ON THE EXISTENCE OF BICONTINUOUS CUBIC PHASES IN DIOLEOYLPHOSPHATIDYLETHANOLAMINE

Aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) exhibit a well-known phase transition from a lamellar liquid-crystalline (L(alpha)) to an inverted hexagonal (H-II) phase as the temperature is raised above 13 degrees C. Recently, it has been reported [1, 2] that a bicontinuous cubic phase can be induced in this system, when the lipid is rapidly cycled between these two phases. Their X-ray diffraction results indicated that the cubic lattice is most consistent with either the Pn3m or Pn3 space group. In studies on similar systems [3, 4], two cubic phases could be detected in the L(alpha)-H-II transition region, however. One aim of the present study was to investigate, if still further bicontinuous cubic phases can be induced in simple DOPE dispersions by changing the experimental conditions, Indeed, we found that two cubic lattices may be induced when the lipid system is cycled between -5 and 15 degrees C more than thousand times. The second goal of the present investigation was to study the temperature dependence of the lattice parameters, the stability and transformations of these cubic phases over a wide temperature range. It has been discussed that the presence of cubic phases may be a general feature of H-II-forming lipids in the region of their lamellar to hexagonal phase transition region and that non-lamellar lipid structures might play an important role as transient intermediates in biochemical processes.