The Study On Properties Of Water Molecules At The Biomembrane Interface

The properties of water near the biological membrane interface are of essential importance for the realization of function of biological membrane. The self-assembly of lipid bilayer membrane, osmosis of water molecules, drug transport across biological membrane and hydration force related to fusion of cell membranes strongly associate to the properties of water near the membrane interface. The properties of membranes strongly affect in turn the properties of water near the membrane interface. In-depth understanding the interaction between water molecules and biological membrane would be helpful in understanding the structure and function of biological membrane. With the development of computer technology and molecular dynamics simulations, many force fields have be proposed and applied to simulate the structure of lipid bilayer. The molecular dynamics simulation of biological membrane promotes the understanding of structure and functional features of biological membrane. However, the developed force fields generally do not involve the properties of water molecules near lipid bilayer/water interface. It is worth exploring whether those force fields are appropriate to investigate the properties of water near the membrane interface. In the work, we firstly employed the method of molecular dynamics simulation and different force fields to investigate the behaviors of water molecules near the interface between biological membrane and water. We then apply the force field, which is considered to be most appropriate to study the properties of water near membrane interface, to investigate the effects of different cations on the dipole orientation of water molecules near bilayer/water interfaces. The contents are arranged as following:The first chapter mainly introduces some basic conceptions of biomembrane as well as relevant research background and achievements about the properties of water near the biological membrane interface.The second chapter introduces the basic knowledge of molecular dynamic simulations and the simulations software used in our investigation.The third chapter introduces the simulation results of our first work. We have carefully studied the behaviors of interfacial water by using different force fields. We analyze the H-bond energies of the eight oxygen atoms in the lipid headgroup. The results of molecular dynamic simulations show that the every force field used in our simulations are proved to be correct because they can give the same membrane structures of lipid bilayer which is in good agreement with the results of experiments. However, we find that the behaviors of the interfacial waters are different from each other when we apply different force fields. We find that:for union force fields, the H-bond energy of double bonded oxygen atom is higher than that of single bonded oxygen atom in the same group. For all-atom force field, the H-bond energy of oxygen atom in the phosphate group has the similar result. However, for C=O1 and C=O2 groups, the H-bond energy of double bonded oxygen atom is smaller than that of single bonded oxygen atom in the same group because the repulsive force between oxygen and hydrogen atoms appears. H-bond energy of oxygen atom in the phosphate group has a slightly wide distribution compared to C=O1 and C=O2 groups. For different force fields, the H-bond energies distributions of 033 and 034 are completely overlap, reflecting the symmetry of phosphate group in DPPC molecule. The total H-bond energy distribution of a group generally exhibits several peaks. Furthermore, the number of peaks corresponds to the H-bond number of corresponding group. By analyzing the distributions of H-bond energy and H-bond numbers of different groups, we find that the results from Berger and Slipid fields are close to the experimental results. These results would be helpful in understanding the behaviors of water near interface of the lipid bilayer and provide a guide for making the appropriate choice of force filed in simulations of lipid bilayer.The fourth chapter introduces the simulation results of our second work. In the chapter, we investigate the effects of different cations on the dipole orientation of water molecules near lipid bilayer/water interfaces. We find that cations have no significant influence on the radial distribution functions of water molecules near the choline group. However, cations can alter the radial distribution functions of water molecules near phosphate group because cations can penetrate into membrane interior and managed to assemble in phosphate group. The dipole orientation of water molecules trends to orientate to the bulk water due to the electrostatic absorbing interaction between water molecules and cations which have penetrated into membrane interior. The effects of different cations on the dipole orientation of water molecules, reside time, average area of every lipid molecule follow the reverse Hofmeister series due to the reason that the degree of different cations adsorbed to the membrane interior follow the same reverse Hofmeister series. The penetration of cations into membrane interior leads to the dehydration of lipid molecule and restructure the hydrogen bonding network. It will result in a change of the dipole orientation of water molecules at the membrane interface.