Solvation Behaviors Of Water And Its Mixture With Protic Ionic Liquid At The Interface With Phospholipid Bilayers From Molecular Simulation Studies

Biomembranes,as the protective barrier of cells or organelles,play a key role in the cellular functions of living organisms.Meanwhile,the functions of bio membranes are closely related to the behaviors of interfacial solvents.In this context,a detailed understanding of the behaviors of solvents at the interface with bio membranes is of great importance to interpret the function of bio membrane.In this thesis,we employed the molecular dynamics（MD）simulation method to systematically study the solvation structures,kinetic properties,hydrogen bond behavior and vibration spectral properties of water and its mixture with protic ionic liquid at the interface with the 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine（POPC）membrane.In chapter 2,MD simulations have been performed to explore the solvation structures,kinetic properties,hydrogen bonding behaviors and spectral properties of water molecules at the interface with POPC membrane.The simulation results show that the interfacial water molecules can form strong hydrogen bond（HB）interactions with the oxygen atoms in POPC molecule,particularly those with the non-bridging oxygen atoms connecting the phosphorus atom.Such enhanced HB would lead to that the interfacial water molecules can exhibit a certain red shift in the mid-IR region for the O–H stretching mode but a more obvious blue shift in the far-IR region for the particular vibration mode of the water-water HBs with respect to the corresponding bulk values.In addition,our simulation results also reveal the restricted translational and rotational motions of hydration waters at the interface of POPC bilayers.The further analysis demonstrates that there are two main kinds of hydration water molecules at the interface,where one is the minority of hydration water molecules（i.e.,IW1）forming HBs with the oxygen atoms of POPC molecules,while the other is the majority of hydration water molecules（IW2）having no HBs with the POPC molecules.Our molecular-level simulation results demonstrate that the shifts in both mid-and far-IR regions of hydration water molecules are almost completely dominated by the minor IW1（less than 40%）rather than the major IW2 since the IW1-IW1 HBs are significantly enhanced while the IW2-IW2 HBs display a bulk-like characteristic compared to the bulk water.Accordingly,more red or blue shifts are observed in the mid-and far-IR sub-spectra of the IW1 molecules while those of the IW2 molecules are much less.In chapter 3,the MD simulations have been carried out to investigate the solvation structures,kinetic properties and hydrogen bonding behaviors of water/ionic liquid mixtures with different concentrations at the interface with phospholipid membrane.Our simulation results clearly demonstrate that all studied HBs have the following strength order:POPC–H₂O>POPC–EA⁺>H₂O–H₂O>NO₃^––H₂O>EA⁺–H₂O>EA⁺–NO₃^–HBs.Furthermore,increasing the EAN concentration can be favorable to a further enhancement of all these HB strength at the interface and in the bulk phase,which lead to slower rotations of EA⁺,NO₃^–,and H₂O.To this end,we suppose that the relevant HB networks around the EA⁺,NO₃^–,and H₂O would be changed considerably with the increase of EAN protic ionic liquid.In this work,our simulation results provide molecular-level insights into the concentration-dependent HB behavior of protic ionic liquid-water mixtures at the interface with phospholipid bilayer,which is of great importance for scientist to understand the influence of ionic liquid on the function of cell membranes.