Monolayers bolaamphiphile

Although the bulk of PDA sensors involve vesicles and Langmuir monolayers, a few examples of responsive PDA assemblies based on bolaamphiphiles and diyne silica nanocomposites have been reported (Lu et al. 2001 Song et al. 2001, 2004 Yang et al. 2003 Peng et al. 2006). Although these materials have not been broadly utilized for analyte sensing, they do exhibit the thermochromic, solvatochro-mic, and pH responsive behavior seen with monolayers and liposomes and hold promise for future development. 

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]. 

Figure 19. (A) Monensin modified channel forming unit 15, negatively charged a,(o-bifunctional amphiphile 16a and neutral one 16b, capable of forming monolayered membrane and positively charged bolaamphiphiles 17 as a sealing agent of the channel. (B) Model of channel formation by 15 in the monolayered membrane composed of 16 and the proposed blocking mode by 17." ...

Figure 4.1. Schematic view of bilayer forming one- and two-tail one-headed amphiphiles and two-headed bolaamphiphile that can form a monolayer.

Masuda, M., and Shimizu, T. (2004), Lipid nanotubes and microtubes experimental evidence for unsymmetrical monolayer membrane formation from unsymmetrical bolaamphiphiles, Langmuir, 20(14), 5969-5977. 

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 . 

Figure 4.3 Vesicular monolayer lipid membranes (MLM) made of bolaamphiphiles with a charge on the inner surface are stable, because charged head groups cannot cross the hydrophobic membranes. Dissociation andfusion of MLMs are slow.

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 4.5 A small part of a monolayered vesicle membrane made by a one-sided precipitation of the given bipyridinium bolaamphiphile 2 with perchlorate.

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. 

Covesicles of the cationic nitrobenzoate 6 and DODAC, or corresponding DPP-analogues, e.g. 7, are hydrolysed at pH 8. Nitrophenolate absorption appears at 400 nm. The outer benzoate esters at the outer vesicle surface are hydrolysed within minutes and the same head groups on the inner surface survive for 1-15 hours (Figure 4.9). Detailed kinetics of flip-flop dynamics and OH permeation have been evaluated in these systems. Monolayer lipid membranes made of macrocyclic bolaamphiphiles showed enhanced dynamic stability. ... 

Figure 4.24 A monensin-based, dianionic bolaamphiphile assembles to form hydrated ion pores in monolayered vesicle membranes. They can be reversibly closed by a,(xi-diamino bolaamphiphiles. ...

Figure 5.12 Electron micrograph of a monolayered micellar fibre (diameter 2.5 0.4 nmj made of the bolaamphiphile 9 at pH 10. Negatively stained with phoshotungstate.

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. 

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 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).

Bohme, P., Hicke, H.-G., BOttcher, C., Fuhrhop, J.-H. (1995). Reactive and rigid molecular monolayers of bisaroylazide diamide bolaamphiphiles on polyacrylonitrile surfaces, J. Am. Chem. Soc. 117 5824. 

Figure 21. Schematic drawing of a SmA phase built up by monolayers of bolaamphiphiles.

Porphyrin bolaamphiphiles have been shown to form platelets with a thickness of 8 A in bulk water. Well-defined monolayers have been obtained with bolaamphiphilic porphyrins with two positive charges on each edge. Charge repulsion obviously first prevents crystallization and second unlimited growth of the isolated sheets. Their surface area is a few [ixn. In aqueous solution lecithin bilayers of unlimited extension (myelin figures) are known to have erne s below 10" M and can thus be considered as noncovalent polymers. ... 

The potential applications of bolaamphiphiles include the formation of monolayer vesicles for drug/gene delivery, ultra thin monolayer membranes, inclusion of functionalities into membranes, and disruption of biological membranes [59a]. 

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