VESICLE FORMATION OF DI-NORMAL-ALKYL PHOSPHATES - LIQUID-CRYSTALLINE BEHAVIOR, MYELINIZATION, COUNTERION INFLUENCE, AND STABILITY

The addition of Ca+ to vesicular suspensions of synthetic di-n-alkyl phosphates (DAP) has a devastating effect on the structure of the vesicles. The vesicles transform into large tubular structures, which upon addition of the Ca2+-binding agent EDTA, revert again to large vesicles. DAP salts other than Ca2+ ((CnH2n+1O)2PO2-M+, With M+ = Na+, K+, TMA+; while n = 10, 12,14,16,18) readily form vesicles in water when stirred above an amphiphile specific temperature. Vesicle formation was studied with a variety of techniques including turbidity measurements, encapsulation methods, optical polarization microscopy, differential scanning calorimetry, electron microscopy, and a fluorescence assay to monitor lipid mixing. All investigated DAP salts (except Ca2+) form lamellar lyotropic phases in water. From these lamellar phases myelin tubules can grow, the formation of which depends on the temperature. The alkyl chain length and the counterion of the amphiphile determine the temperature (T(myelin)) above which myelin tubules can form. These myelin structures do not spontaneously transform into vesicles. However, vesicles do form when mechanical energy is imparted into the system. The ability to form myelin structures (and indirectly vesicles) appears to be regulated by hydration properties of the DAP crystals, which are mainly determined by the counterion and the alkyl chain length. The nature of the counterion is also important for the vesicle stability, which increases with increasing counterion dissociation (TMA+ > K+ > Na+). Thermodynamically the DAP vesicles are unstable; they slowly transform into a crystalline phase. Upon warming, these crystals can dissolve and vesicles are formed. This reversible transition from vesicle to crystal is accompanied by little membrane mixing. This implies that, upon transformation into the crystalline state, the vesicles maintain their identity and molecular randomization does not occur.