Effects Of The Charge And Unsaturation Degree Of Lipids On Microstructure And Dynamical Properties Of Cell Membranes

Cell membrane is the bilayer mostly composed of lipids and proteins on the surface of cell.Cell membrane segregates the internal organization of cell from its external environment and plays an important role in the transport process of energy and substance between the exterior and interior of the cell.Since the lateralorganization and dynamics of cell membrane are very crucial for many cellular processes,revealing the microstructure and dynamical properties of cell membrane is very important to understand the biological function of cell membranes and the microscopic mechanisms of some biological processes or diseases.Some studies have shown that the lateral organization of lipids in cell membrane is inhomogeneous and can separate to form micro domains.However,due to the complex structural and dynamical properties of cell membranes,it is difficult to directly observe and study the micro domains in real cell membrane.The microscopic mechanism of the lateral organization and dynamics behaviors of lipids(especially the charged lipids)in cell membranes is not well understood,yet.In this thesis,using the molecular dynamics method based on coarse-grained MARTINI force field,we systematically study the combined effects of charge and unsaturation degree of lipids on microcosmic structural and dynamical properties of cell membrane.We mainly simulate the ternary lipid bilayers composed of saturated lipids,unsaturated lipids and cholesterol molecules.Here,the lipids are either charged or neutral.Though varying the charge properties and unsaturation degree of lipids and analysing the area per lipid,diffusion coefficient,and order degree of lipids,we systematically reveal how the charge and the unsaturation degree of lipids influence the microscopic structure and dynamical properties of membrane together from the molecular level.Our results mainly consist of the following parts:(1)When the unsaturation degree of the unsaturated lipids is low,the electrostatic interaction between the charged lipids can inhibit the phase separation of lipid bilayer.Especially,when the saturated lipids are charged and the unsaturated lipids are neutral,the distribution of lipids in the lipid bilayer is approximately uniform.In this case,the microstructure of lipid bilayer membrane is greatly affected by the electrostatic interaction between charged lipids.(2)When the unsaturation degree of unsaturated lipids is high,the lipids in both the neutral lipid bilayer and the charged lipid bilayers can separate strongly into liquid-ordered and liquid-disordered domains.In such a case,the microstructure of membrane is dominated by the unsaturation of the unsaturated lipids,and the charge of lipids nearly has no effect on the phase separation of lipid bilayers.(3)Though comparing the lateral diffusion coefficients of lipids in these lipid bilayers,we find that the electrostatic interaction between the charge lipids can enhance the lateral diffusion of lipids in the cell membrane.In particular,in the lipid bilayer containing unsaturated lipids with low unsaturation degree,the effect of electrostatic interaction on lateral diffusion of lipids is very significant.In the lipid bilayer containing unsaturated lipids with high unsaturation,due to the restriction of the strong phase separation,the effect of electrostatic interaction on lipid lateral diffusion is not very obvious,but the diffusion of lipids in charged lipid bilayers still slightly increases compared with the neutral lipid bilayer.This study provides some theoretical insights into understanding the microscopic domain structure and the lateral diffusion of lipid molecules in the cell membrane.