Molecular dynamics simulations of phospholipids in water under external electric field

Phospholipids, the main components of cell membranes, are amphiphilic molecules and can form bilayer membrane structure spontaneously in water. Cell membrane provides an barrier against the substance exchange between the inside and outside of a cell. However, the permeability of cell membrane can be greatly increased by applying an electric field, because water pores in membrane are induced by the external electric field and this phenomenon is called electroporation. By using molecular dynamic (MD) simulations, we have studied effects of membrane deformability and thermal motion on electroporation. Our results show that stiffer membrane can inhibit electroporation. The deformability of membrane can also affect the direction of the initial water penetration by prohibiting the rotation of water molecules entering the hydrophobic region of membrane. Defects in the hydrophobic parts of membrane provide the energy–favorable locations for water intrusion. Thermal motion of lipids induces the formation of defects in membrane. The higher the temperature, the more quickly the pore forms in the applied electric field. The simulation system just after electroporation is not at equilibrium. If the simulation time is extended to make the system reach steady state in the applied electric field, membrane reorientation and phase transition occurs just like the behaviors of block copolymers in external electric field. The water/membrane interfaces in different final structures are prefer to be parallel to the direction of the external electric field. The results are consistent with the understanding on the universal behaviors of dielectric interfaces in external electric field. Membrane electroporation, deformation, fusion, fracture and pore-resealing are observed during membrane reorientation and phase transition. Although electroporation is an effective method to introduce gene and drugs into cells, it has the risk to cause cell death, especially under high electric field. To reduce any potential membrane damage in electroporation, an applied electric field should be as weak as possible. The strength of an external electric field might be reduced if water molecules can be pre–buried in the hydrophobic region of membrane by acoustic shock waves. MD simulations are conducted on the water/membrane/water system with water molecules pre-embedded inside the hydrophobic region of membranes. Results approve the above expectation, showing that a minute external electric field can maintain a water pore in membrane and membrane reorientation still occurs even in the minimal external electric field. MD simulations are also conducted to study the self-assembled structures in different lipid concentration and different strength of external electric field, starting from randomly distributed phospholipids and water mixtures. Results show that external electric field can shift the self-assembled phases, which provides an effective method to control the lyotropic liquid crystalline phases of amphiphilic molecules