Vesicles unsymmetric

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. 

The immisdbility of CF2- and CHi-chains was also utilized for the preparation of unsymmetric vesicle membranes. The hydrophobic parts of bolaamphiphile 5 with fatty acid and fluorocarbon sulfonate halves do not mix and all the fluorosulfonate halves were on the outer side of the monolayered vesicle membrane (Figure 4.7). The sulfonate head group was once more localized by the metachromatic effect, the hydrophobic parts with F- and H-substituted spin labels. 

Fuhrhop et al.35 demonstrated that incorporation of quinone moieties into either the head groups or within the hydrophobic spacer of a bolaphile gave redox-active membranes (Figure 9). Unsymmetrical bo-laamphiphiles were used as models for photosynthetic electron acceptors (e.g. 29) and vesicle formation was verified by electron micrograph. The anticipated membrane organization would position the quinone moieties on the vesicle exterior. The location of the quinone moieties in this series was verified by their quantitative reduction by borohydride. Since boro-hydride does not diffuse through lipid membranes these membrane quinones must be part of the vesicle exterior. 

A macrocyclic and unsymmetrical bolaamphiphile with a large succinic acid headgroup on one end and a smaller sulfonate headgroup on the other is also water soluble at pH >8 and forms vesicles upon acidification to pH 4.5. In this case, however, the precipitated large succinic acid headgroups are located on the outer surface, all small sulfonate headgroups on the inner surface. This has been... 

Fuhrhop. J.-H., Mathieu, J. (1983). An unsymmetric monolayer vesicle membrane, J. Chem. Soc., Chem. Commun., p. 144. 

Unsymmetrical derivatization of a cryptand can afford yet another class of compounds known as gemini-amphiphiles (193) as shown in Scheme 9. However, vesicles from these surfactant molecules are not stable for more than 1 h at 298 K. 

These observations of the effect of spacers in mixed lipid systems led to the synthesis of an unsymmetrical lipid, in which the alkyl chain at -1 position contained a diacetylenic moiety and sn 2 position contained the spacer chain (see Figure 5). This lipid was dispersed in water to form vesicles, but did not show any polymerization enhancement. This result suggests that symmetric short chain PCs are required for any enhancement in polymerization. 

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