Addition of (tri-)block copolymers to phospholipid vesicles: a study of the molecular morphology and structure by using hydrophobic dye molecules as bilayer probes

A dispersion of soybean lecithin in water leads to the formation of multilamellar vesicles (MLVs), which on sonication break down into small unilamellar vesicles of approximately 50 nm in diameter. The addition of polymeric molecules in the liposomal system is thought to provide the liposomes with a steric barrier. The molecules used were (tri-) block copolymers (Synperonics) containing a central hydrophobic part (polypropylene oxide) and two hydrophilic chains (polyethylene oxide). The aim of this work was to study whether it was possible to anchor deep inside the lipid bilayer the copolymer hydrophobic block. The exact localization of the copolymer molecules was investigated using a multiprobe technique. The full spectra of two hydrophobic dyes, namely Nile red (NIL) and Pinacyanol chloride (PCYN), were compared while solubilized inside the liposome bilayer. The sensitivity of their spectral characteristics to polarity and self-aggregation produced a monitor of the bilayer microenvironment.