The use of hydrophobic dye molecules in monitoring the liposome bilayer microenvironment and locating block copolymers added to enhance liposome steric stability

The dispersion of soybean lecithin in water leads to the formation of multilamellar vesicles (MLVs), which on sonication (4hrs approx.) break down to small unilamellar vesicles of similar to 50nm diameter. The addition of polymeric molecules in the liposomal system provides increased steric stabilization. The molecules used were (tri-)block copolymers (Synperonics) containing a central hydrophobic part (polypropylene oxide-PPO) and two hydrophillic chains (polyethylene oxide-PEG) extending from either side. The interaction of these molecules with the vesicle bilayer is thought to be of upmost importance to the mechanical stress, thermodynamic restrictions and steric stability that may be induced. The exact localisation of the copolymer molecules was attempted using a multiprobe technique. The full spectrum 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 molecular mobility produced a monitor of the bilayer micropolarity and fluidity, The relatively high hydrophobicity of Nile red (NIL) provides an accurate polarity sensor of the bilayer microenvironment. The formation of Pinacyanol chloride (PCYN) dimers (and their respective peak) was directly related to the distance between the dye molecules. Shifts of the maximum absorbance (lambda(max)) for both dyes showed that the bilayer environment was becoming more apolar with increasing copolymer concentration. The absorbance intensity decreased with increasing copolymer concentration, denoting a reduction in the solubilization of both dyes and therefore of the bilayer population. The absorbance peak of Pinacyanol chloride (PCYN) due to dimer formation increased at moderate copolymer concentrations, showing signs of possible incorporation inside the bilayer. These experiments provided information about the bilayer structure. Adding block copolymers at an optimum concentration may increase the stability of the liposome by incorporation, following various models proposed. However, at high content of copolymer some bilayer solubilization and mixed micelle formation may occur.