A description of the phospholipid arrangement intermediate to the humidity produced L<inf>α</inf>and H<inf>II</inf>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 (Lα), 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 (HII). X-ray scattering density profiles are derived for both structures. The presence of a stable intermediate rippled structure between the Lαand HIIphases is demonstrated and a model is presented which involves a smooth structural transition between the two extremes described. The transition from Lαto HIIphase 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. © 1989.