EFFECT OF ALKYL CHAIN ASYMMETRY ON THE FUSION AND CRYSTALLIZATION BEHAVIOR OF VESICLES FORMED FROM DI-N-ALKYL PHOSPHATES

Fusion of vesicles formed from synthetic, asymmetric (i.e mixed-chain) sodium di-n-alkyl phosphates (1-6) has been studied with a resonance energy transfer assay for lipid mixing and with transmission electron microscopy. Fusion was induced by Ca2+ ions above the Lalpha --> Lbeta phase transition temperatures of the different bilayers We ha e found that two competing events take place simultaneously but independently from each other These are fusion and formation of tubules, which have been characterized as anhydrous crystals. Both processes are fast. The asymmetric di-n-alkyl phosphates with alkyl chains that differ by only two methylene groups (1 and 2)showed initial fusion rates comparable to that of vesicles formed from (symmetric) di-n-dodecyl phosphate. With an increase in the difference in chain length (3, 4, and 5), the fusion rates increase, indicating that chain asymmetry decreases bilayer stability by reducing alkyl chain packing in the bilayer. However, when the difference in chain length becomes eight methylene groups (6), fusion is effectively suppressed. The sizes of the crystals formed from 1 to 6 turned out to be rather different, but no simple relationship could be found with the extent of asymmetry of the alkyl chains.