Bridges of Calcium Bicarbonate Tightly Couple Dipolar Lipid Membranes

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 CaCl 2 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.

Interactions between zwitterionic membranes in complex electrolytes

“…We first consider neutral membranes with vanishing net charge, i.e., σ s± = σ s and σ m = 0. Equation (26) becomes…”

“…The electrostatic component of the intermembrane forces in charged multilamellar lipid bilayers was measured also in electrolyte solutions in the presence of monovalent [21][22][23] as well as multivalent salts [24] and divalent buffers [25]. More recently, the effect of bicarbonate anions in the presence of previously adsorbed calcium cations on the interactions between dipolar lipid membranes have been investigated in detail [26]. In these systems many different mechanisms can be operating concurrently and it is highly nontrivial to select for the purely electrostatic effects as opposed to nonelectrostatic ionic binding, van der Waals interaction changes or the coupling between the structural and electrostatic interactions.…”

Contribution of dipolar bridging to phospholipid membrane interactions: A mean-field analysis

“…In Appendix A, we show that the ZPB equation ( 12) can be expressed as a generalized Poisson identity. The iterative solution of the ZPB equation can be carried out by first setting integral (16) to I = 1 and updating the value of I at each iterative step until numerical convergence is achieved. This solution scheme should be applied with the electrostatic boundary conditions…”

“…Among these unconventional electrostatic effects, the repulsion between the overall neutral dipolar membranes has been identified first in Parsegian's experiments with egg lecithin bilayers as hydration interaction. 9,10 Subsequently, these experiments have been extended to understand the effect of monovalent salt, [11][12][13] divalent buffers, 14 and multivalent ions 15,16 on zwitterionic membrane interactions.…”

Contribution of dipolar bridging to phospholipid membrane interactions: a mean-field analysis

“…Among these unconventional electrostatic effects, the repulsion between overall neutral dipolar membranes has been identified first in Parsegian's experiments with egg lecithin bilayers as hydration interaction [9,10]. Subsequently, these experiments have been extended to understand the effect of monovalent salt [11][12][13], divalent buffers [14], and multivalent ions [15,16] on zwitterionic membrane interactions.…”