FORMATION OF LIPID TUBULE MICROSTRUCTURES - TIME-RESOLVED FREEZE-FRACTURE ELECTRON-MICROSCOPY AND X-RAY CHARACTERIZATION

The tubule-forming lipid 1,2-bis (10,12-tricosadiynoyl)-sn-glycero-3-p hosphocholine has been dispersed in water as a suspension of large ves icles. The temperature of the dispersion was dropped in small steps fr om 45-degrees-C to 37-degrees-C, and at each step aliquots of the samp le were flash-frozen in liquid propane for freeze-fracture replication . In this temperature range most of the vesicles collapse, losing thei r internal water, and transform to tubules. The vesicles display surfa ce roughness at 43-degrees-C, which becomes interspersed with smooth r egions at 40-degrees-C. At 37-degrees-C the smooth areas appear to bec ome predominant and the vesicles roll themselves up into the hollow cy lindrical form of tubules. This process is partly reversible, since re warming this preparation to 50-degrees-C leads to reformation of vesic les that display some surface roughness. Low and wide-angle X-ray scat tering was used to follow the disorder-order lipid hydrocarbon chain t ransition on cooling for samples prepared with tightly packed tubules (centrifugation pellet) or highly concentrated lipid-water lamellar ph ases. In both cases the transition started at about 40-degrees-C, bein g rapidly achieved in the case of the tubule pellet but showing coexis tence for several days at room temperature of disordered and ordered l ipid in the case of the lamellar sample.