Cubic phases in membrane lipids.

On the basis of data obtained by time-resolved X-ray diffraction, we consider in the present article the occurrence and formation pathways of inverted bicontinuous cubic phases, or bilayer cubic phases, Q (II)(B) , in diluted dispersions of lipids representing major biomembrane lipid classes [phosphatidylethanolamines (PEs), mixtures of PEs and phosphatidylcholines (PCs) with other lipids, glycolipids]. We show that Q (II)(B) formation proceeds much more easily upon cooling from the H(II) phase than upon heating or isothermal conversion from the L(α) phase, thus identifying an indirect but faster route for Q (II)(B) phase induction in lipids. The data collected consistently show that the ability to convert into cubic phase upon temperature cycling appears to be a general property of all lipids exhibiting an L(α) ↔ H(II) phase transition. Admixtures of charged phospholipids, both anionic and cationic, strongly facilitate Q (II)(B) formation in PEs. Their effect may be attributed to increased electrostatic repulsion between the lipid bilayers that reduces the unbinding energy and facilitates the dissipation of the L(α) phase required for its conversion into bilayer cubic phase.