Absorption and Picosecond Fluorescence Characteristics o f Chlorophyll Vesicles as a Function of Temperature

Liposome, Chlorophyll, Photosynthesis, Fluorescence Lifetime, Lipid Phase Change With chlorophyll a-dipalmitoylphosphatidylcholine-liposomes, the absorption increases at 706 and 450 nm, and decreases at 660 and 420 nm, as the temperature is lowered. As the temperature is increased opposite changes are observed. A lipid phase change occurs at 34°. The pigment to lipid ratio is 1 to 5 in the liposome. With chlorophyll a-soy bean lecithin-liposomes the absorption increases at 706, 680 and 440 nm, and decreases at 650 and 430 nm, as the temperature is lowered. As the temperature is increased, opposite changes are observed. A lipid phase change occurs at 26-27 °C. The pigment to lipid ratio is 1 to 13. The spectral change at 706 nm is identified with aggregated chlorophyll. The concentration o f chlorophyll aggregate increases as the temperature is lowered, and decreases as the temperature is raised. Fluorescence decay from chlorophyll a-soy bean lecithin-liposomes is biphasic. The lifetimes of freshly prepared liposomes are 121 ± 4 ps and 1400 + 200 ps. The relative contribution o f the fast and slow fluorescence components are modified by temperature. Heating results in an increase in both lifetimes, and an increase in fluorescence from the long lived component. These changes are interpreted as resulting from a decrease in energy transfer and concentration quenching. The origin of the biphasic fluorescence and spectral transformations in liposomes, and the possible re­ lation between in vitro and in vivo picosecond fluorescence is discussed.