Lipid Corralling and Polymer Squeeze-out in Membranes*

Victims of electrical trauma suffer extensive loss of structural integrity of cell membranes. Stable structural defects -- ``pores'' in the range of 0.1 mm -- have been demonstrated in electroporated cell membranes. Poloxamer 188, a triblock copolymer of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) is known to help seal electroporated cell membranes, arresting the leakage of intracellular materials of the damaged cell. Using a monolayer to mimic the outer leaflet of the cell membrane, we have examined the interaction between the poloxamer and zwitterionic and anionic phospholipid monolayers. With synchrotron x-ray reflectivity and grazing-incidence x-ray diffraction, both the out-of-plane and in-plane structures of mixed phospholipid-poloxamer 188 monolayers were investigated at the air-water interface. P188 selectively inserts into low lipid-density regions of the membrane and ``corrals'' lipid molecules to pack tightly, leading to unexpected Bragg peaks at low nominal lipid density and inducing the film to separate into P188-rich and -poor phases. At tighter lipid packing, the once inserted P188 is squeezed out, providing a route for the poloxamer to gracefully exit when the membrane integrity is restored. Cryo-electron microscopy shows that the poloxamer can associate with the lipid in a reversible two-state fashion, depending on the physical state of the lipid. At temperatures above the main transition temperature (T$_{M})$ where the lipid is fluid, the poloxamer can incorporate itself into lipid vesicles, resulting in a more monodispersity vesicle population. At temperatures below T$_{M, }$the poloxamer cannot penetrate into the gel-like lipid layer, but instead corrals the lipid molecules to self-assemble into bilayer disks with the edges stabilized by the poloxamer.