Bolaamphiphiles micelles

In this section the emphasis will be on assemblies prepared from amphiphilic dendrimers. Amphiphilic dendrimers, carrying both hydrophobic and hydrophilic regions within one molecule, tend to self-assemble into a large variety of different aggregates depending on their structure. The dendritic amphiphiles investigated so far include unimolecular micelles, bolaamphiphiles, superam-phiphiles and various other AB and ABA block copolymers. 

Alkyl chain(s) with carbohydrates at both termini (bolaamphiphiles) have also been reported since they are potential building blocks for the construction of membrane mimetics with a single monolayer [78]. Bisgluconamide and lactobionamides 6b (sug=Glc-A or Lac-A, X=NH) were studied for their crystalline properties and their arrangements in water [39, 40]. Alkyl-a,cc)-dimannitol 6b (sug=Man-ol, n=16-22) [66] or bolaamphiphiles with identical or different carbohydrates at both ends of the alkyl chain 6a (sug=D-Glc/, D-Galp, DL-Xyl-oI) were found to form micelles and lyotropic liquid crystals as well [41]. 

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. 

Recent reports on monomeric and polymerized bolaamphiphiles1 provide evidence for their potential application in the broader field of molecular organizates (1,2). Thus monomeric bolaamphiphiles have been employed in the formation of monolayer lipid membranes or vesicles (1-3). formation of micelles (4-5) and also for spanning bilayer membranes (1-6) The latter process has resulted in the stabilization of membranes. 

Amphiphiles, molecules possessing both hydrophilic (head group) and lipophilic (tail group) moieties,1 have formed the cornerstone of work on structure and functional application of micelles and membranes. One logical modification of the amphiphilic structure would simply be the addition of a second hydrophilic group to the other end of the lipophilic moiety to form the novel class of surfactants called bolaamphiphiles.2... 

Beyond these initial examples, basic questions remain concerning the conformation of bolaamphiphile within micelles. [30] There is evidence that a bent conformation is adopted. 

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). 

Figure 7.32 Organization of bolaamphiphiles in (a) a monolayer membrane, (b) a U-bent bilayer membrane and (c) a U-bent micelle.

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