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 L(c) phases, the wide-angle reflections index on an oblique lattice with spacings s(10)≠s(01)≠s(11), and each chain has four nearest neighbours. For orthorhombic (O(⊥)) packing in L(c) phases, and the rotationally symmetric equivalent in metastable low-temperature L(β) phases (mL(β)), the wide-angle reflections index on a centred rectangular lattice with spacings s(20)<s(11), and each chain has four nearest neighbours. For distorted hexagonal packing in the L(c') subgel, and in L(β') gel phases and the metastable low-temperature equivalent (mL(β')), the wide-angle reflections index on a centred rectangular lattice with spacings s(20)>s(11), and each chain has two nearest neighbours. For hexagonal packing in the L(β), P(β') and interdigitated L(β)(i) gel phases, the wide-angle reflections index on a centred rectangular lattice with spacings s(20)=s(11), 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, a(ch) and b(ch), and the cross-sectional area per chain, A(ch), 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.