Giant Collective Fluctuations of Charged Membranes at the Lamellar-to-Vesicle Unbinding Transition. 1. Characterization of a New Lipid Morphology by SANS, SAXS, and Electron Microscopy

We show the existence of a new and unexpected morphology of charged phospholipid membranes composed of pure dioleoylphosphatidylserine (DOPS, Na⁺) in the absence of added salt. This new morphology is characterized by giant collective fluctuations of locally lamellar stacks of membranes at the lamellar-to-vesicle unbinding transition. The DOPS dispersions have been studied in pure water and at rest using complementary techniques:  small-angle scattering (neutron and X-ray) and freeze-fracture electron microscopy. In the present case of strong unscreened electrostatic interaction, we show that the order-to-disorder transition associated with the lamellar phase unbinding into vesicles is hidden by the presence of this intermediate state of strongly coupled undulating membranes where electrostatic and entropic forces compete. The giant collective fluctuations produce a texture analogous to that of an oyster shell as observed by freeze-fracture electron microscopy. Three microstructures are encountered along the dilution line of bulk samples:  an L<INF>α</INF> lamellar phase with linear swelling, the lamellar oyster shell state characterized by giant collective fluctuations, and finally stable single- or multilayer vesicles formed spontaneously by a peeling mechanism of the fluctuating membranes as suggested by the period of the undulations close to the size of the vesicles.