Synkinon

We substantiate the new notions of synkineses and synkinons for several important types of molecular assemblies as described in this book. 

An electron donor or acceptor site is usually needed in organic synthons for covalent synthesis. The covalent connection of both leads to a new molecule in an essentially irreversible synthetic reaction. Organic synkinons for non-covalent synkinesis usually contain a hydrophilic and a hydrophobic part and/or proton donor or acceptor sites. Non-covalent connection of such amphiphiles leads to a molecular assembly in a reversible synkinetic reaction. Amphiphiles are not only surface active molecules ( surfactant, detergent ), but much more important, they create surfaces. This becomes particularly evident in microemulsions and in suspensions of vesicles and micellar fibres, but is also true in nanoholes and pores, on monolayer surfaces and for many other supramolecular structures. 

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 1.1 Schematic models of a few typical synkinons for some membranes and molecular assemblies. Green indicates hydrophobic blue is hydrophilic. In a few...

In this chapter we introduce compounds which have been successfully applied in the construction of supramolecular assemblies. Only the amphiphiles which have been prepared in sufficient quantities have been admitted milligram quantities being considered unacceptable as starting materials for the preparation, analysis and application of assemblies. Experience proves that complicated dyes, pore builders, receptors etc. never reappear in the literature after their syntheses and spectroscopic properties have been reported. On the other hand, such easily attainable synkinons and surfactants around the ten gram scale need not, of course, be too simple. On the contrary, they may contain all the components of the chiral pool, i.e. amino acids, carbohydrates, steroids etc., as well as all commercial dyes of interest such as protoporphyrin, phthalo-cyanines, carotenes, viologen and quinones. 

Ammonium salts with two different alkyl chains were prepared directly via subsequent alkylations of dimethylamine with primary bromides and crystallization. Commercial hexadecyl-methylamine can be conveniently applied in the same way in order to convey functionality to cationic synkinons. A recent example describes subsequent alkylations with a small functional and a long-chain primary bromide (Scheme 2.4). A-acylated / -phenylenediamine was also alkylated at the second nitrogen atom which had two different alkyl chains, with or without extra functionality . After deacylation, this head group can be diazotized or coupled oxidatively with various heterocycles in water (Scheme 2.4). Photoactive and coloured membrane surfaces are thus obtained. Phenylene-diamine, pyridine and in particular A-methyl-4,4-bipyridinium chloride are relatively weak nucleophiles. Substitution of bromides is slow and the more reactive iodides can rarely be obtained commercially, but the selection of nitromethanes as solvent for bromide substitution is of great help as well as the addition of sodium iodide to enforce a Finkelstein reaction or a combination of both. 

The cyclic peptide cjcfo-[(D-Ala-Glu-D-Ala-Gln)2l with an even number of alternating d- and L-amino acids adopted a low-energy, ring-shaped flat conformation in which all backbone amide functionalities lay almost perpendicularly to the plane of the structure. This synkinon allows subunits to stack in an... 

The simplest and most common synkinons are non-branched, saturated fatty acids from C12 to C18 (trivial names lauroyl Cl2, myristoyl Cl4, palmitoyl or cetyl Cl 6, stearoyl Cl8) and their sodium, ammonium and potassium salts (also known as soaps ). Laurie, myristic, palmitic and stearic acids are barely soluble in water at 20°C (5.5, 2.0,0.7 and 0.3 mg/L) and 60°C (8.7, 3.4, 1.2 and 0.5 mg/L), each ethylene group lowering the solubility by a factor of 2-3. The solubilities of the corresponding sodium and potassium salts are, however, in the order of several grams per litre. Even in highly concentrated emulsions of soaps in distilled water ( 30% w/w), precipitation of solids is often not observed. Bivalent fatty acid salts, however, are just as insoluble as free fatty acids only 1.4 mg of calcium stearate dissolves in 1 L of water. ... 

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. 

Fitting of Charges. Well-defined heterodimers and stacks are formed with the placement of at least two charges on the synkinons. Stack formation can be stopped at the dimer stage by alkyl substituents (Figs. 1.5.4 and 6.6.2)... 

Invert soaps do not appear in nature but are important synkinons in the preparation of artificial membrane structures. The most common application of such monolayers is a cosmetic one. For centuries people smeared fats on their hair to make it shiny, but the hair then stuck together. Invert soaps adsorb strongly to hair proteins and provide them with the elegant luster of a monolayer and the fullness of non-greasy and non-polar hair. Hair with a nonsticky hydrocarbon monolayer on the surface looks irresistibly shiny, fluffy, and clean. Combinations with polymers, such as silicones, proteins, and poly(vinyl pyrrolidone) then help to build even more body in leave-in conditioning products. 

The peptide cyclo [(D-Ala- L-Glu- D-Ala- L-Glu)2] with an even number of alternating d- and L-amino acids adopts a flat conformation. This macrocyclic synkinon forms extended stacks in water and vesicle membranes. A contiguous P-sheet in the form of tubules may reach a length of a few hundred nanometers and an inner diameter of about 7-8 A in water (Fig. 9.5.4). The tubules also condense to form bundles of about 100 parallel strands. In lipid bilayers these peptide nanotubes are aligned parallel to the hydrocarbon chain and act as ion channels at their rigid outer surfaces. Their regulation by molecular stoppers, applied potentials, etc. has not been achieved so far (Kim et al., 1998). 

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