Lipid matrix properties in cationic membranes interacting with anionic polyelectrolytes: A solid-state NMR approach

A combined solid-state NMR and small-angle X-ray scattering approach was employed to investigate morphology and lateral packing properties of mixed zwitterionic and cationic lipid membranes in the presence of an anionic polyelectrolyte. Binary mixtures of zwitterionic DMPC and cationic DODAP form stable lipid bilayers over a wide range of mixing ratios. Addition of negatively charged dextran sulfate (DS) leads to the formation of densely packed lipid bilayer stacks with the polyelectrolyte sandwiched between opposing lipid surfaces. Bridging of polyelectrolyte strands between bilayer surfaces provides attractive forces to overcompensate electrostatic and hydration repulsion, resulting in close approach of apposing membranes. The equilibrium surface distance is controlled by a balance of electrostatic, hydration, bridging, and entropic forces. Lipid molecules in the mixed membranes remain homogeneously distributed in both the absence and presence of DS. DS binds exclusively to cationic DODAP. As a result of DS binding, lipid chain order is equally increased throughout the lipid bilayer. Therefore, a partial penetration of the polyelectrolyte into the hydrophobic core of the membrane is unlikely. Increased order parameters correspond to a reduction of lateral lipid packing density equivalent to an average reduction of area per molecule of 2.9 Angstrom (2). We suggest that this area reduction is the result of membrane surface dehydration induced by the polyelectrolyte that creates attractive forces between bilayers.