Studies of Archaebacterial Bipolar Tetraether Liposomes by Perylene Fluorescence

Abstract

Membrane packing and dynamics of bipolar tetraether liposomes composed of the polar lipid fraction E (PLFE) from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius have been studied by perylene fluorescence. At a probe-to-PLFE lipid ratio of 1:400, we have detected an unusual fluorescence intensity increase with increasing temperature, while the fluorescence lifetime changed little. As the ratio was decreased, the intensity anomaly was diminished. At 1:3200 and 1:6400, the anomaly disappeared. A remarkable perylene intensity anomaly was also observed in bilayers composed of saturated monopolar diester phosphatidylcholines at their main phase transition temperatures. These results suggest that the intensity anomaly may be due to probe aggregation caused by tight membrane packing. At the same probe-to-lipid ratio (1:400), however, 1,2-diphytanoyl- sn-glycero-3-phosphocholine (DPhPC) and 1,2-diphytanoyl- sn-glycero-3-phosphoglycerol (DPhPG) liposomes did not exhibit any intensity anomaly with increasing temperature. This suggests that DPhPC and DPhPG liposomes are more loosely packed than PLFE liposomes; thus the branched methyl groups are not the contributing factor of the tight membrane packing found in PLFE liposomes. Using a multiexcitation method, we have also determined the average ( R), in-plane ( R ip), and out-of-plane ( R op) rotational rates of perylene in PLFE liposomes at various temperatures (20–65°C). R and R ip, determined at two different probe-to-lipid ratios (1:400 and 1:3200), both undergo an abrupt increase when the temperature is elevated to ∼48°C. These data suggest that PLFE liposomes are rigid and tightly packed at low temperatures, but they begin to possess appreciable “membrane fluidity” at temperatures close to the minimum growth temperature (∼50°C) of thermoacidophilic archaebacteria.