Bolaamphiphile

The size of carotenoid aggregates have been determined by dynamic light scattering (DLS), a noninvasive method (Santos and Castanho 1996). DLS also allows distinguishing between spherical or cylindrical aggregates. The hydrodynamic radii rH of hydrophilic carotenoids in water are given in Table 3.1. Size and molecular structure of the bolaamphiphiles crocin, 3.7, and Cardax,... 

Jorgenson et al. reported [125] the synthesis of bolaamphiphile 46 in which a tetrathiafulvalene (TTF), a substrate currently of importance in fields such as organomagnetism and molecular electronics, was incorporated within the central lipophilic region. Gel formation of 46 (Fig. 20) in either DMF-water or ethanol-water solution was noted which bear similarities with the above findings of Newkome and coworkers. 

Host-guest systems made from dendritic materials have potential in the areas of membrane transport and drug delivery [68, 84, 85]. In a recent report [136] Tomalia and coworkers investigated structural aspects of a series of PAM AM bolaamphiphiles (e.g., 50) with a hydrophobic diamino do decane core unit. Fluorescence emission of added dye (nile red) was significantly enhanced in an aqueous medium in the presence of 50 unlike the cases when 51 and 52 were added (Fig. 23). Addition of anion surfactants to this mixture generated supramolecular assemblies which enhanced their ability (ca.by 10-fold) to accommodate nile red (53). Further increase in emission was noted by decreasing the pH from the normal value of 11 for PAMAM dendrimers to 7. At lower pH values the... 

Figure 5.10 Schematic overview of self-assembly process of high-axial-ratio nanostructures using bolaamphiphilic monomers. Arrows indicate hydrogen bond functionalities. Reprinted from Ref. 53 with permission of Wiley-VCH.

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. 

Bolaamphiphiles are molecules carrying two polar end-groups separated by an hydrophobic spacer [97, 98], Several different types of bolaamphiphiles have been reported (Figure 16.8). 

Figure 16.8 Three different kinds of bolaamphiphiles containing different apo-lar spacers and/or polar head groups. The cartoons representthe orthogonal and nonorthogonal arrangements proposed to explain the fiber structures observed in solution...

Auer et al. [134] presented an example for multilayer formation and controlled deposition of functionalized nanoparticles on SAM of mercaptohexadecanoic acid (MHA) using electrostatic interactions. As a pH-sensitive switchable linker between the SAM of MHA and negatively charged gold nanoparticles, bis-benzami-dine bolaamphiphiles having different alkyl spacers were used [135]. This strategy resulted in a potentially tunable and switchable property of the entire assembly. For example, the kinetics of adsorption as well as the final particle layer thickness can be controlled by the kind of bis-benzamidine used as the linker (Fig. 9.16). 

Nakazawa, 1. Suda, S. Masuda, M. Asai, M. Shimizu, T. pH-dependent reversible polymers formed from cyclic sugar- and aromatic boronic acid-based bolaamphiphiles. Chem. Commun. 2000, 881-882. 

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. 

Song J, Cheng Q, Kopta S, Stevens RC. Modulating artificial membrane morphology pH-induced chromatic transition and nanostructural transformation of a bolaamphiphilic conjugated polymer from blue helical ribbons to red nanofibers. J Am Chem Soc... 

Song J, Cisar JS, Bertozzi CR. Functional self-assembling bolaamphiphilic polydiacetylenes as colorimetric sensor scaffolds. J Am Chem Soc 2004 126 8459-8465. 

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.

Besides the T-shaped bolaamphiphiles covered here, there are also T-shaped polyphiles with a reversed position of hydrophilic and lipophilic units, see [311-313],... 

Fig. 51 Examples of T-shaped bolaamphiphiles with non-polar lateral RF/RH-chains showing the effect of fluorination on mesophase types and transition temperatures (77° C) [316-318, Lehmann A, Prehm M, Tschierske C, unpublished results, 319]...

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