid droplet composed of 850 DPPCs formed two shapes in water, one is sphere vesicle, the other oblate vesicle. Simulation experiment and theoretical derivation showed that, oblate vesicles will become spherical vesicles eventually if simulation time is long enough. The research of two morphological vesicles is valuable for the potential application in drug delivery carrier.