Effect of electrostatic repulsion on the morphology and thermotropic transitions of anionic phospholipids
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Samples of dimyristoylphosphatidylglycerol and dimyristoylphosphatidylserine which exhibit highly cooperative phase transitions in suspension with 0.5 M NaCl, 10 mM sodium phosphate, pH 7.0, demonstrate a marked broadening of the phase transition when suspended in distilled water at pH 7. The cooperativity of the thermal transition of dimyristoylphosphatidic acid at pH 7.0, in contrast, was little affected by the presence or absence of 0.5 M NaCl. The most dramatic changes in phase transition properties were observed with phosphatidylglycerol. The morphology of phosphatidylglycerol was also altered by NaCl. At high salt concentrations, typical multilamellar structures were observed by freeze-fracture electron microscopy. However, in distilled water the large multilamellar structures are disrupted, with opened shells and smaller particles of 20-30 nm diameter being observed, in addition to a few larger vesicles. Structures of differing morphology could be partially separated from aqueous 'solutions' of phosphatidylglycerol by ultracentrifugation. The supernate contained only small particles. The results are discussed in terms of the large effective head group volume resulting from strong electrostatic repulsion at low ionic strengths. This allows the anionic lipids, particularly phosphatidylglycerol, to form hydrophilic surfaces with high curvature so as to seal the bilayer edges of discs or shells. Thus, certain anionic lipids in distilled water destabilize bilayers by a mechanism which is opposite to that found with lipids which destabilize bilayers by forming the hexagonal phase.
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