Fibril chirality

Fig. 10 (a-f) Hierarchical self-assembly model for chiral rod-like units A curly tape (c ), a twisted ribbon (d ), a fibril (e ) and a fibre (f). Adapted from Aggeli et al. [20], Copyright 2001 National Academy of Sciences, USA... 

Figure 5.24 Model of hierarchical self-assembly of chiral rodlike monomers.109 (a) Local arrangements (c-f) and corresponding global equilibrium conformations (c -f) for hierarchical selfassembling structures formed in solutions of chiral molecules (a), which have complementary donor and acceptor groups, shown by arrows, via which they interact and align to form tapes (c). Black and the white surfaces of rod (a) are reflected in sides of helical tape (c), which is chosen to curl toward black side (c ). (b) Phase diagram of solution of twisted ribbons that form fibrils. Scaled variables relative helix pitch of isolated ribbons h hh /a. relative side-by-side attraction energy between fibrils eaur/e. Reprinted with permission from Ref. 109. Copyright 2001 by the National Academy of Sciences, U.S.A.

Crown ether binaphthyl derivatives 128-131 (Scheme 71) were synthesized and investigated by Akagi [139], Compounds 128-131 were used to induce chiral nematic phases (N ) in liquid crystals. It was found that the helical twisting power increased with decreasing ring size. Helical polyacetylenes were synthesized in the N phases. It was found that the interdistance between the fibril bundles of the helical polyacetylene was equal to the half-helical pitch of the N liquid... 

Aggeli, A., Nyrkova, I. A., Bell, M., etal., Hierarchical self-assembly of chiral rod-like molecules as amodel for peptide beta-sheet tapes, ribbons, fibrils, and fibers. Proc. Natl. Acad. Sci. U. S. A. 2001,98, 11857-11862. 

Nyrkova, I.A., Semenov, A.N., Aggeli, A., and Boden, N. "Fibril stability in solutions of twisted beta-sheet peptides a new kind of micellization in chiral systems". Eur. Phys.. B 17(3), 481-497 (2000b). 

In the case of oligopeptides which form dynamic antiparallel /1-sheet tapes (Sect. 2.1.5), chirality of the amino-acids is directly responsible for the twisting of the tapes [142,143]. Moreover, the lateral aggregation of the tapes can lead to the formation of fibrils of discrete thickness, and is in fact controlled by the twisting of the tapes [144-146]. 

As another example of a helical polyacetylene, the single-handed helical polyacetylene fibril, whose structure was studied by SEM, was prepared by the polymerization of acetylene within a chiral nematic liquid crystalline phase.192... 

Figure 14.21 (a) a typical gel passing the inversion test , (b) SEM micrograph of a dried gel (xerogel) made from a chiral gelator showing helical fibrils. ... 

Unique side-chain conformation responsible for chirality and azimuthal orientation in the molecular alignment of skin collagen was reported. This alignment may account for the axial periodicity and chiral appearance of skin collagen fibrils. However, its implications in transdermal permeabihty are not yet known [20]. 

Peptides composed of various coded and noncoded amino acid residues self-assemble to form various types of supramolecular architectures, including supramolecular helices and sheets, nanotubes, nanorods, nanovesicles, and nanofibers. The higher-order self-assembly of supramolecular (3-sheets or supramolecular helices composed of short synthetic acyclic peptides leads to the formation of amyloid-like fibrils. Synthetic cyclic peptides were used in supramolecular chemistry as molecular scaffolding for artificial receptors, so as to host various chiral and achiral ions and other small neutral substrates. Cyclic peptides also self-assemble like their acyclic counterparts to form supramolecular structures, including hollow nanotubes. Self-assembling cyclic peptides can be served as artificial ion channels, and some of them exhibit potential antimicrobial activities against drug-resistant bacteria. 

A similar model can be considered even when a system of polymers forms microfibrils or filaments distributed with a constant interdistance. The separation of filaments can be controlled by the balance of several forces hydration, electrical doublelayer repulsion and van der Walls attraction. This conception derives from a paper by Elliot and Rome (1969) who interpreted in that way the interdistance control between muscle cell filaments. We do not know the exact shape of the isopotential surfaces corresponding to that system of forces in the vicinity of fibrils formed by a set of chiral pol5nners, but they must show in many cases a helicoidal symmetry. The complete system can closely resemble the model suggested by Rudall (1955). 

It has been generally accepted that polyacetylene has a planar structure, irrespective of cis and trans forms, due to ir-conjugation between the sp hybridized carbon atoms in the polymer chain [1,4]. If it were possible to modify such a planar structure of pofyacetylene into a hehcal one [13], one might expect novel magnetic and optical properties [14]. Here, we report polymerization of acetylene in asymmetric reaction field constructed with chiral nematic LCs, and show that pofyacetylene films formed by helical chains and fibrils can be synthesized [15]. Polymerization mechanism giving hehcal structure from primary to higher order and hierarchical spiral morphology is discussed. 

Scanning electron microscope (SEM) images of polyacetylene films show that multidomains of spiral morphology are formed (Figure 3.4a), and each domain is composed of helical structure of bundle of fibrils with one-handed screwed direction (Figure 3.4b). The multidomain-type fibril morphology of polyacetylene seems to replicate the chiral nematic LC during interfacial acetylene polymerization. 

Closer observation of SEM images indicates that helical polyacetylenes synthesized in the (R)- and (S)-chiral nematic LCs form screwed bundles of fibrils and screwed fibrils with counterclockwise and clockwise directions, respectively (Figure 3.5). This result implies that the screw direction of helical polyacetylene is controllable by choosing the helicity, i.e., optical configuration of the chiral dopant, so far as the chiral nematic LC induced by the chiral dopant is employed as an asymmetric polymerization solvent. In addition, it is clear that the screw directions of bundle and fibrils are opposite to those of the (R-l)- and (S-1)-chiral nematic LCs used as solvents. This is an unexpected and even surprising result, requiring a sound interpretation that is to be discussed later. 

The bundles of fibrils for helical polyacetylenes synthesized in the (fi-2)- and (S-2)-chiral nematic LCs are screwed counterclockwise and clockwise, respectively The screw directions of helical polyacetylene are opposite to those of the corresponding (fi-2)- and (S-2)-chiral nematic LCs whose directions are confirmed to be clockwise and counterclockwise, respectively, through the miscibility test with choles-teryl oleyl carbonate. This is the same situation as that of the (fi-1)- and (S-l)-chiral nematic LCs including (fi)- and (S)-PCH506-Binol. 

From the above mentioned results, it can be remarked that counterclockwise and clockwise helical polyacetylene chains are formed in (R)- and (S)-chiral nematic LCs, respectively, and that these hehcal chains are bundled through van der Waals interactions to form hehcal fibrils with the opposite screw directions to those of the chiral nematic LCs. The bundles of fibrils further form the spiral morphology with various sizes of domains (Figure 3.9). 

Helical polyacetylene was synthesized in asymmetric reaction field consisting of chiral nematic LC. The chiral nematic LC was prepared by adding a chiroptical binaphthyl derivative as a chiral dopant to a mixture of two nematic LCs. Acetylene polymerizations were carried out using the catalyst Ti(0-n-Bu)4— EtsAl dissolved in the chiral nematic LC solvent. The polyacetylene film consisted of clockwise or counterclockwise helical structure of fibrils in SEM. Cotton effect was observed in the region of ir — ir transition of the polyacetylene chain in CD spectrum. The high-electrical conductivities of 1.5 1.8 x... 

To illustrate the diversity of chiral inorganic objects that can be obtained by transcription of chiral self-assembled fibers as organic templates, even double-helical silica has been produced. Gels of a mix of sugar-based gela-tors produce double-helical silica nanotubes by transcription (Fig. 26) [180]. In addition, gels of gemini smfactant 23 (Scheme 4) produce double-helical fibrils of silica [181]. In the latter case, the continuous variation of the pitch... 

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