Abstract.-Both membranes of Mycoplasma laidlawii and water dispersions of protein-free membrane lipids exhibit thermal phase transitions that can be detected by differential scanning calorimetry. The transition temperatures are lowered by increased unsaturation in the fatty acid residues, but in each case they are the same for membranes and lipids. The transitions resemble those observed for synthetic lipids in the lamellar phase in water, which arise from melting of the hydrocarbon chains within the phospholipid bilayers. Such melts are cooperative phenomena and would be greatly perturbed by apolar binding to protein. Thus the identity of membrane and lipid transition temperatures suggests that in the membranes, as in water, the lipids are in the bilayer conformation in which the hydrocarbon chains associate with each other rather than with proteins. Observations of morphological changes indicate that osmotic imbalance occurs when the membrane transition temperature exceeds the growth temperature, and that for transport processes to function properly the hydrocarbon chains must be in a liquid-like state.After many years of research, knowledge of the molecular organization of biological membranes remains meager. Although the concept of a phospholipid bilayer bounded on each side by protein is accepted by some investigators, others question the basic assumptions of the bilayer model and suggest alternative models in which the association between lipid and protein is hydrophobic rather than polar.1 If in fact lipids exist in membranes as bilayers, some unique property of a bilayer array might be detectable in membranes by a direct physical technique. Such a property is the reversible thermotropic gel-liquid crystal phase transition observed in phospholipid myelin forms in water. It has been studied by differential scanning calorimetry, differential thermal analysis, nuclear magnetic resonance spectroscopy, X-ray diffraction, and light microscopy; it arises from the melting of the hydrocarbon interiors of lipid bilayers.2-' Unlike transitions between liquid-crystalline phospholipid mesophases,5 the melt does not result in a molecular rearrangement and the lipids exist in the lamellar conformation both above and below the transition temperature. As in the case of bulk hydrocarbons, the melting point varies with unsaturation and chain lengths of the fatty acids in the phospholipids. Because cholesterol interferes, to detect a phase change above the ice point in a membrane an organism containing rather saturated fatty acids but little or no cholesterol must be chosen.The membranes of Mycoplasma laidlawii satisfy these requirements. Previous studies of this organism have shown that the cell membrane contains no choles-104

The membrane lipids in living Mycoplasma laidlawii exhibit a phase transition characteristic of that from crystal to liquid crystal within the bilayer conformation. The transition occurs at the same temperature in viable organisms, membranes isolated from the organisms, and isolated membrane lipids. The enthalpy of the transition in the membrane is compared with that of an aqueous suspension of isolated membrane lipids. The result is consistent with presence of an extended lipid bilayer in the native membrane.

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