Browsing by Author "Käs, Josef"

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    Shape transformations of giant vesicles : extreme sensitivity to bilayer asymmetry
    (1990) Berndl, Karin; Käs, Josef; Lipowsky, Reinhard; Sackmann, Erich; Seifert, Udo
    Shape transformations of vesicles of lecithin (DMPC) in water are induced by changing the temperature which effectively changes the volume-to-area ratio. Three different routes are found which include i) symmetric-asymmetric re-entrant transitions from a dumbbell to pear-shaped state, ii) the expulsion of a smaller vesicle (budding), and iii) discocytestomatocyte transitions. All of these shape transformations are explained within a model for the bending energy of the bilayer which assumes i) that the two monolayers do not exchange lipid molecules, and ii) that the two adjacent monolayers exhibit a small difference in their thermal expansivities which is easily produced, e.g., by residual impurities.
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    Vesicle shapes and shape transformations: a systematic study
    (1991) Berndl, Karin; Käs, Josef; Lipowsky, Reinhard; Sackmann, Erich; Seifert, Udo
    In our contribution we report a systematic experimental and theoretical study on shape transformations. In order to avoid the above mentioned complications we have investigated vesicles which consist of electrically neutral lipids (that is phosphatidylcholine) in Millipore water. We find, that even for such a simple system a change in temperature can lead to three different types of shape transformations. Theoretically, we discuss shape transformations within two well established curvature models, (i) the bilayer coupling model of Svetina and Zeks and (ii) the spontaneous curvature model of Helfrichs.A comparison leads to the conclusion that the observed shape transformations can well be explained within the bilayer coupling model provided a small asymmetry in the thermal expansivities of both monolayers is assumed. In some cases, such an asymmetry is not required.