Abstract
The interaction between lipid membranes and ions is associated with a range of key physiological processes. Most earlier studies have focused on the interaction of lipids with cations, while the specific effects of the anions have been largely overlooked. Owing to dissolved atmospheric carbon dioxide, bicarbonate is an important ubiquitous anion in aqueous media. In this paper, we report on the effect of bicarbonate anions on the interactions between dipolar lipid membranes in the presence of previously adsorbed calcium cations. Using a combination of solution X-ray scattering, osmotic stress, and molecular dynamics simulations, we followed the interactions between 1,2-didodecanoyl-sn-glycero-3-phosphocholine (DLPC) lipid membranes that were dialyzed against CaCl2 solutions in the presence and absence of bicarbonate anions. Calcium cations adsorbed onto DLPC membranes, charge them, and lead to their swelling. In the presence of bicarbonate anions, however, the calcium cations can tightly couple one dipolar DLPC membrane to the other and form a highly condensed and dehydrated lamellar phase with a repeat distance of 3.45 ± 0.02 nm. Similar tight condensation and dehydration has only been observed between charged membranes in the presence of multivalent counterions. Bridging between bilayers by calcium bicarbonate complexes induced this arrangement. Furthermore, in this condensed phase, lipid molecules and adsorbed ions were arranged in a two-dimensional oblique lattice.
Highlights
Ion−membrane interactions strongly influence membranes' structure and dynamics, as well as their stability, fusion, protein binding properties, and integration
Tightly condensed lamellar phase of DLPC membranes after dialysis against CaCl2 solution
In the swollen phase, characterized in earlier studies,[3,15,22,29,31,32] the calcium ions adsorb onto the DLPC membranes, charge the membrane, and create a net repulsive interaction that swells the lamellar phase
Summary
Ion−membrane interactions strongly influence membranes' structure and dynamics, as well as their stability, fusion, protein binding properties, and integration. By contrast, when multivalent cations[21] (such as calcium or magnesium)[22] are added, strong electrostatic spatial correlations ensue between the cations and negatively charged membranes These ions are strongly correlated[23] and can bridge apposed lipids and generate attraction between likecharged bilayers.[14,24−26] Even when in the liquid phase, dipolar lipids with saturated tails (e.g., 1,2-didodecanoyl-sn-glycero-3phosphocholine, DLPC) can readily adsorb multivalent cations such as calcium.[15,22,27−32] The binding is considerably weaker if one of the tails (e.g., 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine) or both tails are unsaturated.[15] These adsorbed ions, in turn, charge the bilayers and swell the lamellar phase.[15,33,34] Interestingly, the bound cations may reorient the zwitterionic headgroups,[35−38] slightly decrease the area per headgroup, and increase membrane thickness[39] and the gel-to-liquid phase transition temperature.[36,40] It has been shown that the charge density of a membrane adsorbed with calcium cations can be regulated by diluting the DLPC/Ca2+ dispersion[15] or by applying osmotic stress.[14,22]. After the formation of the tightly condensed phase, the lipids within the bilayers were arranged in a two-dimensional (2D) oblique gel-phase, rather similar to other saturated lipids in the gel-phase.[16−19,47]
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