Abstract
The nanoscale organization and dynamics of lipid molecules in self-assembled membranes is central to the biological function of cells and in the technological development of synthetic lipid structures as well as in devices such as biosensors. Here, we explore the nanoscale molecular arrangement and dynamics of lipids assembled in monolayers at the surface of highly ordered pyrolytic graphite (HOPG), in different ionic solutions, and under electrical potentials. Using a combination of atomic force microscopy and fluorescence recovery after photobleaching, we show that HOPG is able to support fully formed and fluid lipid membranes, but mesoscale order and corrugations can be observed depending on the type of the lipid considered (1,2-dioleoyl- sn-glycero-3-phosphocholine, 1,2-dioleoyl- sn-glycero-3-phospho-l-serine (DOPS), and 1,2-dioleoyl-3-trimethylammoniumpropane) and the ion present (Na+, Ca2+, Cl-). Interfacial solvation forces and ion-specific effects dominate over the electrostatic changes induced by moderate electric fields (±1.0 V vs Ag/AgCl reference electrode) with particularly marked effects in the presence of calcium, and for DOPS. Our results provide insights into the interplay between the molecular, ionic, and electrostatic interactions and the formation of dynamical ordered structures in fluid lipid membranes.
Highlights
Biological membranes are primarily composed of lipids and membrane proteins self-assembled into a complex two-dimensional structure that can evolve in response to external stimuli and actively support the cell function.[1]
As many membrane components are electrically charged, external electric fields can provide a driving force for the motion of components based on electrophoresis, electroosmosis, and hydrodynamic flow.[5−9] The central role of biological membranes in cell function is underlined by the fact that they constitute a primary drug target,[10] and countless studies have investigated the functional aspects of the nanoscale organization of different proteins and lipids.[11,12]
In the absence of potential, DOPS forms a hemimicelle-like arrangement reminiscent of detergents on highly ordered pyrolytic graphite (HOPG).[98−100] Applying an electrical potential exacerbates the apparent amplitude of the corrugations but not their overall shape, suggesting a subtle effect involving water molecules and the lipid headgroups, as electrostatics alone cannot explain the symmetry between ±1.0 V (Figure 5, H2O)
Summary
Biological membranes (biomembranes) are primarily composed of lipids and membrane proteins self-assembled into a complex two-dimensional structure that can evolve in response to external stimuli and actively support the cell function.[1] By efficiently separating the inside of the cell from the surrounding environment, biomembranes are able to actively sustain a significant transmembrane electrical potential, the primary source of energy for the cell. Few studies have been able to explore the molecular-level details of membranes under an electrical potential, and investigations are typically conducted on membrane fragments or synthetic model membranes supported on a flat solid.[16,17] Changes in the membrane molecular assembly and the structure of its components in response to electric fields can provide important clues about how the transmembrane potentials impact the behavior and function of living cells. The use of supported membranes is helpful for high-resolution studies because it limits the spatial fluctuations natural to cell membranes, and the support can be directly used as an electrode for imposing a controlled transmembrane potential.[18,19] Supported membranes are routinely used as model systems for nanoscale investigations with techniques such as fluorescence and super-resolution microscopies[20] and atomic force microscopy (AFM).[21,22] AFM can simultaneously offer molecular-level topographical and mechanical information about the membranes;[23−25] it can be combined with most optical microscopies and is fully compatible with the electrochemical measurements on membranes.[26−29]
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