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Lipowsky R.,Max Planck Institute of Colloids and Interfaces
Advances in Colloid and Interface Science | Year: 2014

Biomimetic membranes are fluid and can undergo two different elastic deformations, bending and stretching. The bending of a membrane is primarily governed by two elastic parameters: its spontaneous (or preferred) curvature m and its bending rigidity κ. These two parameters define an intrinsic tension scale, the spontaneous tension 2 κm2. Membrane stretching and compression, on the other hand, are determined by the mechanical tension acting within the membrane. For vesicle membranes, the two elastic deformations are coupled via the enclosed vesicle volume even in the absence of mechanical forces as shown here by minimizing the combined bending and stretching energy with respect to membrane area for fixed vesicle volume. As a consequence, the mechanical tension within a vesicle membrane depends on the spontaneous curvature and on the bending rigidity. This interdependence, which is difficult to grasp intuitively, is then illustrated for a variety of simple vesicle shapes. Depending on the vesicle morphology, the magnitude of the mechanical tension can be comparable to or can be much smaller than the spontaneous tension. © 2014 The Author. Published by Elsevier B.V. Source

Lipowsky R.,Max Planck Institute of Colloids and Interfaces
Faraday Discussions | Year: 2012

Recent experimental studies on supported lipid bilayers and giant vesicles have shown that uni-lamellar membrane systems can undergo spontaneous tubulation, i.e., can form membrane tubules or nanotubes without the application of external forces. In the case of supported lipid bilayers, the tube formation was induced by the adsorption of antimicrobial peptides. In the case of giant vesicles, spontaneous tubulation was observed after the polymer solution inside the vesicles underwent phase separation into two aqueous phases. Here, these processes are studied theoretically and shown to be driven by membrane tension generated by spontaneous curvature. The latter curvature is estimated for different types of adsorbing particles, such as ions, small molecules, and macromolecules, that differ in their size and in their adsorption kinetics. When the two sides of the membranes are exposed to two different concentrations of these particles, the membranes will acquire a spontaneous (or preferred) curvature. Particularly large spontaneous curvatures are induced by the adsorption of amphipathic peptides and BAR domain proteins. Another mechanism that induces spontaneous curvature is provided by different depletion layers in front of the two sides of the membranes. Irrespective of its molecular origin, a spontaneous curvature is predicted to generate a tension in weakly curved membranes, a 'spontaneous' tension that can vary over several orders of magnitudes and can be as high as 1 mJ m-2. The concept of spontaneous tension is first used to explain the spontaneous tubulation of supported lipid bilayers when exposed to adsorbing particles. This tubulation process is energetically preferred when the spontaneous tension exceeds the adhesive strength of the underlying solid support. Furthermore, in the case of giant vesicles, the spontaneous tension can balance the osmotic pressure difference between the interior and exterior aqueous compartment. The vesicles are then able to form stable cylindrical nanotubes that protrude into the vesicle interior as observed recently for membranes in contact with two aqueous polymer phases. In these latter systems, the vesicle membranes are governed by two spontaneous tensions that can be directly measured since they are intimately related to the effective and intrinsic contact angles. This journal is © The Royal Society of Chemistry. Source

Dimova R.,Max Planck Institute of Colloids and Interfaces
Advances in Colloid and Interface Science | Year: 2014

This review gives a brief overview of experimental approaches used to assess the bending rigidity of membranes. Emphasis is placed on techniques based on the use of giant unilamellar vesicles. We summarize the effect on the bending rigidity of membranes as a function of membrane composition, presence of various inclusions in the bilayer and molecules and ions in the bathing solutions. Examples for the impact of temperature, cholesterol, some peptides and proteins, sugars and salts are provided and the literature data are discussed critically. Future directions, open questions and possible developments in this research field are also included. © 2014 Elsevier B.V. Source

Vollhardt D.,Max Planck Institute of Colloids and Interfaces
Current Opinion in Colloid and Interface Science | Year: 2014

Knowledge of the mesoscopic morphology of condensed phase domains formed after the main phase transition in the two-phase coexistence region of Langmuir monolayers progressed rapidly with the development of the highly-sensitive imaging techniques, particularly by Brewster angle microscopy (BAM). Latest developments of commercial BAM instruments have been developed to a high technical level and allow upgrading to imaging ellipsometers which combine optical microscopy and ellipsometry and make the assessment of small layered structures or patterned thin films possible. A large variety of condensed phase domains different in mesoscopic sizes and shapes as well as their textural features has been observed which depend sensitively on the chemical structure of the amphiphilic monolayer and the system conditions, such as surface pressure and temperature. This unsuspected morphological variety of condensed phase domains has been proven not only in Langmuir monolayers but also in adsorbed monolayers (Gibbs monolayers), in Langmuir monolayers penetrated by dissolved surfactants or in adequate molecular recognition systems. The inner textures of domains can be explained on the basis of their geometry and the two-dimensional lattice in dependence of the tilt angle of the alkyl chains and gave rise to the development of a geometric concept on the basis of the molecular packing. New knowledge has been gained about non-equilibrium structures and their transition kinetics into the equilibrium state. Combined results obtained recently by BAM have enhanced the understanding of molecular organization in phase diagrams and binary mixtures. Recent advances in model studies about chiral discrimination effects and of the highly specific structural changes of host-monolayers by recognition of non-surface active guest-components have made progress. Semi-empirical quantum chemical methods have been used to gain insight into the role of different types of interactions involved in the main characteristics of mesoscopic length scale aggregates of mimetic systems. © 2014 Elsevier Ltd. Source

Laurino P.,Max Planck Institute of Colloids and Interfaces
Nature protocols | Year: 2011

A detailed protocol for the large-scale synthesis of carbohydrate and dihydrolipoic acid (DHLA)-coated CdSe/ZnS and CdTe/ZnS nanoparticles using continuous flow reactors is described here. Three continuous flow microreaction systems, operating at three different temperatures, are used for the synthesis of mannose-, galactose- or DHLA-functionalized quantum dots (QDs). In the first step of synthesis, the CdSe and CdTe nanoparticles are prepared. The size and spectral properties of the CdSe core of the nanoparticles are controlled by adjustment of the residence time and the temperature. As a second step, the zinc sulfide capping under homogenous conditions is carried out at a substantially lower temperature than is required for nanoparticle growth in batch processes. Finally, the trioctylphosphine/oleic acid ligand is effectively replaced with either carbohydrate PEG-thiol moieties or DHLA at 60 °C. This new protocol allows the synthesis of biologically active fluorescent QDs in 4 d. Source

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