Gel state surface properties of phosphatidylcholine liposomes as measured with merocyanine 540

The surface properties of liposomes composed by saturated phosphatidylcholines and their mixtures with cholesterol in the gel state have been studied using merocyanine 540 as a fluorescent and optical probe. A new absorption peak at 450 nm and a new fluorescent band at 630 nm were observed when the dye was added to suspensions of DMPC multilamellar liposomes in the gel state. These peaks were also observed in membranes with different lipid compositions in conditions in which the P beta' and the L beta' phases were present. The increase of temperature above the main transition temperature of DMPC or the incorporation of 35% cholesterol into DMPC bilayers at 13 degrees C caused the disappearance of these peaks. The changes in the absorption and fluorescent spectra upon addition of cholesterol resembles very well the phase diagrams reported by Mortensen et al. ((1988) Biochim. Biophys. Acta 945, 221-245) indicating that the corrugated structures characteristic of the L beta' and the P beta' phases have different surface properties related to the partitioning of amphiphilic dies.