The edge energy of a lipid bilayer is considered to be the crucial parameter controlling the formation of closed vesicles. It is considered to be modulated by amphiphiles accumulating at the edge as described by a Gibbs isotherm. The approach is tested for the system egg-lecithin -taurochenodesoxycholate. The parameters entered into the Gibbs' isotherm are determined by dynamic light scattering in the regime of mixed micelles (vanishing edge tension). Adjustment of an appropriate finite edge tension then leads to the discovery of metastable discs after sonication which close to vesicles spontaneously. This disc-vesicle transition is observed by electron microscopy. It is described in terms of a phenomenological potential profile. Estimates of the intrinsic edge tension and of the elastic modulus of an open lipid bilayer are obtained.

2014 La bicouche phospholipidique structure de base des membranes biologiques est un modèle approprié de système bidimensionel. Les défauts au sein de ces bicouches sont étudiés par microscopie électronique par la méthode de cryo-décapage. On peut différencier des défauts topologiques et non topologiques. Le premier type est analysé par la théorie de l'homotopie. Deux systèmes de défauts, un abélien et un non-abélien, ont été trouvés, ce qui est la conséquence d'une brisure de symétrie. L'effet de petites quantités d'impuretés sur la structure des défauts a été étudié. Un aperçu de l'intérêt biologique possible des défauts est donné dans la derniere partie. Abstract. 2014 The phospholipid bilayer is the basic structure of biological membranes as well as a suitable model of a two-dimensional system. Defects within these bilayers are studied electron microscopically by application of the freeze fracture method Topological and non-topological defects can be distinguished The first type is analysed in terms of the homotopy theory. Two defect systems, an Abelian and a non-Abelian, are found which are a consequence of a symmetry breaking. The effect of small amounts of impurities on the defect structure is studied An outline of the possible biological relevance of defects is given in the last part.

The relaxation of a lecithin suspension after sonication is studied by electron microscopy. It is shown that vesicles are formed in a two-stage process: Fragmentation into open sheets of bilayer under the action of sound and subsequent spontaneous closure of the planar disks into closed shells. A prerequisite of the discovery is a slowing down of the relaxation process by the addition of a moderate amount of cholate as an 'edge-actant'.Vesicle Lecithin

The lateral mobility of the lipid analog N-4-nitrobenzo-2-oxa-1,3 diazole phosphatidylethanolamine and of the integral protein glycophorin in giant dimyristoylphosphatidylcholine vesicles was studied by the photobleaching technique. Above the temperature of the chain-melting transition (Tm = 23 degrees C), the diffusion coefficient, Dp, of the protein [Dp = (4 +/- 2) X 10(-8) cm2/s at 30 degrees C] was within the experimental errors equal to the corresponding values DL of the lipid analog. In the P beta 1 phase the diffusion of lipid and glycophorin was studied as a function of the probe and the protein concentration. (a) At low lipid-probe content (cL less than 5 mmol/mol of total lipid), approximately 20% of the probe diffuses fast (D approximately equal to 10(-8) - 10(-9) cm2/s), while the mobility of the rest is strongly reduced (D less than 10(-10) cm2/s). At a higher concentration (cp approximately 20 mmol), all probe is immobilized (D less than 10(-10) cm2/s). (b) Incorporation of glycophorin up to cp = 0.4 mmol/mol of total lipid leads to a gradual increase of the fraction of mobile lipid probe due to the lateral-phase separation into a pure P beta 1 phase and a fraction of lipid that is fluidized by strong hydrophilic lipid-protein interaction. (c) The diffusion of the glycophorin molecules is characterized by a slow and a fast fraction. The latter increases with increasing protein content, which is again due to the lateral-phase separation caused by the hydrophilic lipid-protein interaction. The results are interpreted in terms of a fast transport along linear defects in the P beta 1 phase, which form quasi-fluid paths for a nearly one dimensional and thus very effective transport. Evidence for this interpretation of the diffusion measurements is provided by freeze-fracture electron microscopy.

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