Detection of pH-Tunable “Raft Switches” Using Fluorescence Markers in Rigid Liposome Membranes

Lipid membrane heterogeneities in the form of lipid domains may alter the permeability of membranes. In nature, membrane heterogeneities are prevalent in cells, therefore, understanding the physics of domain formation could be of great significance. We are testing the effect of pH on the domain formation in high glass transition temperature (T_g) anionic liposomes. The liposomes are labeled with Pyrene, a fluorophore that has different maxima emission spectra for ?monomers' and ?excimers'. Measurements are obtained using fluorescence spectrometry, and a lower monomer to excimer ratio corresponds to greater domain formation. We observed that as the pH is dropped domain formation increases. We have also seen that these domain formations are reversible, i.e. if the pH is increased the domains disappear, and if the pH is decreased, the domains reform. We call these reversible domains ?raft switches'. As a control for these experiments, the same experiments were performed with low T_g, zwitterionic liposomes and it was observed that there is no change in the monomer to excimer ratio with change in pH. A potential application of these liposomes is their use as drug delivery carriers to enhance drug availability at the target site because they are engineered to form domains (and have, therefore, enhanced permeability) only at pH's that correspond to the endosomal pathway.