Properties of galactocerebroside layers transferred to glassy carbon electrodes: effect of an applied electric field

Electrostatic field effects on membrane domain segregation and on lateral diffusion

Natural membranes are organized structures of neutral and charged molecules bearing dipole moments which generate local non-homogeneous electric fields. When subjected to such fields, the molecules experience net forces that can modify the lipid and protein organization, thus modulating cell activities and influencing (or even dominating) the biological functions. The energetics of electrostatic interactions in membranes is a long-range effect which can vary over distance within r^-1 to r^-3. In the case of a dipole interacting with a plane of dipoles, e.g. a protein interacting with a lipid domain, the interaction is stronger than two punctual dipoles and depends on the size of the domain. In this article, we review several contributions on how electrostatic interactions in the membrane plane can modulate the phase behavior, surface topography and mechanical properties in monolayers and bilayers.

Composition-driven surface domain structuring mediated by sphingolipids and membrane-active proteins. Above the nano- but under the micro-scale: mesoscopic biochemical/structural cross-talk in biomembranes

Biomembranes contain a wide variety of lipids and proteins within an essentially two-dimensional structure. The coexistence of such a large number of molecular species causes local tensions that frequently relax into a phase or compositional immiscibility along the lateral and transverse planes of the interface. As a consequence, a substantial microheterogeneity of the surface topography develops and that depends not only on the lipid-protein composition, but also on the lateral and transverse tensions generated as a consequence of molecular interactions. The presence of proteins, and immiscibility among lipids, constitute major perturbing factors for the membrane sculpturing both in terms of its surface topography and dynamics. In this work, we will summarize some recent evidences for the involvement of membrane-associated, both extrinsic and amphitropic, proteins as well as membrane-active phosphohydrolytic enzymes and sphingolipids in driving lateral segregation of phase domains thus determining long-range surface topography.

Effects of divalent cations on the formation and structure of solid supported lipid films

The interaction of glassy carbon-supported thin wetting films of lecithin with some divalent cations is investigated by impedimetry and voltammetry. The influence of Ca2+, Mg2+, and Mn2+ on the film structure is explored in two different cases--the divalent cations are added to the electrolyte either before or after the formation of the film. When the film has been previously formed, the addition of divalent cations in millimolar concentrations leads to changes in the passive electrical parameters and the blocking properties of the films. On the one hand the dielectric properties of the film measured in 0.1 M KCl seem to improve after the interaction with divalent cations--the film capacitance decreases, the resistance and resistivity of the film increase. On the other hand the increase of the redox current in the presence of 1 mM Fe(CN)6(3-/4-) in the electrolyte suggests the formation of some defects in the lipid structure of the film after the action of divalent cations. It is shown that the amount of these defects could be significantly decreased when the divalent cations are present in the electrolyte solution before the film formation. The effect of divalent cations on the film stability is tested by applying negative potential to the film. In 0.1 M KCl the films are not stable at potential of - 0.8 V (vs. Ag/AgCl) and are destroyed. The addition of divalent cations stabilizes the films and at certain millimolar concentrations the films remain intact after the action of the negative potential. The effect of Mn2+ is more pronounced, the Ca2+ and Mg2+ have smaller commensurate effect. It is proposed that the changes in the films' properties could be related with more tight packing of the lipid molecules with the divalent cations inserted in the film and that some defects could be opened during the rearrangement of the lipids when the film has been previously formed.

Biophysics of sphingolipids II. Glycosphingolipids: An assortment of multiple structural information transducers at the membrane surface

Glycosphingolipids are ubiquitous components of animal cell membranes. They are constituted by the basic structure of ceramide with its hydroxyl group linked to single carbohydrates or oligosaccharide chains of different complexity. The combination of the properties of their hydrocarbon moiety with those derived from the variety and complexity of their hydrophilic polar head groups confers to these lipids an extraordinary capacity for molecular-to-supramolecular transduction across the lateral/transverse planes in biomembranes and beyond. In our opinion, most of the advances made over the last decade on the biophysical behavior of glycosphingolipids can be organized into three related aspects of increasing structural complexity: (1) intrinsic codes: local molecular interactions of glycosphingolipids translated into structural self-organization. (2) Surface topography: projection of molecular shape and miscibility of glycosphingolipids into formation of coexisting membrane domains. (3) Beyond the membrane interface: glycosphingolipid as modulators of structural topology, bilayer recombination and surface biocatalysis.

Effect of externally applied electrostatic fields on the surface topography of ceramide-enriched domains in mixed monolayers with sphingomyelin

Lipid and protein molecules anisotropically oriented at a hydrocarbon-aqueous interface configure a dynamic array of self-organized molecular dipoles. Electrostatic fields applied to lipid monolayers have been shown to induce in-plane migration of domains or phase separation in a homogeneous system. In this work, we have investigated the effect of externally applied electrostatic fields on the distribution of the condensed ceramide-enriched domains in mixed monolayers with sphingomyelin. In these monolayers, the lipids segregate in different phases at all pressures. This allows analyzing by epifluorescence microscopy the effect of the electrostatic field at all lateral pressure because coexistence of lipid domains in condensed state are always present. Our observations indicate that a positive potential applied to an electrode placed over the monolayer promotes a repulsion of the ceramide-enriched domains which is rather insensitive to the film composition, depends inversely on the lateral pressure and exhibits threshold dependence on the in-plane elasticity.