ADDITION OF (TRI-)BLOCK COPOLYMERS TO PHOSPHOLIPID-VESICLES - A STUDYOF THE MOLECULAR MORPHOLOGY AND STRUCTURE BY USING HYDROPHOBIC DYE MOLECULES AS BILAYER PROBES

A dispersion of soybean lecithin in water leads lo the formation of mu ltilamellar 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-) bl ock copolymers (Synperonics) containing a central hydrophobic part (po lypropylene oxide) and two hydrophilic chains (polyethylene oxide), Th e aim of this work was to study whether it was possible to anchor deep inside the lipid bilayer the copolymer hydrophobic block, The exact l ocalization of the copolymer molecules was investigated using a multip robe technique, The full spectra of two hydrophobic dyes, namely Nile red (NIL) and Pinacyanol chloride (PCYN), were compared while solubili zed inside the liposome bilayer, The sensitivity of their spectral cha racteristics to polarity and self-aggregation produced a monitor of th e bilayer microenvironment, The more hydrophobic NIL proved an accurat e polarity sensor of the bilayer microenvironment and the formation of PCYN dimers ard nonabsorbing aggregates can be directly related to th e local (bilayer) concentration of the dye and the volume available to the solubilized dye molecules, Shifts of the maximum absorbance (lamb da(max)) for both dyes showed that the bilayer environment was becomin g more apolar with increasing copolymer concentration, The absorbance peak of PCYN due to dimer/aggregate formation increased at moderate co polymer concentrations, indicating that the polymer is incorporated in side the lipid bilayer. (C) 1997 Academic Press.