Effect of poly(ethylene glycol) on the Ca2+-induced fusion of didodecyl phosphate vesicles

This paper reports a study of the effect of the dehydrating agent poly(ethylene glycol) (PEG) on didodecyl phosphate (DDP) bilayers and on the fusion activity of DDP vesicles as a function of the molecular weight of PEG. PEG 8K in a concentration of 10 wt % does not induce fusion. However, Ca2+-induced fusion is promoted as reflected by a lowering of the Ca2+ threshold concentration. This effect can most likely be attributed to the dehydrating capacity of the polymer. Interestingly, low concentrations (0.1 wt %) of PEG 20 K induce a moderate fusion capacity. At higher concentrations (0.5 wt %) fusion is inhibited, irrespective of the presence of Ca2+. These molecular weight dependent effects can be rationalized by taking into account that the clouding temperature differs for PEGs of different molecular weights. In the case of PEG 20K a microscopic phase separation will occur at the bilayer-water interface because PEG-PEG interactions and presumably PEG-DDP interactions are favored over PEG-water interactions. As a consequence, the DDP vesicle surface becomes covered with PEG 20K, resulting in a steric stabilization of the vesicles. This will impede or prevent, depending on the polymer concentration, the vesicles from approaching each other sufficiently close for fusion to occur.