Bolaamphiphile asymmetric

A new family of crown-ether-based bolaamphiphiles, 21, that aggregate into a previously unknown type of bolaamphisome was prepared by Gokel and coworkers [50]. Evidence was presented on vesicle formation from the aqueous suspension of such bolaform amphiphiles. Several reports describing the synthesis of various macrocyclic models related to archael compounds have also appeared in the literature [51]. Fyles et al. prepared a novel series of asymmetric bis-macrocyclic bolaphiles, 22, and evaluated their transport activities in vesicles and planar bilayer membranes [52]. 

The same maleic acid macrolide was also used to produce bolaamphiphiles with two different head groups in high yield. For this purpose, the macrolide was first dissolved in 2-propanol and reacted with an alkaline solution of 2-mercapto-succinic acid. The solvent was then evaporated and the residue extracted with acetone to remove a minimal percent of the remaining macrolide. The product, which contained only one mercaptosuccinic acid substituent, was redissolved in hot 2-propanol/water 4 1 and then sodium bisulfite added. A bolaamphiphile with a large dicarboxylic acid head group and a small sulfonate head group was thus obtained in an almost quantitative yield . Of course, it is also possible to esterify the maleic acid with bolaamphiphile alcohols in order to obtain asymmetric, non-cyclic bolaamphiphiles (Scheme 2.10). 

Another type of asymmetric bolaamphiphile not only varies the head groups but also partitions off the hydrophobic core into a hydrocarbon and a per-... 

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 4.6 a) The bolaamphiphile 3 with a large and a small head group assembles to form asymmetric vesicle membranes. 

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. 

Figure 2.5.14 Asymmetrical monolayered vesicle membranes have been obtained from the two bolaamphiphiles shown, (a) All large headgroups are on the outer surface, all small headgroups at the inside of the vesicle. This phenomenon is by no means universal. It has to be tested for each individual asymmetrical bolaamphiphile. In most cases there is only a small difference in the localization of different headgroups. In (b) the metachromatic effect of polyanions on methylene blue aggregation on the sulfonated membrane outside is indicated (see text above).

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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