Determination of the size distribution of liposomes by SEC fractionation, and PCS analysis and enzymatic assay of lipid content

Abstract

In this study, small liposomes obtained by high-pressure homogenization were fractionated according to their particle sizes by size exclusion chromatography (SEC). The subfractions were analyzed by photon correlation spectroscopy (PCS) as well as enzymatic phosphatidylcholine (PC) assay for their particle sizes and lipid contents, respectively. For small egg PC-liposomes, a size range of 15 nm to 60 nm was found, with 80% of the vesicles being smaller than 30 nm in size. This is in contradiction to a mean size of 85 +/- 32 nm as indicated by PCS without fractionation. The PCS technique appears to underestimate very small particles below 30 nm if (few) bigger particles are present. The PCS particle size analysis of unfractionated hydrogenated egg PC/cholesterol-liposomes (2:1, mole/mole) by PCS did not yield any significant results. On fractionation, however, a particle size range of 40 nm to 120 nm was determined in a reproducible manner. Our results indicate that the combination of size exclusion fractionation with subsequent photon correlation spectroscopic particle size analysis and enzymatic PC assay can give both more detailed and more reliable insight into the particle size distribution of small liposomes than PCS alone.