A description of the phospholipid arrangement intermediate to the humidity produced L_α and H_II phases in dioleoylphosphatidylcholine and its modification by dioleoylphosphatidylethanolamine as studied by X-ray diffraction

X-ray diffraction from ordered multilayers has been used to study the structure of the lipid-water system in orientated multilayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 30°C as a function of water content. At high levels of water content the structure is lamellar (L_α), comprising equidistant lipid bilayers of constant thickness separated by layers of water. The low water content structure is a two-dimensional hexagonal array with narrow water channels (H_II). X-ray scattering density profiles are derived for both structures. The presence of a stable intermediate rippled structure between the L_α and H_II phases is demonstrated and a model is presented which involves a smooth structural transition between the two extremes described. The transition from L_α to H_II phase is related to the bilayer thickness and not to any other variable parameter such as water content or composition. The effect of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) on the structure and phase transition characteristics of the system is reported.