Ribbons, twisted

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 5.3 Electron micrographs of diastereomers of A-octyl-D-aldonamide (a) helical rods from D-Glu-8 (1, bar = 50 nm), (b) rolled-up sheets from D-Man-8 (2, bar = 300 nm), and (c) twisted ribbons from D-Gal-8 (3, bar = 300 nm). Reprinted with permission from Ref. 31. Copyright 1990 by the American Chemical Society.

Figure 5.20 TEM images of twisted ribbons observed in diacetylenic amino-acid-derivatized lipids in methanol-water solution (a) Glu-PDA (34) (b, c) Gln-PDA (35). Bar = 0.6 xm. Reprinted with permission from Ref. 95. Copyright 2000 by the American Chemical Society.

Addition of 5% ganglioside Gmi into the L-Glu-Bis-3 resulted in the appearance of vesicles along with twisted ribbons, while addition of nonchiral 10,12-docosadiynedioic acid caused the formation of platelets.97 These results affirm the importance of packing geometry, along with head group chirality, for the formation of helical structures. 

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.

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

Figure 5.38 (a) Negative-stained transmission electron micrograph of nanotubules formed from equimolar mixture of DCg PC and DNPC (2 mM total lipid concentration) stored at 4°C just prior to deposition, (b) Freeze-fracture electron micrograph of twisted ribbons at 27°C. Bars = 100 nm. Reprinted with permission from Ref. 153. Copyright 2001 by the American Chemical Society. 

Furthermore, Oda et al. pointed out that there are two topologically distinct types of chiral bilayers, as shown in Figure 5.46.165 Helical ribbons (helix A) have cylindrical curvature with an inner face and an outer face and are the precursors of tubules. These are, for example, the same structures that are observed in the diacetylenic lipid systems discussed in Section 4.1. By contrast, twisted ribbons (helix B) have Gaussian saddlelike curvature, with two equally curved faces and a C2 symmetry axis. They are similar to the aldonamide and peptide ribbons discussed in Sections 2 and 3, respectively. The twisted ribbons in the tartrate-gemini surfactant system were found to be stable in water for alkyl chains with 14-16 carbons. Only micelles form... 

Figure 5.45 Cationic gemini amphiphiles having chiral counterions. TEM images of representative twisted ribbons formed by 16-2-16 tartrate (49 + 50) at 0.1% in water for various values of enantiomeric excess (a) 0 (racemate) (b) 0.5 (c) 1 (pure L) (d) 1 (pure L) in presence of 1 eq sodium L-tartrate. Bar = 100 nm. Reprinted with permission from Ref. 165. Copyright 1999 by Macmillan Magazines.

Figure 5.46 Schematic representation of helical and twisted ribbons as discussed in Ref. 165. Top Platelet or flat ribbon. Helical ribbons (helix A), precursors of tubules, feature inner and outer faces. Twisted ribbons (helix B), formed by some gemini surfactant tartrate complexes, have equally curved faces and C2 symmetry axis. Bottom Consequences of cylindrical and saddlelike curvatures in multilayered structures. In stack of cylindrical sheets, contact area from one layer to next varies. This is not the case for saddlelike curvature, which is thus favored when the layers are coordinated. Reprinted with permission from Ref. 165. Copyright 1999 by Macmillan Magazines.

In this chapter, we have surveyed a wide range of chiral molecules that self-assemble into helical structures. The molecules include aldonamides, cere-brosides, amino acid amphiphiles, peptides, phospholipids, gemini surfactants, and biological and synthetic biles. In all of these systems, researchers observe helical ribbons and tubules, often with helical markings. In certain cases, researchers also observe twisted ribbons, which are variations on helical ribbons with Gaussian rather than cylindrical curvature. These structures have a large-scale helicity which manifests the chirality of the constituent molecules. 

Fig. 2. Electron micrographs highlighting the polymorphism of amyloid fibrils. (A) A single human calcitonin protofibril with a diameter of 4 nm (adapted from Bauer et al., 1995). (B) Different morphologies present in a transthyretin fibril preparation. Black arrowheads show oligomers of different sizes, the black arrow points to a 9- to 10-nm-wide fibril, and the white arrowhead marks an 4-nm-wide fibril (adapted from Cardoso et al., 2002). (C-F) Human amylin fibril ribbons (adapted from Goldsbury et al., 1997). (C) A single 5-nm-wide protofibril. (D-F) Ribbons containing two (D), three (E), or five (F) 5-nm-wide protofibrils. (G) A twisted ribbon made of four 5-nm-wide protofibril subunits of Api-40 (adapted from Goldsbury et al., 2000b). Scale bar, 50 nm (A-G).

For A/i it is worth noting that while single 5-nm protofibrils were rarely imaged by EM or SFM, the thinnest single fibrils had a diameter around 8-9 nm and were termed protofibrils by many researchers in the field (Goldsbury et al., 2005 Harper et al., 1997, 1999 Lambert et al, 1998 Nielsen et al., 1999 Walsh et al., 1997, 1999). Flat and twisted ribbons formed from 5-nm-wide subunits, as seen with human amylin, were also depicted for AjSi 4o (Fig- 2G Goldsbury et al., 2000b, 2005). 

A/1 it was possible to follow the growth of twisted ribbons, with a periodic twist of 80-130 nm, by depositing seeds on mica prior to the injection of a fresh peptide solution (Fig. 4C Goldsbury et al., 2005). In the case of human amylin, it was even possible to observe by time-lapse SFM how fibrils are formed from an oligomeric nucleus by initial growth in height from... 

Fig. 26a,b. High resolution height SFM-micrographs of a 14-ABG-PS on mica [86] b Twisted ribbon structure of polypeptide /i-sheets [ 167]. The plectoneme conformation is caused by the backward folding of the torsionally stressed molecules [86]. Insert in (a) depicts a plectoneme supercoil... 

Figure 3. Model of (p a)s TIM barrel from triose phosphate isomerase. Numbered arrow and twisted ribbon structures are beta sheets and alpha helicies, respectively. (Adapted and reproduced from Ref. 66 with permission. Cop3night 1984 American Association for the Advancement of Science.)...

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