Dynamic excitations in membranes induced by optical tweezers.

We present the phenomenology of transformations in lipid bilayers that are excited by laser tweezers. A variety of dynamic instabilities and shape transformations are observed, including the pearling instability, expulsion of vesicles, and more exotic ones, such as the formation of passages. Our physical picture of the laser-membrane interaction is based on the generation of tension in the bilayer and loss of surface area. Although tension is the origin of the pearling instability, it does not suffice to explain expulsion of vesicles, where we observe opening of giant pores and creeping motion of bilayers. We present a quantitative theoretical framework to understand most of the observed phenomenology. The main hypothesis is that lipid is pulled into the optical trap by the familiar dielectric effect, is disrupted, and finally is repackaged into an optically unresolvable suspension of colloidal particles. This suspension, in turn, can produce osmotic pressure and depletion forces, driving the observed transformations.

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abstract: Surface forces between molecularly smooth mica sheets were measured in Na+ and K+ aqueous salt solutions (at ∼10−3 M) both at 21°C and, for the case of Na+, at 65°C. Additional short-range repulsive forces were found to be the same at both temperatures. In very low concentrations of electrolyte solution (∼5 × 10−6M) and in hydrochloric acid solutions (up to 1.2 × 10−3M) these forces were completely absent-giving force curves in very good agreement with DLVO theory. Analysis of these force-interaction results appears to prove conclusively the existence of counterion hydration forces on mica. These forces apparently also prevent both bubble coalescence and coagulation of latex colloids at high concentrations of electrolyte. The total interaction between charged colloidal surfaces should involve, as apparently in the case of mica, both short-range repulsive forces due to bound partially hydrated ions and a longer range repulsion due to hydrated counterions in the compressed double-layer. These “hydration” forces should be present in many colloidal systems and be dominant in those with high negative charge densities and little possibility of hydrogen bonding to adjacent water layers. Surfaces consisting of ionic species (e.g., Zwitterionic lecithin bilayers) should give rise to repulsive forces due to the surface ion hydration effects only.

title: Instability and deformation of a spherical vesicle by pressure.
abstract: The infinitesimal stability of a spherical vesicle (closed membrane) is studied as a function of the pressure difference between the outer and inner media. It is found that above some threshold pressure the spherical vesicle can be deformed into a shape associated with $l\mathrm{th}$-order spherical harmonics. The comparison with numerical examples calculated previously by Deuling and Helfrich shows good agreement. Some applications to red blood cells are discussed.

title: Unbinding transition of a biological model membrane.
abstract: We have discovered a temperature-driven unbinding transition of fluid membranes in samples of digalactosyldiacylglycerol (DGDG) swollen in 0. 1M NaC1 aqueous solution. Single bilayers were observed by phase contrast light microscopy. The transition is reversible and without apparent hysteresis. Its temperature is only a few C lower for the single membranes of two vesicles than for multilayer systems, but it varies strongly among samples. The bending rigidity of DGDG bilayers was also measured and found to be (0. 12-0.21) x10 ' erg.

title: Local unbinding of pinched membranes.
abstract: Loosely bound membranes exhibit an unusual elastic response when pinched together by optical tweezers, locally unbinding to a large intermembrane distance. Tweezing a stack of many bound membranes produces extreme local swelling in the vicinity of the tweezing point. We introduce a model that incorporates bending elasticity, fluctuations, and intermembrane interactions to calculate the membrane profiles subject to a local pinch. Theoretically, we find strongly overshooting profiles in agreement with experiment. We predict scaling behavior of the overshoot with the pinch strength and size. PACS numbers: 82.65.Dp, 68. 10.Et, 87.22.Bt Adhesion of biological membranes involves both the attraction of two homogeneous surfaces  and the interactions of specific molecular binding sites [2,3]. Experiments suggest that binding sites are dominant locally. However, the overall binding of two membranes with localized "chemical stickers ' is strongly influenced by the elastic response and fluctuations of the embedding bilayer . To study the overall membrane response to a single binding site, we study a simplified model system by locally pinching together two membranes with optical tweezers [5,6]. This system can also model "local collisions, " which are important in the study of interacting, flexible membranes and the understanding of their unbinding transition [7,8]. Previously, we showed that laser tweezers remove area from the membrane, inducing tension in membranes with limited area and thus driving shape transitions. In the present work the tweezers are used primarily as a mechanical tool, clamping at a point two large planar membranes, which effectively have unlimited area and allow only a negligible buildup of tension. We studied bilayers composed of dimyristoyl-phosphatidylcholine (DMPC) (Sigma) produced by a standard technique [9]. Our experimental setup is described elsewhere [10,11]. Typical working conditions were 30 C and laser intensities from 30 up to 150 mW. We selected sets of two or more nearly Oat membranes, which were sections either of huge vesicles or of lamellar sheets that extended from the bottom of the cell to its top. We chose sections of membranes that appeared to be bound or loosely bound, at a distance of -1 p, m. The membranes are pinched by the laser tweezers in the — 0.5 p, m region of the trap. Regardless of the

title: Membrane-induced interactions between inclusions
abstract: We investigate theoretically the effect of embedded inclusions on membrane structure, and the corresponding membrane-induced interactions between inclusions. We find that the membrane thickness, which is perturbed from its equilibrium value by the coupling to the embedded inclusions, decays non-monotonically with distance from the inclusion boundary. As a result, the membrane-induced interactions between inclusions vary non monotonically with spacing. The periodicity of the perturbation profile, as well as the strength and range of the induced interactions, are proportional to the ratio of the amphiphile bending modulus and compressibility

title: Anchor ring-vesicle membranes.
abstract: A family of the exact and analytical solutions of the equilibrium shape equation of vesicle membranes is found. They are anchor rings with generating circles of radii in the ratio 1/ \ensuremath{\surd}2 . It is shown that these ring vesicles are stable for some negative values of their spontaneous curvatures, such that experimental construction of such vesicles seems possible. A discussion shows that a positive Gauss-curvature elastic modulus favors the formation of these special vesicles.

title: Conformal degeneracy and conformal diffusion of vesicles.
abstract: The shape of vesicles with genus g=2, i.e., with two holes or two handles, is studied in the framework of curvature models. These vesicles exhibit a new phase which also persists for higher genus g>2. In this phase, the ground state of the vesicle is conformally degenerate even when the volume, the area, and the total mean curvature of the vesicle are kept constant. It is predicted that such vesicles undergo a new type of diffusive motion, termed conformal diffusion, which should be observable in experiments as pronounced shape fluctuations.

title: Shape Transformations of Giant Vesicles: Extreme Sensitivity to Bilayer Asymmetry
abstract: 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 a pear-shaped state, ii) the expulsion of a smaller vesicle (budding), and iii) discocyte–stomatocyte 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 adjacent monolayers exhibit a small difference in their thermal expansivities which is easily produced, e.g., by residual impurities.

title: Shape transitions and shape stability of giant phospholipid vesicles in pure water induced by area-to-volume changes.
abstract: Shape transformations of vesicles of dimyristoylphosphatidylcholine (= DMPC) and palmitoyloleylphosphatidylcholine (= POPC) in ion-free water were induced by changing the area-to-volume ratio via temperature variations. Depending on the pretreatment we find several types of shape changes for DMPC (in pure water) at increasing area-to-volume ratio: (a) budding transitions leading to the formation of a chain of vesicles at further increase of the area-to-volume ratio, (b) discocyte-stomatocyte transitions, (c) reentrant dumbbell-pear-dumbbell transitions, and (d) spontaneous blebbing and/or tether formation of spherical vesicles. Beside these transitions a more exotic dumbbell-discocyte transition (e) was found which proceeded via local instabilities. Pears, discocytes, and stomatocytes are stable with respect to small temperature variations unless the excess area is close to values corresponding to limiting shapes of budded vesicles where temperature variations of less than or equal to 0.1 degree C lead to spontaneous budding to the inside or the outside. For POPC we observed only budding transitions to the inside leading either to chains of vesicles or to distributions of equally sized daughter vesicles protruding to the inside of the vesicle. Preliminary experiments concerning the effect of solutes are also reported. The first three types of shape transitions can be explained in terms of the bilayer coupling model assuming small differences in thermal expansivities of the two monolayers. This does not hold for the observed instabilities close to the limiting shapes.