Calcium-induced fusion of didodecylphosphate vesicles: the lamellar to hexagonal II (HII) phase transition

• Published in: The Journal of membrane biology Vol. 95; no. 3; p. 255
• Main Authors: Rupert, L A, van Breemen, J F, van Bruggen, E F, Engberts, J B, Hoekstra, D
• Format: Journal Article
• Language: English
• Published: United States 01.01.1987
• Subjects: Calcium; Freeze Fracturing; Lipid Bilayers; Magnetic Resonance Spectroscopy; Microscopy, Electron; Models, Biological; Models, Molecular; Molecular Conformation; Organophosphates; Organophosphorus Compounds
• ISSN: 0022-2631
• DOI: 10.1007/BF01869487

Electron microscopic techniques have been employed to investigate the ability of didodecylphosphate vesicles (diameter approx. 900 A) to fuse in the presence of Ca2+. As revealed by negative staining, Ca2+ induces extensive fusion and large vesicles with diameters up to 7000 A are formed. In a process secondary to fusion, the fused vesicles display a tendency to flatten and are subsequently transformed into extended tubular structures. Freeze-fracture electron microscopy, in conjunction with 31P NMR and selected area electron diffraction measurements indicate that the tubes are packed in a hexagonal (HII) array and that the amphiphiles are converted from the lamellar to the hexagonal HII phase. The relationship between membrane fusion and the lamellar-to-hexagonal phase transition is discussed in terms of formation and abundance of transiently stable inverted micellar intermediates at contact regions between two interacting membranes. A model for the conversion of the (vesicular) lamellar into the (tubular) hexagonal HII phase is presented, taking into account the molecular shape of the amphiphile. The relevance of using simple synthetic amphiphiles as models for phospholipid bilayers and complex biomembrane behavior is briefly discussed.