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.

Picosecond and Terahertz Perturbation of Interfacial Water and Electropermeabilization of Biological Membranes

“…First, we are working at frequencies or on time scales where the relative dielectric permittivity of water is much less than 80 (Barthel et al 1990). Second, the intrusion of individual water molecules from the complex dielectric landscape of the membrane interface to the low-permittivity environment of the lipid bilayer interior is a phenomenon much better treated in atomic detail, where macroscale material properties like permittivity are of limited utility (Tokman et al 2013). …”

“…What happens in the simulations is contrary to these expectations. The stabilization of intruding interfacial water by the applied electric field (Tokman et al 2013) is not reversed when the field direction changes. The water bridges grow, rather than shrink, cycle after cycle, drawing the phospholipid head groups after them.…”

“…Molecular dynamics simulations provide a view of one type of permeabilizing structure, the lipid electropore (Tieleman 2004; Gurtovenko and Vattulainen 2005; Tarek 2005; Böckmann et al 2008; Levine and Vernier 2010), consistent with experimental observations (Melikov et al 2001; Naumowicz and Figaszewski 2013; Koronkiwicz et al 2002) and continuum models (Zimmermann et al 1974; Abidor et al 1979; Chizmadzhev and Abidor 1980; Weaver and Chizmadzhev 1996). The formation of these nanometer-scale, conductive pores is initiated by the electric-field-stabilized intrusion of interfacial water into the membrane interior, which leads to the construction of energy-minimized water bridges across the lipid bilayer and the reorganization of the membrane phospholipids along the water columns to form the hydrophilic wall of the pore (Tokman et al 2013). Although lipid electropores are only one component of the electropermeabilized cell membrane, they are the first structures to form after the application of a porating electric field, and it may be that for very short pulses or very high-frequency EMF they are the dominant structures.…”

Electroporation-Mediated Gene Delivery

“…Computer simulations using recent advances in molecular dynamics approaches to describe membrane behavior under the influence of an applied electric field have suggested that once the membrane reaches the critical point, water molecules begin to gain access across the membrane and an inverted hydrophilic pore can be formed (Figure 2) (Delemotte & Tarek, 2012; Ho, Casciola, Levine, & Vernier, 2013; Ho, Levine, & Vernier, 2013; Kramar et al, 2012; Polak et al, 2013; Tokman et al, 2013). With continued application in the field, this metastable hydrophilic pore can transition to a larger, more stable pore that can allow the transport of larger charged macromolecules.…”

Electrical analysis of cell membrane poration induced by an intense nanosecond pulsed electric field, using an atomistic-to-continuum method

“…Based on an energetic analysis, Tokman and co-authors recently suggested that pore formation in response to an intense electric field is driven by the reorganization of water molecules in columnar stacks at the membrane interface [14]. They showed that an electric field of sufficient intensity stabilizes columnar water dipole arrays oriented along the applied field while it destabilizes planar water configuration perpendicular to the field direction.…”