Bolaamphiphile solubility

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. 

Firstly we have to differentiate between monolayer (MLM) and bilayer (BLM) lipid membranes in vesicles. MLMs are composed of bolaamphiphiles these are amphiphiles which carry two head groups, namely one on each end of a hydrophobic core. Two head groups instead of one renders the amphiphile more water-soluble. Two short alkyl chains with 12 or more methylene groups, or one long chain with more than 24 hydrophobic atoms must be employed in order to obtain amphiphiles with a low critical vesicular concentration ( cvc < 10 M). The general abbreviation cmc is, however, usually applied instead of cvc . 

In the MLM and BLM vesicles, the cmc is small (< 10 M). In MLMs, the solubility of the individual monomers may be relatively large, if they contain charged head groups. Once the bolaamphiphiles are entrapped in a vesicular assembly they cannot escape, as the polar head group would have to pass through an apolar membrane, which is an unlikely process. At pH = 9, for example, the diacetic acid la dissolves reasonably well as the dianion in water. 

In nature, asymmetry is achieved through membrane dissolved proteins. In lipid membrane systems without proteins, only monolayers made of bola-amphiphiles allow a totally asymmetric arrangement of head groups. The simplest asymmetry to be achieved is dependent on the one-sided precipitation of bolaamphiphiles. a,to-Dicarboxylic acids, for example, are often soluble at pH > 8 and spontaneously form vesicles upon acidification to pH 5. At a lower pH, all carboxyl groups become protonated and one usually observes ill-defined precipitates . 

Figure 4.10 Symmetric vesicles with reactive head groups turn asymmetric when a water-soluble, membrane-inactive reagent reacts only with the outer surface. Flip-flip usually takes hours and can be completely suppressed in MLMs made of charged bolaamphiphiles. 

Spherical vesicles (see Sec. 2.5.4) are made by the same kind of amphiphiles that form micelles. Highly soluble amphiphiles (e.g., sodium salts of fatty acids or soaps) form micelles badly soluble amphiphiles (e.g., free fatty acids) give vesicles or crystallize. Amphiphilic monomers with two or three long alkyl chains are often totally water insoluble as monomers but dissolve well as vesicular assemblies. Vesicles usually collapse upon drying (Fig. 1.5.8a), but one isolable monolayer vesicle made of rigid carotenoid bolaamphiphiles has also been reported (Fig. 5.5). Hydrogen bond chains convert spherical vesicles to tubules. Such tubules can again be isolated in the dry form and can be stored. They are particularly stable if monolayer membranes are used (Fig. 1.5.8b). 

MLM vesicles can be made of bolaamphiphiles with two identical head-groups or two different headgroups. Water-soluble bipyridinium tetrabromide... 

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... 

The nucleophilic reactivity of cysteine has been exploited in Michael reactions with quinones. One example is a water-soluble naphthoquinone, which has been entrapped in chlorophyll-containing vesicles in order to study light-induced electron transfer through a membrane from glutathione to the quinone (Fore, 1983). Another example is an asymmetrical vesicle membrane made of a cysteine quinone carboxylate bolaamphiphile, where all the quinone is localized on the outer surface of the vesicle (see Scheme 7.2.6 Scheme 9.5.1). 

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