Ultraslow dynamics of a complex amphiphile within the phospholipid bilayer: Effect of the lipid pre-transition

The shape and intensity of fluorescence emission spectra of Merocyanine 540 embedded in dipalmitoylphosphatidylcholine bilayers differ depending on the thermal history of the sample. This apparent hysteresis in fluorescence emission was most prominent in the temperature range of 20 to 35°C. Analysis of kinetic and temperature cycling experiments suggested that Merocyanine 540 slowly (half time of about 30min) assumes a metastable configuration as temperature is raised above the phospholipid pre-transition point. When the sample was cooled below the pre-transition temperature, the metastable state slowly depopulated (half time of about 15min). The rate of merocyanine exchange among these states was influenced more by membrane lipid mobility than by lipid order since cholesterol increased the rate of transition to the metastable state by a factor of 11.