Lipids coupled to polyelectrolyte multilayers:: Ultraslow diffusion and the dynamics of electrostatic interactions

The lateral diffusion in a model membrane consisting of phospholipid monolayers attached to a polymer cushion is studied by means of fluorescence recovery after pattern photobleaching (FRAP). The polymer support is formed by multilayers of oppositely charged polyelectrolytes prepared by layer-by-layer self-assembly. Lipid diffusion coefficients in layers in air are very small, on the order of D approximate to 10(-12) cm(2) s (-1), indicating strong electrostatic coupling. Comparing phosphatidylcholine lipids with different chain orders,(DMPC, DOPC and phosphatidic acid (DOPA), it is found that the lipid headgroup determines the lateral diffusion, with negligible influence of the chain. The sign of the charge of the last polyelectrolyte layer does not influence D, which is attributed to lipid binding to extending loops of internal polyelectrolyte layers. The dependence of D on temperature can be described by an activation behavior with activation energies on the order of 10 kT. No hysteresis occurs between diffusion measurements during heating or cooling cycles. Slow kinetics of chain rearrangement are observed at high T. Despite strong electrostatic interactions at the internal interface, evidence of a large amount of molecular flexibility of the polymer chains in layers is provided. The activation energies for a zwitterionic headgroup are larger than for an ionic headgroup, consistent with interactions of the zwitterionic head with both positive and negative polymer segments.