Hexagonal packing of lipid acyl chains and membrane plasticity.

Electron microscope and electron diffraction observations on microcrystals of pure lipids (a phosphatidylethanolamine, two phosphatidylcholines, a phosphatidic acid, and a galactocerebroside) reveal an extreme flexibility of lipid layers when the acyl chains are hexagonally packed (d100 = 4.17 A). This is corroborated by similar observations on wet bilayers of a lecithin. It is shown that more "crystalline" polymethylene packings do not impart such plasticity to lipid layers and are therefore an unsuitable structural matrix for dynamic biological membranes.