Fibers, amphiphilic molecules

The purpose of this study is to follow the aggregation process from the small amphiphile molecule to the infinite network of Figure 3. For a critical density of fibers, viscosity diverges while an elastic modulus appears (15,16). 

It is apparent that the solubilization of amphiphilic molecules with chiral head groups produces aggregates such as braided fibers, helices, ribbons, rods, or tubules. When these organizations are sufficiently extended, they start to get entangled and develop the ability to entrap solvent molecules in the resulting reticulum. Many such instances eventually lead to the formation of gels. This phe-... 

Fig. 6 Micellar fibers with chiral packing of amphiphilic molecules...

In the last Section 6.4 new supramolecular approaches to construct synthetic biohybrid catalysts are described. So-called giant amphiphiles composed of a (hydrophilic) enzyme headgroup and a synthetic apolar tail have been prepared. These biohybrid amphiphilic compounds self-assemble in water to yield enzyme fibers and enzyme reaction vessels, which have been studied with respect to their catalytic properties. As part of this project, catalytic studies on single enzyme molecules have also been carried out, providing information on how enzymes really work. These latter studies have the potential to allow us to investigate in precise detail how slight modifications ofthe enzyme, e.g., by attaching a polymer tail, or a specific mutation, actually infiuence the catalytic activity. 

Bhattacharya etal. [73] reported gelation behavior of several L-phenylalanine-based amphiphiles. To explore the impact of molecular structures on gelation of L-phenylalanine derivatives, Bhattacharya and co-workers synthesized as many as twelve L-phenylalanine-based mono- and bi-polar derivatives and solubilized each of these in selected solvents. The formation of gel was found to depend on the concentration of the gelling agent, solvents and the temperature. The SEM and TEM studies suggested the formation of intertwined threads and fibers juxtaposed by slender filaments, which also produced a network with pores, which probably held the solvent molecules due to surface tension in the gel. 

In another application electrospinning [189] of PS-fo-P4VP(PDP)i.o supra-molecules was used to produce internally structured fibers with diameters in the range of 200-400 nm. Due to the block copolymer sample selected, self-assembly resulted in spherical P4VP(PDP) domains with the well-known internal lamellar structure. After the PDP was extracted from the fibers using methanol, porous fibers were obtained [190]. With this method, the thickness of the fibers can be tuned by adjusting the spinning conditions, and the size and nature of the pores can be controlled by the choice of block copolymer and amount of amphiphile. 

Hydroxy stearic acid and polybenzylglutamate were the classical cases of relatively simple molecules that produced twisted fibers, till the burst of reports on other examples, viz. amino acid amphiphiles, diacetylenic phospholipids, and gluconamides started in the mid-1980s. The handedness of chiral assemblies can be determined from electron micrographs, provided that care is taken to manipulate grids, specimens, films, and image scanners in a consistent way. It was not till the... 

Aqueous dispersion of a triple-chain glutamic acid diester monoamide shows beautiful helical fibers under the light microscope, which slowly close to form tubules. Above 50°C the fibers melt and rearrange to vesicles. Twisted ribbons made of similar molecules with azobenzene in the hydrophobic core polymerize upon UV irradiation. This leads to the disappearance of an induced CD spectrum of surface adsorbed methyl orange. Alanine-derived amphiphiles with azobenzene units in the chain also form helical fibers in water. They absorb at 350 run when the number of the methylene groups in the outer chain is even and at 320 nm when odd. The chromophore in the fiber is presumably less stacked in even-numbered alkyl chain assemblies (Fig. 9.5.i). 

Figure 6.3 Self-assembled nanostructures based on P-sheet (a) peptides packing into sheets and fibers based on hydrophobic interactions on one face of the molecule, and complementary ionic interaction on the other (b) peptides with alternating hydrophilic and hydrophobic residues assemble into P-sheet structures (left) that form twisted ribbons (right) and bundle into larger fibers and (c) self-assembly based on amphiphilic triblock peptides, where the central hydrophobic block forces self-assembly via hydrophobic interactions between molecules and hydrogen bonding along the fiber axis.

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