Modelling polymer-induced adhesion between charged membranes: theoretical and computational approach

Abstract

In the main part of my thesis I describe by analytical techniques and extensive coarse grained MD simulations the adhesion between two identical planar charged surfaces embedded in a uncharged polymer-containing electrolyte solution. The solution mimics physiological fluids: It contains 0.1 M of mono-valent ions and a small number of divalent cations that form tight bonds with the headgroups of the charged lipids. The components have heterogeneous spatial distributions. I extended existing analytical theories based on the Poisson-Boltzmann equation by introducing the polymer-ions and solvent competition between polymer-ions and water-ions solvation energy. The resulting analytical model allowed one to calculate the polymer and electrolyte profiles, the fraction of bound ions and the inter-membrane forces as a function of the surface charge density, fluid composition and solvation energies of ions. Results highlight the relevant role of the ion distribution in determining the exclusion of neutral polymer from the membrane gap provided the polymer dielectric permittivity is smaller than the solvent one (salting-out mechanism). The model shows a strong coupling between osmotic forces, surface potential and salting-out effects of the slightly polar polymer chains. It highlights some of the key differences in the behaviour of monomeric and polymeric mixed solvents and their responses to Coulomb interactions. Our main findings are: a) the onset of long-ranged ion-induced polymer depletion force that increases with surface charge density and b) a polymer-modified repulsive Coulomb force that increases with surface charge density. Overall, the system exhibits homeostatic behaviour, resulting in robustness against variations in the amount of charges. It could explain why non-adsorbing uncharged polymers are so effective in inducing adhesion of both neutral and charged colloidal particles. Our MD simulations confirm these conjectures providing a molecular picture of the phenomenon and move on by following the spontaneous evolution of the modelled system. Next, I removed the constraint of a planar shape and leave the membranes free to fluctuate. Again, both analytical models and MD simulations have been employed and the data have been compared with the experimental findings. Results evidence the onset and growth of inter-membrane distance fluctuations that sometimes form point-like adhesion sites stabilized by the formation of cations-assisted bridges connecting juxtaposed anionic membranes. The lateral growth of these adhesion sites eventually leads to the poration and merging of the contacting lipid bilayers.