Observation of inverted cubic phase in hydrated dioleoylphosphatidylethanolamine membranes

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Abstract
We report the observation of an inverted cubic phase in aqueous dispersions of 1,2-dioleoyl-sn-glycero-2-phosphoethanolamine (DOPE) by small-angle X-ray diffraction. DOPE is a paradigm in the study of nonlamellar phases in biological systems: it exhibits a well-known phase transition from the lamellar (L.alpha.) to the inverted hexagonal phase (HII) as the temperature is raised. The transition is observed to occur rapidly when a DOPE dispersion is heated from 2.degree.C, where the L.alpha. phase is stable, to 15.degree.C where the HII phase is stable. We report on the induction of a crystallographically well-defined cubic lattice that is slowly formed when the lipid dispersion is rapidly cycled between -5 and 15.degree.C hundreds of times. Once formed, the cubic lattice is stable at 4.degree. C for several weeks and exhibits the same remarkable metastability that characterize other cubic phases in lipid-water systems. X-ray diffraction indicates that the cubic lattice is most consistent with either the Pn3m or Pn3 space group. Tests of lipid purity after induction of the cubic indicate the lipid is at least 98% pure. The cubic lattice can be destroyed and the system reset by cycling the specimen several times between -30 and 2.degree.C. The kinetics of the formation of the cubic are dependent on the thermal history of the sample, overall water concentration, and the extreme temperatures of the cycle. The presence of cubic phases may be a general feature of HII-forming lipids and is discussed in terms of the spontaneous radius of curvature of lipid monolayers [Gruner, S. M. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 3665-3669].

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