Vesicles consisting of a bilayer membrane of amphiphilic lipid molecules are remarkably flexible surfaces that show an amazing variety of shapes of different symmetry and topology. Owing to the fluidity of the membrane, shape transitions such as budding can be induced by temperature changes or the action of optical tweezers. Thermally excited shape fluctuations are both strong and slow enough to be visible by video microscopy. Depending on the physical conditions, vesicles adhere to and unbind from each other or a substrate. This article describes the systematic physical theory developed to understand the static and dynamic aspects of membrane and vesicle configurations. The preferred shapes arise from a competition between curvature energy, which derives from the bending elasticity of the membrane, geometrical constraints such as fixed surface area and fixed enclosed volume, and a signature of the bilayer aspect. These shapes of lowest energy are arranged into phase diagrams, which separate regi...

Configurations of fluid membranes and vesicles

The Part and the Whole. The principle of self-organization for the creation of functional units is not an invention of modern natural sciences. It was already a basic idea of the ancient philosophies in Asia and Europe: only the mutuality of the parts creates the whole and its ability to function. Translated into the language of chemistry this means: the self-organization of molecules leads to supramolecular systems and is responsible for their functions. Thermotropic and lyotropic liquid crystals are such functional units, formed by self-organization. As highly oriented systems, they exhibit new properties. The importance of lyotropic liquid crystals for the life sciences has been known for a long time. They are a prerequisite for the development of life and the ability of cells to function. In materials sciences this concept of function through organization led to the development of new liquid-crystalline materials. From the point of view of macromolecular chemistry, this review tries to combine these two different fields and especially hopes to stimulate their interaction and joint treatment. To exemplify this, the molecular architecture of polymeric organized systems will be discussed. Polymeric liquid crystals combine the ability to undergo spontaneous self-organization–typical of liquid-crystalline phases–with the polymer-specific property of stabilizing these ordered states. As new materials, polymeric liquid crystals have already been investigated intensively. As model systems for biomembranes as well as for the simulation of biomembrane processes, they so far have been little discussed. The intention of this review article is to show that polymer science is able to contribute to the simulation of cellular processes such as the stabilization of biomembranes, specific surface recognition, or even the “uncorking” of cells. Polymer science, having an old tradition as an inter-disciplinary field, can no longer restrict itself to common plastics. Attempts to reach new horizons have already begun. The borderland between liquid crystals and cells will certainly play an important role. Basic requirements to work in this frontier area between organic chemistry, membrane biology, life science, and materials science will be the delight in scientific adventures as well as the courage to go ahead. The most important prerequisite will be the willingness to cooperate with disciplines which so far have not really accepted each other. From this point of view, this review does not aim at giving defined answers. It wants instead to encourage the scientific venture: too often we cling to painfully acquired knowledge, fearing adventures.

Molecular Architecture and Function of Polymeric Oriented Systems: Models for the Study of Organization, Surface Recognition, and Dynamics of Biomembranes

Phase separation within the lipid bilayer of vesicles can lead to the formation of domains that affect the equilibrium shape of these vesicles. As a result of the competition between the bending energy of the bilayer and the line energy of the domain boundaries, the domains induce the formation of buds if their size is sufficiently large. This phenomenon of domain-induced budding is studied both for freely adapting and for fixed volume. The phase diagrams show that the constraint on the volume acts against the budding process but will in general not suppress it. In many situations, domain-induced budding leads to limit shapes for which the bud consists of a closed sphere that is connected to the ``mother'' vesicle by an infinitesimal neck. This neck is characterized by a general and simple neck condition for the mean curvature of the membrane segments adjacent to the neck. Budding also occurs if the Gaussian bending energy is taken into account. The effect of the Gaussian curvature energy is to change the structure and the stability of those shapes that exhibit small necks. \textcopyright{} 1996 The American Physical Society.

/pdf/shape-transformations-of-vesicles-with-intramembrane-domains-4xm4u8tl47.pdf

Shape transformations of vesicles with intramembrane domains

Der Teil und das Ganze. Nicht erst die moderne Naturwissenschaft hat die Selbstorganisation fur den Aufbau von Funktionseinheiten entdeckt, schon in den alten Philosophien Asiens und Europas ist sie ein Grundgedanke: Erst die Gemeinsamkeit der Teile schafft das Ganze und seine Funktionsfahigkeit. Ubersetzt in die Sprache der Chemie bedeutet dies: Die Selbstorganisation von Molekulen fuhrt zu supramolekularen Systemen. Thermotrope und lyotrope Flussigkristalle sind solche Funktionseinheiten, die sich durch Selbstorganisation bilden und als hochorientierte Systeme neue Eigenschaften aufweisen. Die Bedeutung lyotroper Flussigkristalle ist in den Biowissenschaften seit langem bekannt. Sie sind eine Voraussetzung fur die Entstehung des Lebens und die Funktionsfahigkeit von Zellen. In den Materialwissenschaften hat das Konzept Funktion durch Organisation in den letzten Jahren bereits zur Entwicklung neuer flussigkristalliner Materialien gefuhrt. Aus der Sicht der Makromolekularen Chemie soll am Beispiel der molekularen Architektur von Polymersystemen versucht werden, diese beiden Bereiche gemeinsam zu betrachten und besonders zu ihrer gemeinsamen Bearbeitung anzuregen. Polymere Flussigkristalle vereinigen die Fahigkeit zur spontanen Selbstorganisation, die den flussigkristallinen Zustand auszeichnet, mit polymerspezifischen Eigenschaften, die eine Stabilisierung von Ordnungszustanden zulassen. Als neue Materialien wurden auch diese bereits intensiv untersucht. Als Modellsysteme fur Biomembranen und zur Simulation von Biomembranprozessen hingegen wurden sie bisher nur wenig diskutiert. Intention dieses Beitrags ist es, zu zeigen, das die Makromolekulare Chemie helfen kann, Zellprozesse wie die Stabilisierung von Biomembranen, die spezifische Oberflachenerkennung oder gar die „Entkorkung” von Zellen zu simulieren. Von jeher interdisziplinar arbeitend, kann sich die Polymerwissenschaft schon lange nicht mehr an den klassischen Massenkunststoffen festhalten. Der Aufbruch zu neuen Forschungsbereichen hat begonnen. Die Verbindung von Flussigkristall- und Biomembranforschung scheint uns besonders wichtig. Eine der Grundvoraussetzungen, dieses Grenzgebiet zwischen Organischer Chemie und Membranbiologie oder, allgemeiner, zwischen Biowissenschaften und Materialwissenschaften zu bearbeiten, wird die Bereitschaft zu einer engen Kooperation mit Nachbardisziplinen sein, die sich bisher oft fremd gegenuberstanden. Dieser Beitrag ist weit davon entfernt, definitive Antworten zu geben; er soll aber Mut machen zum wissenschaftlichen Wagnis, denn oft halten wir aus Angst vor dem Abenteuer am muhsam erworbenen Wissen zu fest.

Molekulare Architektur und Funktion von polymeren orientierten Systemen – Modelle für das Studium von Organisation, Oberflächenerkennung und Dynamik bei Biomembranen

https://www.cell.com/biophysj/pdf/S0006-3495(93)81203-7.pdf

Budding and fission of vesicles.

Abstract The lateral lipid organization and the local surface curvature in small and giant bilayer vesicles of binary lipid mixtures were investigated. Mixtures of the following lipids are studied: Cholesterin, di-alcyl-lecithins, dioleyl-lecithin and di-alcyl-phosphatidic acid. The latter is considered as being representative of charged natural lipids (e.g. phosphatidyl serine or cardiolipin). Three different experimental methods are compared: 1) The excimerfluorescence method, 2) the spin label technique and 3) the freeze fracture electron microscopy. The latter two methods yield information on the size of lipid precipitations. The surface curvature may be studied by electron microscopy. The essential experimental results are: (1) Mixtures of smectic phases of different symmetry undergo lateral phase separation. (2) The phase separation leads to a domain-like lateral lipid organization. (3) The domain structure is often accompanied by a variation in local curvature. (4) In membranes containing a charged lipid component, the domain structure may also be triggered by external charges (such as surface proteins). (5) two types of domains are observed: Circular domains (diameters of the order of several 100 Å) occur in mixtures of non-tilted fluid and rigid phases. Elongated domains (width ∼ 100 Å) are observed in mixtures of tilted and nontilted phases. This domain pattern is characteristic for mixtures of lecithins and cholesterols. The domain structure is explained by combining the theory of spinodal decomposition of alloys with the essential result of the orientational elastic model of membranes. The domain size calculated from this model agrees well with the experimental result. The periodic ripple structure observed between the pre-and the main transition is explained by generalizing the concept of spinodal decomposition to include the separation of phases which are only distinguishable (e. g. by the tilt angle). The width of the domains in equilibrium is explained in terms of the spontaneous curvature of the decoupled monolayer of a bilayer. Good agreement with the experimental result is obtained. The ripple phase is only a special case of the surface induced domain structures in ordinary liquid crystals. The defect structure of the ripple phase has been analysed in terms of its symmetry. A symmetry rule is established which leads to a model of lipid orientation. Below the pretransition no periodic domain pattern is observed under normal conditions. But upon cooling a bilayer very rapidly a defect structure reminiscent of a screw dislocation is ob served. This is expected for coupled biaxial monolayers.

On domain structure and local curvature in lipid bilayers and biological membranes.

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On domain structure and local curvature in lipid bilayers and biological membranes.