Chiral vesicle structure

Addition of 5% ganglioside Gmi into the L-Glu-Bis-3 resulted in the appearance of vesicles along with twisted ribbons, while addition of nonchiral 10,12-docosadiynedioic acid caused the formation of platelets.97 These results affirm the importance of packing geometry, along with head group chirality, for the formation of helical structures. 

The experiments discussed in this chapter have shown that a variety of chiral molecules self-assemble into cylindrical tubules and helical ribbons. These are indeed surprising structures because of their high curvature. One would normally expect the lowest energy state of a bilayer membrane to be flat or to have the minimum curvature needed to close off the edges of the membrane. By contrast, these structures have a high curvature, with a characteristic radius that depends on the material but is always fairly small compared with vesicles or other membrane structures. Thus, the key issue in understanding the formation of tubules and helical ribbons is how to explain the morphology with a characteristic radius. 

Up until 1977, the non-covalent polymeric assemblies found in biological membranes rarely attracted any interest in supramolecular organic chemistry. Pure phospholipids and glycolipids were only synthesized for biophysical chemists who required pure preparations of uniform vesicles, in order to investigate phase transitions, membrane stability and leakiness, and some other physical properties. Only very few attempts were made to deviate from natural membrane lipids and to develop defined artificial membrane systems. In 1977, T. Kunitake published a paper on A Totally Synthetic Bilayer Membrane in which didodecyl dimethylammonium bromide was shown to form stable vesicles. This opened the way to simple and modifiable membrane structures. Since then, organic chemists have prepared numerous monolayer and bilayer membrane structures with hitherto unknown properties and coupled them with redox-active dyes, porous domains and chiral surfaces. Recently, fluid bilayers found in spherical vesicles have also been complemented by crystalline mono-... 

The great diversity of concepts and synkinetic structures which have been realized within the last decade and which is partly represented in this volume, suggests that all kinds of membranes are accessible asymmetric, as thin as 2.0 nm, helical, porous, fluid or solid, chiral on the surface or in the centre, photoreactive etc. etc. This diversity will inevitably grow. A few obvious unsolved problems which need immediate attention can also be detailed e.g. synkinesis of solid micelles and vesicles from concave molecules with at least four hydrogen bonding sites, co-crystallization of porphyrins with solid membrane structures, and evaluation of nanopores as catalytic sites. Many more such target assemblies will undoubtedly be envisioned and successfully syn-kinetized. 

In biological membranes, the chirality of the component molecules is typically not reflected in the structure of the supramolecular assembly, e.g. the monolamellar vesicle that constitutes the biological cell membrane, the multilamellar systems of the thylakoids that are present in the photosynthetic machinery of chloroplasts, and the highly curved inner membranes of the mitochondria. It should be noted that even if the chirality of biological lipid molecules is not expressed in an assembly, the fact that the components are chiral is still important for the following reasons ... 

Converse flexoelectric studies of lyotropic liquid crystals, such as vesicles, is still an active subject. Notably, the sensory mechanism of outer hair cell composite membranes " can be understood by the flexoelectric properties of the lipid bilayer. The converse of this effect, i.e., a voltage-generated curvature, has also been observed and was discussed by Todorov et Another related phenomenon is the ferroelectricity which results from the tilted layered structures of chiral molecules, which has been discussed extensively since the 1980s.Ferroelectric phases are called... 

The systems described above formed linearly extended, fibrous self-assemblies, although the morphological structures depended on amphiphiles. The molecular arrangement in fibers were maintained by the formation of concentric multilamellar bilayer, besides hydrophobic interaction and hydrogen bonding, except C Asp fibers with chiral character. The assemblies underwent temperature-dependent fibril-vesicle transition. The fibril-vesicle transition resulted from the rearrangement of bilayers. Generally, multilamellar layers were diminished in ves-... 

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