Coarse-grained simulation studies of the adsorption of polyelectrolyte complexes upon lipid membranes

Coarse-grained molecular dynamics simulations have been applied to explore the adsorption of oppositely charged polyelectrolyte complexes (PECs) on an electronically neutral dipalmitoylphosphatidylcholine (DPPC) lipid bilayer. The membrane model implied an implicit solvent description, and the DPPC parametrization is capable of reproducing relatively well the main physical properties of the bilayer such as the area per lipid, bending modulus, bilayer thickness, orientation order parameter and internal pressure distribution. It has been furthermore shown that the lipid model can be applied to investigate the dynamics and adsorption structures of PECs with a varying polyanion-to-polycation charge ratio. Irreversible adsorption has been observed for the overcharged PECs, the polyanion beads being in close contact with the choline group and without internalization into the membrane core. The bound PEC undergoes a relaxation phase characterized by steady spreading on the outer leaflet of the membrane. The effect of the charge excess on this phase has been expressed by variable shapes of PECs ranging from oblate discs to prolate spheroids, net charge gathering at the bilayer surface and partial unwinding of the polyanion. The response membrane was characterized by slightly tighter lipid packing upon PEC attachment and an increase of the order parameter in the membrane core following the PEC relaxation phase. Analysis of truncating electrostatic interactions indicated that the cutoff distance did not influence the dynamics of the adsorption process and induced an artificial ordering of the lipid tails.