Interaction Between Nanoparticles And Aggregates Of Amphiphile Molecules

Amphiphile molecules, a type of macromolecules having a special structure, contain both hydrophilic heads and hydrophobic tails. Because of the amphiphile property of these molecules, they can self-assemble into ordered structures in hydrophilic or hydrophobic solvents. With this special property, amphiphile molecules have been widely used in science, industry and even in our daily life. On the other hand, nanoparticles have shown great potential to use in modern medical treatments, including diagnostic, drug delivery, and therapeutic agent. In recent years, increasing number of researches have reported the aggregation behavior and the phase behavior of amphiphile molecules at different conditions. However, little of them reported the self-assembly behaviors of the amphiphile molecules affected by the presence of nanoparticles, which are of sharp local curvature.Biomembrane, which is one of the important parts of cells, is composed by a particular type of amphiphile molecules, lipid molecules. Biomembrane participates in many cellular activities, like cell signal transduction and transportation. Thus, studying on the interaction between the biomembrane and the external nanoparticles is quite important to understand the nature of the cellular life and for the safe application of nanoparticles. However, because of the biomembrane is a dynamic structure with high complexity and most cellular activities taking place in the scale of nanometer or nanosecond, it is difficult to investigate the molecular mechanism of activities participated by both membrane and nanoparticles experimentally. Fortunately, with the development of the computer technology and simulation techniques, computer simulations become as one of important tools to study the biological systems, which can compensate for the blindness of experiment. Therefore, in this work we mainly used the molecular simulation methods, including dissipative particle dynamics (DPD) and lattice Monte Carlo (LMC) and theoretical models (Helfrich theory and perturbation theory) to investigate the interaction between amphiphile molecules and nanoparticles. Our work mainly consists of two parts as follows:1. First, in this work we investigate:(i) the effection of nanoparticles, especially their highly curved surfaces, on the aggregation of the amphiphile molecules; (ii) the phase behavior of the binary mixture in confined space provided by nanoparicles; (iii) the mechanism of the self-assembly of the hydrophobic nanoparticles inside a lipid bilayer that consists of a particular kind of amphiphile molecules, namely lipid molecules.(1) Using lattice Monte Carlo simulations and Helfrich theoretical model calculations, we investigate how the self-assembly of adsorbed amphiphile molecules is affected by the local curvature of solid surface of nanoparticles as well as by the own structure of the amphiphile molecules. Our simulation results reveal that beyond a critical curvature value, solid surface geometry governs the spatial ordering of aggregates and may induce the morphological transitions. The simulation results also show how the curvature of solid surfaces modulates the distribution of aggregates:the anisotropy in local curvature along and perpendicular to cylindrical surfaces tends to generate orientationally ordered cylindrical micelles. To account for the morphological transitions induced by the local curvature of solid surface, we constructe a theoretical model which includes the Helfrich bending energy, the deformation energy of aggregates induced by solid surfaces, and the adsorption energy. The model calculations indicate that on highly curved solid surfaces of nanoparticles the bending energy for bilayer structure sharply increases with surface curvature, which in turn induces the morphological transition from bilayer to cylinderical structure. Our results suggest that the local curvature of nanoparticles provides a means of controlling the spatial organization of amphiphile molecules.(2) In a real cellular system, the cell membrane includes lots of type of lipids and proteins, and the distributions of the lipids are inhomogeneous at the different cellular activities. Hence the phase behaviors of the lipids play a key role in controlling various biological processes. To understand the demixing behavior of lipid mixture, we use first ordered perturbation theoretical method to investigate the phase behavior of binary mixture in both bulk system and slit pore. Calculations indicate that demixing takes place more easily when the dimension of the system is increased. In other word, demixing takes place more difficultly in slit pores when compared to the bulk system.(3) Using N-varied Dissipative Particle Dynamics method (N-varied DPD), we demonstrate that hydrophobic nanoparticles having a diameter compared to the hydrophobic membrane thickness are capable of clustering in the hydrophobic interior of a cell membrane. The results show that except from the isotropic aggregation, an unexpected linear arrangement of spherical nanoparticles, which is not still found from experiments, is identified here. The free-energy costs associated with linear and isotropic aggregations are computed explicitly to interpret aggregation behavior, and the obtained phase diagrams give us a comprehensive understanding of where linear aggregation is expected. In this work we also shows that nanoparticle size and membrane tension play a key role in determining the nanoparticles aggregate, while the effects of nanoparticles concentration and membrane curvature seem to be relatively weak.2. As the second part of the dissertation, we investigate the formation membrane nanotubes induced by moving nanopaandrticles and the pearling transition of membrane tubes induced by nanoparticle adsorption. This part mainly includes:(1) Using the N-varied DPD simulation method, we investigate the kinetic process of the formation of lipid nanotube induced by a moving nanoparticle, which is often controllably implemented experimentally by optical tweezer and magnetism. The simulation results indicate that there exists an energy barrier accompanied the morphology transition for the nanotube formation. We analyze the order of the lipid molecules, the membrane thickness, and the lipid density on the bilayer in the vicinity of membrane tube, and it is found that the energy barrier and the morphology transition are induced by a sudden change of the ordering of lipid molecules. We also find that although the bilayer tension, the radius of the nanotube, and the growth speed of the tubes will all affect the value of the energy barrier, they do not change the position where the morphology transition takes place.(2) A coarse-grained molecular dynamics simulation method that belongs to the class of dissipative particle dynamics schemes with implicit solvent are used to indicate that adsorption of nanoparticles inside a membrane tube and pressure difference across the membrane cooperatively induce membrane tube pearling. We demonstrate that nanoparticle adsorption and aggregation initiate the shape transformation, and pressure difference provides a driving force for pearling instability. Depending on the dynamic coupling of shape transition and nanoparticle aggregation, different shape transitions via four kinds of pearling pathways are recognized, including pearls on a string (i.e., vesicles are interconnected via either a chain or double-chain of nanoparticles) and tube-to-vesicle transition that is dominated kinetically either by nanoparticle-membrane attraction or by pressure difference. Considering the fact that biological membranes are semipermeable and many proteins interact with the membranes, these findings not only provide a mechanism of membrane tube pearling but also demonstrate the importance of osmotic pressure and protein-membrane interaction for many cell activities related to shape transitions.