Lateral order in gel, subgel and crystalline phases of lipid membranes: Wide-angle X-ray scattering

The chain packing in ordered phases of lipid bilayers can be classified according to wide-angle X-ray diffraction (WAXS). For triclinic (T‖) and monoclinic (M‖) packing in crystalline Lc phases, the wide-angle reflections index on an oblique lattice with spacings s10 ≠ s01 ≠ s11, and each chain has four nearest neighbours. For orthorhombic (O⊥) packing in Lc phases, and the rotationally symmetric equivalent in metastable low-temperature Lβ phases (mLβ), the wide-angle reflections index on a centred rectangular lattice with spacings s20 < s11, and each chain has four nearest neighbours. For distorted hexagonal packing in the Lc′ subgel, and in Lβ′ gel phases and the metastable low-temperature equivalent (mLβ′), the wide-angle reflections index on a centred rectangular lattice with spacings s20 > s11, and each chain has two nearest neighbours. For hexagonal packing in the Lβ, Pβ′ and interdigitated Lβi gel phases, the wide-angle reflections index on a centred rectangular lattice with spacings s20 = s11, and each chain has six nearest neighbours. The available WAXS database for phospholipid and glycolipid bilayers is classified here according to the above scheme, by using well-established examples to assign the wide-angle reflections. The nearest and next-nearest neighbour chain–chain spacings, ach and bch, and the cross-sectional area per chain, Ach, are calculated for each phase of each lipid. These parameters determine many of the properties of the ordered lipid phases and, together with the chain tilt, specify the area occupied by the lipid head groups at the surface of the bilayer.

► Chain packing in the ordered phases of phospholipid and glycolipid bilayers is classified according to the symmetry of the subcell determined from wide-angle X-ray diffraction (WAXS).
► Nearest and next nearest neighbour chain spacings and cross-sectional area per chain are presented for a wide range of phospholipids and glycolipids in Lc, Lc’, Lβ, Lβ’, Pβ’ and Lβi (interdigitated) phases.
► Data on headgroup spacings in the subgel phases of phosphatidylcholines are summarised.