All-atom molecular dynamics simulations of the combined effects of different phospholipids and cholesterol content on electroporation

Abstract

The electroporation mechanism could be related to the composition of the plasma membrane, and the combined effect of different phospholipid molecules and cholesterol content on electroporation has rarely been studied nor conclusions drawn. In this paper, we applied all-atom molecular dynamics (MD) simulations to study the effects of phospholipids and cholesterol content on bilayer membrane electroporation. The palmitoyloleoylphosphatidylcholine (POPC) model, palmitoyloleoylphosphatidylethanolamine (POPE) model, and a 1 : 1 mixed model of POPC and POPE called PEPC, were the three basic models used. An electric field of 0.45 V nm^-1 was applied to nine models, which were the three basic models, each with three different cholesterol content values of 0%, 24%, and 40%. The interfacial water molecules moved under the electric field and, once the first water bridge formed, the rest of the water molecules would dramatically flood into the membrane. The simulation showed that a rapid rise in the Z-component of the average dipole moment of the interfacial water molecules (Z-DM) indicated the occurrence of electroporation, and the same increment of Z-DM represented a similar change in the size of the water bridge. With the same cholesterol content, the formation of the first water bridge was the most rapid in the POPC model, regarding the average electroporation time (t _ep), and the average t _ep of the PEPC model was close to that of the POPE model. We speculate that the differences in membrane thickness and initial number of hydrogen bonds of the interfacial water molecules affect the average t _ep for different membrane compositions. Our results reveal the influence of membrane composition on the electroporation mechanism at the molecular level.

Fig. 1. Time spent in each stage of the simulation. T stands for temperature, and P stands for pressure.

Fig. 2. The potential energy and the area per lipid (APL) of the PEPC model during the 10 ns equilibrium process. (a) Potential energy. (b) APL.

Fig. 3. Dynamic process of electroporation in the PEPC model under an electric field. The oxygen and hydrogen atoms are represented by red and white spheres, the gold spheres represent the phosphorus atoms of the phospholipid headgroup, and the blue string-like structures in the middle are the tails of the phospholipid molecules. (a) 0 ns – initial stage. (b) 0.58 ns – water protrusion. (c) 1.15 ns – water bridge. (d) 1.35 ns – after water bridge.

Fig. 4. The Z-DM and the number of H-bonds for the POPE, POPC and PEPC models. The scatter plots are our data. In order to facilitate observation, we fitted the data to get the curves, and marked the time when the first water bridge of the POPC, POPE and PEPC models formed. (a) Z-DM. (b) H-bonds.

Fig. 5. The Z-DM at different points in time and the corresponding top views of the three models under the same Z-DM. In order to observe the size of the water bridge, the top views only showed the phospholipid molecules and hid the water molecules. (a) The corresponding times of the three models at the same Z-DM. (b) Top views of the three models at the same Z-DM.

Fig. 6. The Z-DM and top views of the cholesterol-containing models. (a) The changes of the Z-DM with time. The scatter plots are our data. We fitted the data to get the curves, and the solid circles mark the time when the first water bridges of the POPC, POPE and PEPC models formed. The dotted line indicates the Z-DM value of 0.6 D. From the occurrence of electroporation, increments in the Z-DM of 0.4 D are marked by triangles. (b) The times and corresponding top views of all models when the Z-DM was 0.6 D. (c) The times and corresponding top views of all models when the Z-DM increment was 0.4 D.

Fig. 7. Average t_ep and membrane thickness for all models. The average t_ep and membrane thickness were the average values of 10 repeated simulation results for each model, and the membrane thickness was calculated before the electric field was applied. The dotted lines were made to follow the change in values between models at the same cholesterol content value. (a) Variation of average t_ep with cholesterol content for the POPE, POPC and PEPC phospholipid models. (b) Variation of membrane thickness with cholesterol content for the POPE, POPC and PEPC phospholipid models.

Fig. 8. The initial number of H-bonds and the change in the number of H-bonds for the 40% cholesterol content models under the electric field. The initial number of H-bonds was the average value of the number of H-bonds during the 10 ns equilibrium process. (a) Initial number of H-bonds for all models. (b) The number of H-bonds for the 40% cholesterol models under the electric field. The black circles represent the moment when electroporation occurs. The dotted lines represent the average number of interfacial water H-bonds over the first nanosecond after the electric field is applied.

Fig. 9. Hydrogen bonds between phospholipid molecules and interfacial water molecules in the PEPC model under the electric field at three cholesterol concentrations.