Morphology of the intermediate stages in the lamellar to hexagonal lipid phase transition |
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Authors: | Valery L Borovjagin Juan A Vergara Thomas J McIntosh |
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Institution: | (1) Institute of Biological Physics, Academy of Science of the USSR, 142292 Puschino, Moscow Region, USSR;(2) Department of Anatomy, Duke University Medical Center, 27710 Durham, North Carolina |
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Abstract: | Summary The addition of calcium to suspensions of egg phosphatidylcholine and cardiolipin converts multiwalled liposomes to the hexagonal (HII) phase (Rand, R.P., Sengupta, S. (1972)Biochim. Biophys. Acta
255:484–492). We have studied this lamellar to hexagonal phase transition by freeze-fracture, thin-section electron microscopy, and X-ray diffraction and have morphologically characterized the intermediate stages. The first step in the transition involves the invagination and fusion of bilayers, marked by the appearance of lipidic intramembrane particles and crater-like indentations, as the large liposomes are converted to smaller flattened and elongated vesicles. The next step is the formation of tightly packed hexagonal arrays of tubules, each tubule being about 11 to 15 nm in diameter. These tubules are filled with fluid and a lipid bilayer forms the wall of each cylinder. Finally this tubular bilayer phase is converted to the hexagonal (HII) phase, where the distance between tubes is 5.5 to 7.5 nm. |
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Keywords: | cardiolipin hexagonal phase lamellar phase lipid polymorphism phase transition freeze fracture |
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