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A B S T R A C TSome simple lipid-water systems havc been studied by x-ray scattering techniques, as a function of lipid concentration and temperature. Several liquid-crystalline phascs have been found, and their structure has been determincd: only one of these is lamellar. In all thcsc phases the hydrocarbon part of the lipid molecules has a disordered, liquid-like structure. One biological phospholipid, a human brain extract, has been studied by the samc technique, and two liquid-crystalline phases have been found: a lamcllar phase, built up by an ordcrcd sequence of lipid and water planar sheets, and a hexagonal phase, which is a hexagonal array of circular cylinders, cach cylinder being a thin water channel covered by thc hydrophilic groups of the lipid molecules, the hydrocarbon chains filling the gap between thc cylinders. The interpretation of the elcctron microscope obscrvations of the structure of lipoprotein mcmbranes is discusscd, and somc possible biological implications arc suggested.

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