Electric field-driven water dipoles: nanoscale architecture of electroporation

Abstract

Electroporation is the formation of permeabilizing structures in the cell membrane under the influence of an externally imposed electric field. The resulting increased permeability of the membrane enables a wide range of biological applications, including the delivery of normally excluded substances into cells. While electroporation is used extensively in biology, biotechnology, and medicine, its molecular mechanism is not well understood. This lack of knowledge limits the ability to control and fine-tune the process. In this article we propose a novel molecular mechanism for the electroporation of a lipid bilayer based on energetics analysis. Using molecular dynamics simulations we demonstrate that pore formation is driven by the reorganization of the interfacial water molecules. Our energetics analysis and comparisons of simulations with and without the lipid bilayer show that the process of poration is driven by field-induced reorganization of water dipoles at the water-lipid or water-vacuum interfaces into more energetically favorable configurations, with their molecular dipoles oriented in the external field. Although the contributing role of water in electroporation has been noted previously, here we propose that interfacial water molecules are the main players in the process, its initiators and drivers. The role of the lipid layer, to a first-order approximation, is then reduced to a relatively passive barrier. This new view of electroporation simplifies the study of the problem, and opens up new opportunities in both theoretical modeling of the process and experimental research to better control or to use it in new, innovative ways.

Figure 1. Comparisons of WLW and WVW systems.

Snapshots of the time evolution of water-lipid-water (WLW) and water-vacuum-water (WVW) configurations under an external electric field of 500 MV/m. (a) WLW configuration at times 5.8, 6.7, and 7.3 ns from the start of the simulation with both water molecules (oxygen–red, hydrogen-gray) and lipid molecules (phosphorus-yellow, nitrogen-blue, lipid tail groups–silver) displayed. (b) same WLW data as in (a) but with only water molecules shown. (c) WVW configuration at times 1.157, 1.160, and 1.194 ns.

Figure 2. Pore initiation times.

Average pore initiation times for WLW and WVW systems calculated with three sets of simulations for each configuration.

Figure 3. Protrusion molecules identification.

XZ-projection of the water molecules positions in a typical WVW simulation at the times (a) just before protrusion begins to grow and (b) just before it begins to interact with/attract water molecules from the other side of the gap. Protrusion molecules are colored in red and the rest of the water molecules are shown as blue.

Figure 4. Anti-correlation of protrusion height and total interaction energy.

Graphs demonstrating anti-correlation between the increase of the protrusion height (black curve) and the decrease of the total interaction energy per protrusion molecule of protrusion waters with all other water molecules (both in the protrusion and in bulk) for (a) WLW (red curve) and (b) WVW (blue curve) simulations.

Figure 5. Pearson correlation coefficients.

Histograms of the Pearson correlation coefficients demonstrating anti-correlation between the increase of the protrusion height and the decrease of the total interaction energy per protrusion molecule of protrusion waters with all other water molecules (both in the protrusion and in bulk) for (a) WLW and (b) WVW simulations.

Figure 6. Constituent terms of total interaction energy.

Comparison of constituent terms of the total interaction energy per protrusion molecule between WLW (red curve) and WVW (blue curve) simulations: (a) dipole–external electric field interaction energy, (b) electrostatic interaction energy, (c) Lennard-Jones interaction energy.

Figure 7. Correlation between protrusion height and total interaction energy in WLW.

A graph (a) and a histogram of the correlation coefficient (b) demonstrating positive correlation between the protrusion height growth and the increase in the total interaction energy between the protrusion waters and the lipids in WLW simulations.

Figure 8. Energetic comparison of vertical vs. planar dipole configurations.

Total energies of dipole configurations (I) and (II). Dashed line–sum of dipole-dipole interaction and dipole-electric field interaction terms for a horizontal layer of oriented dipoles (configuration (I)), solid line–sum of dipole-dipole interaction, dipole-electric field interaction, and the total solvation energy required to remove the dipoles from the bulk water for the vertical stack of dipoles (configuration (II)).