Effects on interactions of oppositely charged phospholipid vesicles of covalent attachment of polyethylene glycol oligomers to their surfaces: adhesion, hemifusion, full fusion and "endocytosis".

Oppositely charged giant vesicles are known to adhere, hemifuse and fuse, all of which depend upon the nature of surface contacts. To further understand such interactions, vesicles were surface-modified with polyethylene glycol (PEG), a moiety that reduces surface-surface interactions. Positively charged vesicles were composed of O-ethyldioleoylphosphocholine (EDOPC), dioleoylphosphatidylcholine (DOPC) and a carbocyanine dye (DiO), with and without DPPE-PEG (dipalmitoylphosphatidylethanolamine-N-PEG MW of the PEG portion = 2000). Negatively charged vesicles were composed of dioleoylphosphatidylglycerol (DOPG), DOPC and a rhodamine B dye (Rh-PE), with as well as without DPPE-PEG (MW 2,000). A microscope-mounted electrophoresis chamber allowed selected pairs of vesicles to be brought into contact while color images were collected at video rates (30 frames/s). Data collection focused on effects of PEG on vesicle interactions as a function of the surface charge density. Relative to PEG-free preparations, vesicles containing DPPE-PEG (1) formed larger contact zones, (2) underwent adhesion and fusion processes more slowly (by two to four times) and (3) at high charge density were less susceptible to rupture upon contact. Unexpectedly, PEG-containing vesicles exhibited engulfment of a smaller by a larger vesicle, a process topologically similar to cellular endocytosis. These observations are interpreted to mean that, although initial surface-surface interactions are weakened by the intervening layer of PEG chains, eventual and strong bilayer-bilayer contact is still possible, evidently because the lipid anchors of these chains can diffuse away from the contact zone.