ON THE EXISTENCE OF BICONTINUOUS CUBIC PHASES IN DIOLEOYLPHOSPHATIDYLETHANOLAMINE

Aqueous dispersions of 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolam ine (DOPE) exhibit a well-known phase transition from a lamellar liqui d-crystalline (L(alpha)) to an inverted hexagonal (H-II) phase as the temperature is raised above 13 degrees C. Recently, it has been report ed [1, 2] that a bicontinuous cubic phase can be induced in this syste m, when the lipid is rapidly cycled between these two phases. Their X- ray diffraction results indicated that the cubic lattice is most consi stent with either the Pn3m or Pn3 space group. In studies on similar s ystems [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 inves tigate, if still further bicontinuous cubic phases can be induced in s imple DOPE dispersions by changing the experimental conditions, Indeed , we found that two cubic lattices may be induced when the lipid syste m 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 transformation s of these cubic phases over a wide temperature range. It has been dis cussed 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.