Synkinesis membranes

There are, for example synkinons for the synkinesis of micelles, vesicles, pores, fibres and planar mono- or multilayers. A given synkinon can also be applied for another synkinetic target if the conditions are changed or if the synkinon is chemically modified. The most simple example is stearic acid. At pH 9, it is relatively well-soluble in water and forms spherical micelles. If provided with a hydrogen bonding chiral centre in the hydrophobic chains (12-hydroxystearic acid), it does not only form spherical micelles in water but also assembles into helical fibres in toluene. At pH 4, stearic acid becomes water-insoluble but does not immediately crystallize out spherical vesicles form. A second type of synkinon, which produces perfectly unsymmetrical vesicle membranes, consists of bolaamphiphiles with two dififerent head groups on both ends of a hydrophobic core. Such bolaamphiphiles are also particularly suitable for the stepwise construction of planar multilayered assemblies. 

Figure 4.27 Long-chain bolaamphiphiles with an electroneutral and a cationic head group form membranes around anionic polymers (here DMA) and colloids. Synkinesis of organic-inorganic composite materials in bulk aqueous media can thus be achieved. ...

So far, we have described synkinesis only for molecular mono- and bilayer systems. 3D crystals are, in general, considered to constitute stable, chemically dead systems with no potential for the construction of reactive, supramolecular systems. There are, however, exceptions. First of all, the surface of crystals is, of course, as reactive as any other surface. Crystal engineering is considered here as the solid-state branch of synkinesis. Furthermore crystals with large cavities have recently been prepared. They contain inner surfaces which may have interesting receptor properties in co-crystallization and photochemical fixation processes, which constitute another type of planned synkinesis. Furthermore, spontaneous 3D crystallization may compete with the synkinesis of membranes and the molecular conformations and interactions in crystals are important standards for the study of membranes. The study of 3D crystal structures of amphiphiles is therefore mandatory as a basis for all structural work on molecular assemblies. 

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. 

The other chapters then lead from the simple to the more complex molecular assemblies. Syntheses of simple synkinons are described at first. Micelles made of 10-100 molecules follow in chapter three. It is attempted to show how structurally ill-defined assemblies can be most useful to isolate single and pairs of molecules and that micelles may produce very dynamic reaction systems. A short introduction to covalent micelles, which actually are out of the scope of this book, as well as the discussion of rigid amphiphiles indicate where molecular assembly chemistry should aim at, namely the synkinesis of solid spherical assemblies. Chapter four dealing with vesicles concentrates on asymmetric monolayer membranes and the perforation of membranes with pores and transport systems. The regioselective dissolution of porphyrins and steroids, and some polymerization and photo reactions within vesicle membranes are also described in order to characterize dynamic assemblies. 

Typical target assemblies of synkinesis are noncovalent dimers, charge-transfer complexes, hetero-dimers and -trimers, cyclic or helical oligomers, inherently asymmetrical or helical membranes in the form of vesicles, spheres, tubules, and rods, as well as macroscopic surface monolayers. Pores and domains in vesicle membranes and gaps of molecular size in surface monolayers are other synkinetic targets. They may function as receptors or reactive, enzyme-like surface clefts (Fendler, 1982 Fuhrhop 1982 Israelaehvili, 1992 Fuhrhop and Koning). 

A large variety of totally artificial membranes accessible by synkinesis have structures unknown to biological systems. One may, for example, prepare membranes as thin as 2 nm containing photoactive groups in various positions, and they may show a totally unsymmetrical distribution of two different head-groups on both surfaces of a curved membrane (see Sec. 2.5.3). Natural membranes, on the other hand, are incredibly functional. They perform the complex energy conversion and reproduction processes of life with unsurpassed efficiency and reliability. 

In synkinesis steroids are useful in solution as spacers between two reactive. sites bound to steroid substituents and as matrices on solid. surfaces. The magic of the steroidal skeleton lies in its extreme variability of stiffness and flexibility accompanied by consistent intramolecular distances (Fig. 3.1.1). Steroids also form hydrophobic and amphiphilic domains with pronounced stereochemical control and selective solubility in membrane structures. 

Use the chiral pool in synthesis and synkinesis. The common monosaccharides provide three or four asymmetrical carbon atoms per unit (an orgy in precious chiral center at very low cost). Carbohydrates together with amino acids therefore constitute the principal components of the chiral pool, the most rewarding and renewable source of fine chemicals and stereoselective membrane surfaces. 

In comparison with other textbooks on natural compounds, this book gives strong emphasis to water, membranes, and solid surfaces as reaction media. Synkinesis is much more efficient and useful there than it is in homogeneous organic solutions. Furthermore, electrons, protons, oxygen, and water are taken as the most important reagents in synkinetic assemblies, not the various carbanions and carbonyl compounds of synthetic chemistry. 

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