Abstract
We have studied the electrostatic screening effect of NaCl solutions on the interactions between anionic lipid bilayers in the fluid lamellar phase using a Poisson–Boltzmann-based mean-field approach with constant charge and constant potential limiting charge regulation boundary conditions. The full DLVO potential, including the electrostatic, hydration and van der Waals interactions, was coupled to thermal bending fluctuations of the membranes via a variational Gaussian Ansatz. This allowed us to analyze the coupling between the osmotic pressure and the fluctuation amplitudes and compare them both simultaneously with their measured dependence on the bilayer separation, determined by the small-angle X-ray scattering experiments. High-structural resolution analysis of the scattering data revealed no significant changes of membrane structure as a function of salt concentration. Parsimonious description of our results is consistent with the constant charge limit of the general charge regulation phenomenology, with fully dissociated lipid charge groups, together with a 6-fold reduction of the membranes’ bending rigidity upon increasing NaCl concentration.
Highlights
Lipid bilayers are well-established mimics of biological membranes, enabling the application of an array of biophysical techniques to study their physicochemical properties.[1−4] Significant efforts have been devoted to unraveling the interactions between adjacent membranes,[5] which are remarkably similar to those between other biological macromolecules or between colloids in general.[6−8] Rigid uncharged membranes are well-described within the Derjaguin−Landau− Verwey−Overbeek (DLVO) paradigm where the total interaction potential can be decomposed into an attractive van der Waals part and a repulsive hydration interaction part, respectively,[9] augmented by a short-range steric contribution arising from lipid headgroup collisions of adjacent bilayers at vanishing separations.[10]
The swelling behavior of charged anionic bilayers in salt solutions for four different concentrations of monovalent salt (NaCl), was fitted to the osmotic pressure dependencies of the mean bilayer separation and fluctuation amplitude predictions based on a theoretical model that hinges on a variant of the Feynman−Kleinert variational theory
From these measurements and fits, we found that the equilibrium interlamellar separation decreases with increasing salt concentration, which is expected on grounds that the electrostatic repulsion favoring multilayer swelling is reduced by the Debye ES screening
Summary
Lipid bilayers are well-established mimics of biological membranes, enabling the application of an array of biophysical techniques to study their physicochemical properties.[1−4] Significant efforts have been devoted to unraveling the interactions between adjacent membranes,[5] which are remarkably similar to those between other biological macromolecules or between colloids in general.[6−8] Rigid uncharged membranes are well-described within the Derjaguin−Landau− Verwey−Overbeek (DLVO) paradigm where the total interaction potential can be decomposed into an attractive van der Waals (vdW) part and a repulsive hydration interaction part, respectively,[9] augmented by a short-range steric contribution arising from lipid headgroup collisions of adjacent bilayers at vanishing separations.[10] Both, the vdW and the hydration interactions are ubiquitous and not specific for membrane−membrane interactions, as is sometimes claimed for the latter.[11] Hydration interaction represents a universal, solvent-mediated interaction in a highly structured solvent such as water, observed to occur at small spacings even between completely rigid surfaces and can not be ascribed to thermally excited protrusions.[12]. The PB predictions can sometimes fail even qualitatively for physically interesting situations involving highly charged membranes, or multivalent mobile ions, engendering ES interactions between symmetrically charged surfaces that can turn attractive, defying the common wisdom about ES
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