Effect of temperature and ions on the microviscosity of bilayers from natural phospholipid mixtures

The thermotropic behaviour of two natural extracts of phospholipids differing by their content in saturated fatty acids and acidic polar heads were studied with the aid of the fluorescent probes : perylene and diphenylhexatriene. The main results are that the bilayers are in a fluid state in the range from -- 5 to 50 degrees C, in the presence as well as in the absence of calcium, and that misleading conclusions may be derived with the perylene probe. The effects of pH and Ca2+/H+ interaction on the microviscosity were studied. In the absence of calcium pH increase fluidizes the bilayers ; when Ca2+ is present the microviscosity is constant over a large pH range. Calculations using the Gouy-Chapman theory suggest that the pH-induced microviscosity changes are controlled by the content in phosphadidylserine, diphosphatidylglycerol and phosphatidic acid of the extracts. Similarly the rigidifying effect of Ca2+ seems to be related to the content in these phospholipids. These hypothesis were confirmed by modifying the content of the extracts in these phospholipids. It is proposed that the behaviour of the two studied extracts results from the high insaturation of their fatty acids, which is typical of biological membranes.