| Abstract: | (1) The thermotropic behaviour of dimyristoyl phosphatidylglycerol, phosphatidylserine, phosphatidic acid and phosphatidylcholine was investigated by differential scanning calorimetry and freeze-fracture electron microscopy as a function of pH and of Ca2+ concentration. (2) From the thermotropic behaviour as a function of pH, profiles could be constructed from which apparent pK values of the charged groups of the lipids could be determined. (3) Excess Ca2+ induced a shift of the total phase transition in 14 : 0/14 : 0-glycerophosphocholine and 14 : 0/14 : 0-glycerophosphoglycerol mixtures. In 14 : 0/14 : 0-glycerophosphocholine bilayers containing 16 : 0/16 : 0-glycerophosphoglycerol lateral phase separation was induced by Ca2+. (4) Up to molar ratios of 1 : 2 of 14 : 0/14 : 0-glycerophosphoserine to 14 : 0/14: 0-glycerophosphocholine, excess Ca2+ induced lateral phase separation. Addition to mixtures of higher molar ratios caused segregation into different structures: the liposome organization and the stacked lamellae/cylindrical organization. (5) Addition of excess Ca2+ to mixtures of 14 : 0/14 : 0-glycerophosphocholine and 14 : 0/14 : 0-phosphatidic acid caused, independent of the molar ratio, separation into two structural different organizations. (6) The nature of Ca2+-induced changes in bilayers containing negatively charged phospholipids is strongly dependent on the character of the polar headgroup of the negatively charged phospholipid involved. |
