Effect of pH on the Permeability of Heterogeneous Liposome Membranes

Increased permeability leading to encapsulated content release from heterogeneous (phase-separated) lipid membranes in the form of liposomes, was previously observed in our group. The objective of this study was to evaluate the permeability of liposome membranes to fluorophores that were not encapsulated into the liposomes, but rather, present in the peri-liposomal solution. ?Inverse' loading of liposomes was studied. The permeability of heterogeneous liposome membranes was investigated using fluorescence spectroscopy. Liposomes were composed of two types of lipids: zwitterionic and anionic, allowing for lipid domains, or ?rafts?, to form at low pH. An increase in domain formation correlates with increased membrane permeability and ?inverse' loading of liposomes. Thus it was hypothesized that the rate at which calcein, a fluorescent water soluble dye, is taken up by liposomes would increase with decreasing pH. Measurements were conducted over a five day period. It was observed that for the first three days, more calcein was taken up by liposomes at lower pHs. These studies will enable us to obtain a better understanding of the molecular mechanisms that govern lipid membrane permeability under phase separated conditions. Implications in understanding cellular functions in areas of cell membranes with lipid rafts are relative and potentially significant.