Giant Vesicles Exposed to Aqueous Two-Phase Systems: Membrane Wetting, Budding Processes, and Spontaneous Tubulation

In this review, we describe recent studies of giant unilamellar vesicles exposed to aqueous polymer solutions. These solutions undergo phase separation when the weight fractions of the polymers exceed a few percent. Depending on the lipid and polymer composition, the membrane may be completely or partially wetted by the aqueous phases. The deflation process induces a variety of vesicle shape transformations. One such transformation is droplet-induced budding of the vesicles. Another, particularly striking transformation is the spontaneous tube formation, which reveals a substantial asymmetry and spontaneous curvature of the membranes, arising from the different polymer compositions across the membrane. The tubulation starts with the nucleation of small membrane buds which then grow into necklace-like tubes. When the tube length reaches a certain critical value, the necklace-like tube transforms into a cylindrical one. Analyzing different aspects of the observed vesicle shapes, quantitative estimates of the spontaneous curvature which is found to vary, depending on the lipid composition, between 1/(125 nm) and 1/(600 nm), are obtained. These curvatures are generated by the weak adsorption of poly(ethylene glycol) (PEG) molecules onto the lipid membranes, with a relatively small binding affinity of about 4 kJ mol−1 or 1.6 kBT per PEG molecule for either liquid-ordered or liquid-disordered membranes.