THE EFFECTS OF CHAIN FLEXIBILITY ON THE PROPERTIES OF VESICLES FORMEDFROM DI-N-ALKYL PHOSPHATES

This paper describes a study of the effects of chain flexibility and c hain packing on the properties of vesicles formed from sodium di-n-alk yl phosphates. Three di-n-alkyl phosphates with a constant chain lengt h but with different degrees of unsaturation have been synthesized: di oleyl phosphate (DOP) and dielaidyl phosphate (DEP) having, respective ly, a cis and a trans double bond at C-9 and the saturated distearyl p hosphate (DSP). These surfactants form vesicles, as confirmed by trans mission electron microscopy (EM). The gel to liquid-crystalline phase transition was studied using both fluorescence polarization and differ ential scanning microcalorimetry. According to fluorescence polarizati on, using trans,trans,trans-1,6-diphenyl-hexa-1,3,5-triene (DPH) as a probe, DSP vesicles undergo a cooperative transition from a gel to a l iquid-crystalline state at 72 degrees C. The polarization of the probe captured in DOP or DEP vesicles decreased gradually in the temperatur e range 0-40 degrees C, indicating a non-cooperative phase transition. It appears that the vesicle bilayer is in a gel state below 0 degrees C and in a liquid-crystalline state above 40 degrees C. A temperature -dependent P-31 NMR study failed to identify the exact phase-transitio n temperature. The effect of the fusogenic cation Ca2+ was studied qua litatively using EM. Calcium induces fusion of DEP vesicles but, withi n a short time, tubules are formed which are most probably anhydrous c rystals of the calcium salts. For DSP vesicles the latter process is e xtremely fast and fused vesicles cannot be detected. In contrast, DOP vesicles fuse under the influence of calcium, but no crystallization t akes place. The fused DOP vesicles are stable for more than one week i n the presence of 4 mM Ca2+ stored at room temperature or at 60 degree s C. Addition of EDTA to DOP vesicles leads to chelation of the calciu m ions and to a transition to multilamellar vesicles.