Effect of PEG lipid on fusion and fission of phospholipid vesicles in the process of freeze-thawing

The effects of freeze-thawing on the sizes and size distribution of egg yolk phosphatidylcholine (EggPC) liposomes in the presence of 0-40 mol% distearoylphosphatidylethanolamine-polyethyleneglycol 2000 (DSPE-PEG2000) prepared by detergent removal method were studied by quasielastic light scattering (QELS), gel exclusion chromatography, and freeze-fracture electron microscope. Especially, gel exclusion chromatography successfully provided the size distribution of polydisperse vesicle suspension. The mean diameters of the liposomes, which had originally large size, decreased with increase in the number of freeze-thawing cycles. On the contrary, the mean diameters of the liposomes, which had originally small size, increased with increase in the numbers of freeze-thawing cycles. After freeze-thawing over 10 times, the liposomal mean diameters fell into a range from 80 to 150 nm in spite of their original size. Gel exclusion chromatography showed that in the process of freeze-thawing of the liposomes containing DSPE-PEG 2000, two opponent events, one is fission and the other is fusion, occurred at the same time.