Multiscale
Selforganization

Membranes and Vesicles


  • Membranes are very thin and highly flexible sheets of molecules which provide the basic structural elements for the molecular architecture of biological cells, see membrane cartoons. Even though the molecular composition of biomembranes is rather complex and highly specific, they all exhibit the same universal construction principle: a molecular bilayer of lipids and membrane proteins.

  • Large membranes form closed surfaces or vesicles, which can be directly observed under the microscope. [1] [2] [3] Vesicle membranes exhibit many different shapes and shape transformations such as domain-induced budding [4] [5] [6] and conformal diffusion [7], both of which were first predicted theoretically. Current research activities on membranes and vesicles include:

    • Supramolecular organization of bilayer membranes: The self-assembly of bilayer membranes in water can be studied by computer simulations such as Molecular Dynamics [8] or Dissipative Particle Dynamics [9]. For multi-component membranes, such simulations have shown that the bending rigidity is a nonmonotonic function of membrane composition [10] [11], and demonstrated domain formation and phase separation on small vesicles [11].

    • Membrane adhesion via specific molecular bonds ("molecular recognition"): Biomembranes interact via molecular 'stickers' [12] [13] [14], i.e., via pairs of membrane-anchored adhesion or receptor molecules. The linked molecules still diffuse within the contact area of the membranes [15], which implies that they can form intramembrane clusters and domains [13] [16] . This lateral mobility leads to domain pattern formation within the contact area [17], and to binding cooperativity of the membrane-anchored receptors [18].

    • Membrane curvature induced by macromolecules and nanoparticles: Polymers and other macromolecules that are anchored to the membrane induce a curvature in the adjacent membrane patch. [19] [20] If the non-anchored segments of the polymer are repelled from the membrane, the membrane curves away from the molecule. [21] [22] If the non-anchored segements are adsorbed onto the membrane, the membrane usually curves towards the polymer. [23] Membrane curvature is also induced by non-anchored nanoparticles and macromolecules within the aqueous solution [24] [25] [26]. Particularly dramatic effects arising from membrane/polymer interactions have been recently observed for lipid vesicles in PEG/dextran solutions: these systems involve a ''hidden'' material parameter, the intrinsic contact angle [27], undergo complete-to-partial wetting transitions [28], and form many stable nanotubes [29].

    • Fusion of membranes and vesicles: Membrane fusion is an essential process during virial infection, vesicular trafficking, synaptic transmission etc. The fusion process starts with membrane adhesion and the molecular reorganization of the two adjacent bilayer membranes. The presumably simplest way to induce such a reorganization is via membrane tension. Using Dissipative Particle Dynamics simulations, several pathways for tension-induced fusion have been identified. [30] [31] [32] One pathway is governed by two different energy barriers. [31] The simulation results are consistent with fusion experiments, in which vesicle fusion was monitored with a temporal resolution of 50 microseconds. [33]

  • For more information, see multiscale selforganization of membranes and vesicles.