DISTRIBUTION OF NEGATIVE PHOSPHOLIPIDS IN MIXED VESICLES

Positive metachromatic dyes (methylene blue and acridine orange) interact with mixed vesicles containing phosphatidylcholine and negative phospholipids. The following results have been obtained: (1) the interaction occurs only with external negative phospholipids; (2) the dye to negative phospholipid stoichiometry is 1:1; (3) the presence of the dyes does not perturb the lipid distribution on the external surface. The dye absorbance changes can be utilized to calculate the amount of paired negative phospholipids on the external surface. This value, compared with that obtained from the statistical analysis, gives the degree of association of these lipids. As to the surface distribution of negative phospholipids, it has been found that: (1) in the case of phosphatidylserine and phosphatidylglycerol the amount of paired phospholipids is lower than that statistically calculated, in the case of phosphatidylinositol it is slightly higher, and in the case of phosphatide acid it is the same. (2) The degree of association for the different phospholipids is: phosphatidylinositol > phosphatidc acid > phosphatidylserine > phosphatidylglycerol, at neutral pH and at low ionic strength. The sequence is not altered by changing the phosphatidylcholine/negative phospholipid ratio in the vesicles. (3) The association is higher when the vesicles are in the solid state and decreases at temperatures higher than the transition temperature of the vesicles. (4) The mixing properties seem to be related to the difference in polar head groups rather than in the hydrocarbon chain structure. Regarding the transbilayer distribution in mixed vesicles the charged component is preferentially distributed on the external layer at a high phosphatidylcholine/negative phospholipid ratio. At a low phosphatidylcholine/negative phospholipid ratio, phosphatidylserine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidic acid are preferentially distributed on the internal layer. © 1978, American Chemical Society. All rights reserved.