Some simple lipid-water systems have been studied by x-ray scattering techniques, as a function of lipid concentration and temperature. Several liquid-crystalline phases have been found, and their structure has been determined: only one of these is lamellar. In all these 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 same technique, and two liquid-crystalline phases have been found: a lamellar phase, built up by an ordered sequence of lipid and water planar sheets, and a hexagonal phase, which is a hexagonal array of circular cylinders, each cylinder being a thin water channel covered by the hydrophilic groups of the lipid molecules, the hydrocarbon chains filling the gap between the cylinders. The interpretation of the electron microscope observations of the structure of lipoprotein membranes is discussed, and some possible biological implications are suggested.

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