The effects of chain flexibility on the properties of vesicles formed from di-n-alkyl phosphates

This paper describes a study of the effects of chain flexibility and chain packing on the properties of vesicles formed from sodium di-n-alkyl phosphates. Three di-n-alkyl phosphates with a constant chain length but with different degrees of unsaturation have been synthesized: dioleyl phosphate (DOP) and dielaidyl phosphate (DEP) having, respectively, a cis and a trans double bond at C-9 and the saturated distearyl phosphate (DSP). These surfactants form vesicles, as confirmed by transmission electron microscopy (EM). The gel to liquid-crystalline phase transition was studied using both fluorescence polarization and differential scanning microcalorimetry. According to fluorescence polarization, 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 liquid-crystalline state at 72°C. The polarization of the probe captured in DOP or DEP vesicles decreased gradually in the temperature range 0-40°C, indicating a non-cooperative phase transition. It appears that the vesicle bilayer is in a gel state below 0°C and in a liquid-crystalline state above 40°C. A temperature-dependent 31P NMR study failed to identify the exact phase-transition temperature. The effect of the fusogenic cation Ca2+ was studied qualitatively using EM. Calcium induces fusion of DEP vesicles but, within a short time, tubules are formed which are most probably anhydrous crystals of the calcium salts. For DSP vesicles the latter process is extremely fast and fused vesicles cannot be detected. In contrast, DOP vesicles fuse under the influence of calcium, but no crystallization takes place. The fused DOP vesicles are stable for more than one week in the presence of 4 mM Ca2+ stored at room temperature or at 60°C. Addition of EDTA to DOP vesicles leads to chelation of the calcium ions and to a transition to multilamellar vesicles.