Fibers helical

Lipid Nanotubes and Helical Fibers from Cardanyl Glycolipids... 

Figure 5.5 Computer-generated model of quadruple helix structures made of D-Glu-8 (1) based upon image-processed electron micrograph of helical fibers. Reprinted with permission from Ref. 35. Copyright 1993 by the American Chemical Society.

Figure 5.6 Steric view of sickle structure of D-Glu-8 (1) in helical fibers based upon measurements made using rotational echo double-resonance NMR experiments. The N-C3 torsion angle (>Iy) is —100°, while the C1-C4 torsion angle (>I 4) is —53°. Reprinted from Ref. 40 with permission of The Royal Society of Chemistry on behalf of PCCP Owner Societies. 

Figure 5.11 Confocal laser scanning micrographs of self-assembled helical fibers made of glucosamide bolaamphiphiles Glc-NC( )CN-Glc (14) (a) n = 6, (b) n = 10, and (c) n = 12. Reprinted from Ref. 53 with permission of Wiley-VCH.

Not all fibers yield fibers on comminution. Fibrous varieties of quartz (Si02), for example, are formed from tightly bonded, aligned helical fibers that cannot be separated mechanically (Frondel, 1978). Fibrous calcite (CaCOa), when crushed or ground, breaks into equant grains of rhombic shape. The fragments reflect the cleavage characteristics of the mineral. 

We have tried to explore the gel structure at several microscopic scales the left handed helix conformation, the supramolecular structure, showing the kind of helix association (triple helices, fibers, aggregates,. ..), the role of the solvent. 

Pagel K, Wagner SC, Samedov K, Von Beripsch H, Boettcher C, Koksch B. Random coils, 8-sheet ribbons, and a-helical fibers one peptide adopting three different secondary structures at will. J Am Chem Soc 2006 128 2196-2197. 

R,R)- and (S -cyclohexanediamines 102 self-assemble with cyclohexane-1,2-diol 103 forming helical fibers of 40-70 nm length whilst the racemate produces platelets [53]. 

Lehn and coworkers also discussed the principles of design of self-assembling helical structures on the basis of conformational control. Depending on the solvent used, they were able to obtain helicate fibers and bundles possessing extended molecular channels characterized by hollow cores of limited diameter ofca. 8 A. This finding could form the basis of functional materials for multichannel ion active transport [28]. 

The chiral-bilayer-effect hypothesis has been evoked for the rationalization of the helical fibers formed from enantiomeric or diastereomeric surfactants (Fig. 54) [373], Different packing of the chiral surfactants in the crystals (head-to-tail) and in bilayer or micellar aggregates (tail-to-tail) is the basis for this postulate. Crystallization from aggregates requires an energetically costly, 180°... 

Spontaneous resolved two chiral domains are formed in equal probability. In other words, enantiomeric excess (ee) is zero. We now ask whether ee can be controlled or not. The answer is yes. Several methods used in bent-shaped mesogenic phases will be introduced. The direct method is of course an addition of chiral dopant. Actually this has been shown to be a viable method [6, 61]. Use of chiral surface is also effective [62], By using polyimide with chiral side chains at both substrate surfaces, imbalance of two chiral domains (10% ee) has been achieved. Another method using macroscopic helical structure was demonstrated by Jakli et al. [63]. They used a nonchiral polymer network, which was formed in the N phase. After the polymerization, N compounds were washed out, then bent-core mesogens were introduced. Because of the polymer helical fibers, bent-core mesogen shows a chiral domain. 

The observed structures were explained by a chiral bilayer effect mechanism proposing that only the enantiomerically pure compounds can lead to the formation of helical fibers which in turn slowly rearrange to enantiopolar crystal layers (Scheme 7.1). Within the micellar fibers, the polar head groups are oriented toward the aqueous environment and must therefore go through an energetically unfavourable -slow- dehydration followed by a 180 ° to form the enantiopolar crystals. 

Figure 20.9. Molecular simulations of numerous FKE8 peptides, with hydrophobic phenylalanine on one side and hydrophilic lysines and glutamic acids on the other, undergo self-assembly to form left-handed helical fibers that contain 97 peptides per helical turn, 7 nm in diameter with a 19 nm pitch. The molecular simulated structures are consistent with the experimental observations. Water drives the hydrophobic phenylalanine pack inside the left-handed peptide double helix (Marini et al., 2002 Hwang et al.. 2003).

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