Modulation of elasticity and interactions in charged lipid multibilayers: monovalent salt solutions

TL;DR

This study investigates how NaCl salt solutions influence electrostatic interactions and membrane flexibility in charged lipid bilayers, combining theoretical modeling with experimental data to reveal minimal structural changes but significant rigidity reduction.

Contribution

It introduces a combined theoretical and experimental approach to analyze salt effects on lipid bilayer interactions, emphasizing the constant charge limit and membrane flexibility.

Findings

No significant structural changes with salt concentration

Four-fold reduction in membrane bending rigidity with increased NaCl

Electrostatic, hydration, and van der Waals forces modeled effectively

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 the 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 four-fold reduction of the membranes' bending rigidity upon increasing NaCl concentration.