Hydrodynamic lift on bound vesicles

Bound vesicles subject to lateral forces such as arising from shear flow are investigated theoretically by combining a lubrication analysis of the bound part with a scaling approach to the global motion. A minor inclination of the bound part leads to significant lift due to the additive effects of lateral and tank-treading motions. With increasing shear rate, the vesicle unbinds from the substrate at a critical value. Estimates are in agreement with recent experimental data.Comment: 9 pages, one figur

Geometry of lipid vesicle adhesion

The adhesion of a lipid membrane vesicle to a fixed substrate is examined from a geometrical point of view. This vesicle is described by the Helfrich hamiltonian quadratic in mean curvature; it interacts by contact with the substrate, with an interaction energy proportional to the area of contact. We identify the constraints on the geometry at the boundary of the shared surface. The result is interpreted in terms of the balance of the force normal to this boundary. No assumptions are made either on the symmetry of the vesicle or on that of the substrate. The strong bonding limit as well as the effect of curvature asymmetry on the boundary are discussed.Comment: 7 pages, some major changes in sections III and IV, version published in Physical Review

Fluctuation spectrum of fluid membranes coupled to an elastic meshwork: jump of the effective surface tension at the mesh size

We identify a class of composite membranes: fluid bilayers coupled to an elastic meshwork, that are such that the meshwork's energy is a function $F_\mathrm{el}[A_\xi]$ \textit{not} of the real microscopic membrane area $A$, but of a \textit{smoothed} membrane's area $A_\xi$, which corresponds to the area of the membrane coarse-grained at the mesh size $\xi$. We show that the meshwork modifies the membrane tension $\sigma$ both below and above the scale $\xi$, inducing a tension-jump $\Delta\sigma=dF_\mathrm{el}/dA_\xi$. The predictions of our model account for the fluctuation spectrum of red blood cells membranes coupled to their cytoskeleton. Our results indicate that the cytoskeleton might be under extensional stress, which would provide a means to regulate available membrane area. We also predict an observable tension jump for membranes decorated with polymer "brushes"

A defect of sphingolipid metabolism modifies the properties of normal appearing white matter in multiple sclerosis

Maintaining the appropriate complement and content of lipids in cellular membranes is critical for normal neural function. Accumulating evidence suggests that even subtle perturbations in the lipid content of neurons and myelin can disrupt their function and may contribute to myelin and axonal degradation. In this study, we determined the composition and quantified the content of lipids and sterols in normal appearing white matter (NAWM) and normal appearing grey matter (NAGM) from control and multiple sclerosis brain tissues by electrospray ionization tandem mass spectrometry. Our results suggest that in active-multiple sclerosis, there is a shift in the lipid composition of NAWM and NAGM to a higher phospholipid and lower sphingolipid content. We found that this disturbance in lipid composition was reduced in NAGM but not in NAWM of inactive-multiple sclerosis. The pattern of disturbance in lipid composition suggests a metabolic defect that causes sphingolipids to be shuttled to phospholipid production. Modelling the biophysical consequence of this change in lipid composition of NAWM indicated an increase in the repulsive force between opposing bilayers that could explain decompaction and disruption of myelin structure

Mutual adhesion of lecithin membranes at ultralow tensions

We present an extensive light-microscopic study of mutual adhesion of egg (and dimyristoyl) lecithin bilayers in highly swollen samples, including adhesion brought about by osmotic inflation of vesicles. The lateral tensions associated with adhesion, if below ca. 3 x 10-4 dyn cm-1 , could be read from a rounding of the membrane next to the contact area. The contact angle of symmetric adhesion, i.e. of two membranes under equal tension, had a maximum of 45°. While showing some scatter, it did not seem to depend on tension down to 3 x 10-6 dyn cm-1, the lowest tension observed. The maximum contact angle of single membranes adhering to bundles of membranes was near 70°. The data indicate that mutual adhesion of lecithin membranes is always induced by lateral tension. The constancy of the contact angles appears to be compatible with undulation theory, but their magnitude compels us to postulate an unknown, optically unresolvable roughness of lecithin membranes which serves as an area reservoir.On présente une étude par microscopie optique de l'adhésion mutuelle de doubles couches de lécithine d'œuf ou dimyristoyl dans des échantillons très gonflés. Les tensions latérales permettant l'adhésion, en dessous de 3 x 10-4 dyn cm-1, sont déterminées à partir de la courbure de la membrane à proximité de la zone de contact. L'angle de contact maximal des deux membranes soumises à des tensions égales à une valeur maximale de 45°. En dépit de quelque dispersion, cet angle ne semble pas dépendre de la tension jusqu'à sa valeur observable la plus faible, 3 x 10-6 dyn cm-1. L'angle de contact maximal d'une membrane isolée adhérant à un ensemble de membranes est proche de 70°. Les données indiquent que l'adhésion mutuelle est toujours induite par la tension latérale. La valeur constante des angles de contact semble compatible avec la théorie des ondulations, mais cette valeur nous pousse à postuler une rugosité de la membrane, non détectable optiquement, qui servirait de réserve d'aire

Elastic Torques about Membrane Edges: A Study of Pierced Egg Lecithin Vesicles

The shape of mechanically pierced giant vesicles is studied to obtain the elastic modulus of Gaussian curvature of egg lecithin bilayers. It is argued that such experiments are governed by an apparent modulus, ¯κ(app), not the true modulus of Gaussian curvature, ¯κ. A theory of ¯κ(app) is proposed, regarding the pierced bilayer vesicle as a closed monolayer vesicle. The quantity measured, i.e. ¯κ(app)/κ, where κ is the rigidity, agrees satisfactorily with the theory. We find ¯κ(app) = -(1.9 ± 0.3) · 10(-12) erg (on the basis of κ = (2.3 ± 0.3) · 10(-12) erg). The result may have implications for bilayer fusion

Giant membranes of swollen phosphatidylethanolamines and glycolipids

The swelling of phosphatidylethanolamines and biological glycolipids is studied by phase contrast microscopy. In their fluid phase the bilayers of all the lipids separate in water and, with one proven exception, in NaCl solutions. The very rare mutual adhesion of giant membranes which normally separate is apparently always induced by lateral tensions. Its average contact angles are larger than in the case of egg phosphatidylcholine but, as there, do not depend on tension within the one or two powers of ten covered by the observations. Certain cerebrosides form needles and fibres when the bilayers are in the solid phase

Bilayer bending rigidity of some synthetic lecithins

The curvature elastic modulus of fluid membranes of DMPC, DPPC, and DSPC in water was measured. From the equilibrium distribution of the thermal bending fluctuations of tubular vesicles we obtained values of 2.4, 2.0, and 1.8 × 10-12 erg, respectively, with errors of about 20 %. We also observed some deformations of the fluctuating tubes such as sharp knee-like bends and pronounced short-waved wriggles which apparent elastic energies seem too high for thermal excitation.Nous avons mesuré le module d'élasticité de courbure des membranes fluides de DMPC, DPPC et DSPC dans l'eau. De la distribution d'équilibre des fluctuations thermiques de flexion de vésicules tubulaires, nous avons obtenu les valeurs de 2,4,2,0 et 1,8 x 10-12 erg avec une erreur d'environ 20 %. Nous avons également observé quelques déformations des tubules fluctuantes telles que des « coudes » bien marqués et des ondulations prononcées de courtes longueurs d'onde dont l'énergie élastique fictive nous a paru trop grande pour être excités thermiquement