Lateral organization of lipid membranes induced by amphiphilic polymer inclusions

Polymers belonging to a set of amphiphilic poly(acrylic acid) derivatives of varying hydrophobicity and charge density have recently been shown to slowly break small lipid vesicles and stabilize for hours or days transient mixed states such as membrane sheets, aggregates of mixed micelles, integral membrane proteins complexes, and so forth. We used giant unilamellar vesicles labeled with fluorescent probes to observe the evolution of a whole lipid-membrane including nondisruptive events, during the polymer-induced transition. The effect on the lipid bilayer depended strongly on the chemical structure and the concentration of polymer. Polymers of high hydrophobicity needed hours to disrupt the membranes. Before breakage, we observed intermediate states such as buds and filaments. Using less hydrophobic polymers, formation of flat domains was observed over hours at high polymer concentration (0.5 g/L). A single vesicle combined, over a few tens of micrometers, both curved fluorescent zones and flat zones of different compositions. More dilute conditions preserved the vesicle curvature and its low permeability to Dextran (M-w 9300 g/mol). The bound polymer/lipid ratio was on the order of 50-60 mg/g as measured by capillary electrophoresis analysis. These mixed systems give a unique experimental access to the effect of amphipatic macromolecules on membrane structure and properties. The slow kinetics of reorganization and generic formation of domains are specific features of the macromolecules in contrast with the well-documented effects of short surfactant molecules.