Structural studies of phospholipid cubic phases

Abstract

In our previous work, we have shown that a general feature of saturated phosphatidylethanolamines is that they exhibit lamellar — inverse hexagonal (H_II) phase transitions with increasing temperature, at all water contents. We have found that the effect of decreasing the lipid chainlength to C₁₂(didodecyl) is to cause the appearance of three intermediate bicontinuous cubic phases in different regions of the binary phase diagram. We have identified these phases as being of cubic aspects 4, 8 and 12, corresponding to probable spacegroups Pn3m (Q²²⁴), Im3m (Q²²⁹) and Ia3d (Q²³⁰). Furthermore, we find that for the C₁₆(dihexadecyl) lipid, cubic phases are also induced in qualitatively the same regions of the phase diagram by methylation of the polar headgroup, which increases its hydrophilicity. We have investigated whether the chirality of the lipid influences the chirality of the cubic phase formed, and have found that, at least for these phospholipids, it does not. We have also found that cubic phases are formed by hydrated phosphatidylcholine/fatty acid 1/2 (mol/mol) stoichiometric mixtures, when the chainlength is reduced, and we report preliminary results obtained by X-ray and neutron diffraction on these systems.