Giant phospholipid vesicles obtained by the method of electroformation were observed by the phase contrast microscope. Most of these vesicles contain a protrusion which shortens in a slow shape ...transformation process until it is absorbed into the main vesicle body. We are concerned with the last stages of this shape transformation process, where the protrusions attain a beadlike shape. The number of "beads" decreases one by one in consecutive steps, and it is demonstrated that each such step consists of two distinguishable phases. During the first phase the beadlike shape does not change and the necks connecting the "beads" are narrow. During the second phase the width of the protrusion necks increases. On the basis of the assumption that these shape transformations are driven by the decrease of the equilibrium difference between the outer and the inner membrane monolayers areas, the system behavior is analyzed in terms of the generalized bilayer couple model. The theoretical results confirm the observed time sequence: at a given number of "beads" the protrusion has in the first phase the shape that consists of spheres connected by infinitesimal necks, and during the second phase the protrusion is a single prolate unit with open necks. The discrepancies between the observed and the predicted widths of the necks are interpreted by the repulsive forces between the neighboring "beads" induced by the membrane thermal fluctuations. The analysis presented extends the existing catalog of vesicle shapes to the region of larger differences between the areas of membrane monolayers, and confirms the applicability of the generalized bilayer couple model to the description of the shape behavior of phospholipid vesicles containing beadlike protrusions.PUBLICATION ABSTRACT
We studied the coupling of a polymeric amphiphile undergoing a transition from an expanded to a collapsed state of the hydrophilic polymer chain upon heating above 29°C with free and solid supported ...lipid bilayers. Lateral diffusion measurements and studies of vesicle shape-changes confirmed previous calorimetric studies, suggesting that the amphiphile remains incorporated in the giant vesicles and the supported bilayer during the transition. The two-dimensional hydrodynamic radii of the amphiphile in the expanded and collapsed state was measured by making use of the strong quadratic dependence of the diffusion coefficient on the radius of the diffusant (
D ∞
r
−2) due to the frictional coupling between the bilayer and the supporting substrate. The hydrodynamic radius changed by a factor of 3 during, the transition from the expanded to the collapsed state. Shape-changes of multilamellar and thin-walled vesicles during the transition were observed by phase contrast microscopy. In the expanded state of the polymer, the vesicles exhibit a smooth contour, although non-trivial and non-symmetric shapes, were found. Heating over the collapse transition induces pronounced budding of the thin-walled vesicles, while the multilamellar liposomes exhibit only a transient formation of long wavelength protrusions. The shape-changes are explained in terms of the coupling between phase separation and induced curvature.
Shapes of fluid lipid vesicles are governed by the bending elasticity of their membrane as described by the Area-Difference-Elasticity (ADE) model. These shapes can be quantified using a suitable ...modal representation of the vesicle contour. Prolate vesicles are characterized by a hierarchy in their shape amplitudes. Experimentally, we find an ordering of the amplitudes with mode number both in large (100 nm) as well as giant (10 μm) unilamellar vesicles. Mean shapes are found only within the small energetically stable region of the prolate phase. Our study demonstrates that bending energy concepts may be quantitatively used on cellular length scales ranging from the size of organelles to the plasma membrane.
Amphiphilic membrane active compounds are considered that affect the shapes of cells and cellular organelles by intercalation into the phospholipid part of their membranes. It is taken into ...consideration that amphiphile-membrane interaction modifies membrane mechanical properties. The relationship between membrane mechanical properties and vesicle shapes and the concept of the bilayer couple model are shortly reviewed. Then it is put forward that the strength of the amphiphile-membrane interaction may depend on the lateral packing of phospholipid molecules. It is shown that in such a case the amphiphile molecules bind to the membrane in a cooperative manner. Moreover, the amphiphile binding makes the ratio between the nonlocal and local membrane bending constants to be effectively larger and thus widens the range of possible stable vesicle and cellular shapes.
Vesicles of egg yolk phosphatidylcholine (EYPC) were studied by cryo-transmission electron microscopy. The electron micrographs indicate that, despite the rapidity of cooling, membrane undulations ...are flattened and some vesicles change their shapes before the samples freeze. These artefacts are attributed to the action of the lateral tension that results from the membrane area contraction associated with the temperature drop. Other micrographs represent grainy membranes and angular vesicles. We regard them as the first direct evidence for the superstructure and optically invisible roughness which were recently postulated for these membranes.
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