Self-assembly helical structure

Inorganic double or triple helices are formed by two or three ligand strands wrapped around linearly disposed metal ions [13], Among cyclic transition metal complexes, circular helicates [nJ cH [ ] "cH is a general notation characterizing circular helicates (cH) with n = number of metal ions and m = helicity (m = 2 for a double helix) have specific features and may be considered as toroidal helices [34]. There are two different kinds of circular helical systems. Some structures self-assemble from the metal ions and the ligands only in the presence of an anion, which could act as a template [34,35,64-67], whereas, in other cases, the circular helicates self-assemble from the metal ions and the ligands alone [68-70]. 

The alicyclic structure must have a small amount of twist. This may be built into a rigid alicyclic skeleton, or it may be achieved by means of conformational flexibility in the ring system. It aids the generation of helicity during self-assembly. 

Percec V, Dulcey AE, Peterca M, Hies M, Ladislaw J, Rosen BM, Edlund U, Heiney PA (2005) The internal structure of helical pores self-assembled from dendritic dipeptides is stereochemically programmed and aUosteiicaUy regulated. Angew Chem bit Ed 44 (40) 6516-6521. doi 10.1002/anie.200501331... 

Fig. 25 Proposed structure for the binary self-assembly of a 28/29 (T A 1 1) and b 28/29 (T A = 2 1). Possible elongation mechanism for the helical stacl self-assembled from c 28/29 (T A = 1 1) and d 28/29 (T A = 2 1). Reprinted with permission from [74]. (2006) American Chemical Society...

Spontaneous self assembly of a dinuclear triple helical complex is observed with linked bis-[4,5]-pineno-2,2 -bipyridines. Studies by electrospray mass spectrometry, CD and NMR determined that the major species in solution was a complex of Zn L = 2 3 stoichiometry with a triple helical structure and an enantiomerically pure homochiral configuration at the metal centers. The preference for the formation of one of the possible stereoisomers over the other is of interest.265 Another binuclear triple helical complex is formed from zinc addition to bis[5-(l-methyl-2-(6-methyl-2 -pyridyl)benzimidazolyl)]methane. Spectrophotometric titrations with a zinc solution... 

Two-dimensional planar interpenetrating networks have been formed using the spacer ligand 2,2 -bis-l,6-naphthyridine with a zinc salt.274 Helicate structures have been synthesized which rely heavily on non-covalent interactions in the metal-assisted self-assembly process in solution.275... 

Completely different mechanisms are involved in the self-assembly of the tobacco mosaic virus (TMV). This virus consists of single-strand RNA, which is surrounded by 2,130 identical protein units, each of which consists of 158 amino acid residues. A virus particle, which requires the tobacco plant as a host, has a rodlike structure with helical symmetry ( Stanley needles ). It is 300 nm long, with a diameter of 18nm. The protein and RNA fractions can be separated, and the viral... 

The chiral l,3,5-triazepane-2,6-dione 149 and its ring fused analogue 150 have been shown to form H-bonded helical molecular tapes with P chirality on self assembly in the solid state. With 149, this self assembly proceeds through aromatic-aromatic ring interactions resulting in hollow tubular structures <06CC4069>. 

AG(E)2468>. The application of distinct self-assembly processes to form grid-type and double-helical structures has potential in understanding and controlling molecular information an interesting discussion has been presented <00CEJ2103>. 

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 experiments discussed in this chapter have shown that a variety of chiral molecules self-assemble into cylindrical tubules and helical ribbons. These are indeed surprising structures because of their high curvature. One would normally expect the lowest energy state of a bilayer membrane to be flat or to have the minimum curvature needed to close off the edges of the membrane. By contrast, these structures have a high curvature, with a characteristic radius that depends on the material but is always fairly small compared with vesicles or other membrane structures. Thus, the key issue in understanding the formation of tubules and helical ribbons is how to explain the morphology with a characteristic radius. 

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. 

Figure 6.10 Chiral hexakis-porphyrinato benzene compound 66, which possesses propellerlike structure and can self-assemble to form helical arrays.

There are two levels of self-assembly in the formation of tetra-, penta-and hexa-nuclear products from the poly-bipyridyls (L) 20 and 21 and iron(II) salts FeCl2, FeBr2 or FeS04 - the products are anion-dependent. The coordination of three bpy units, from different ligand molecules, to the Fe2+ centers produces a helical structure interaction of these helical strands with anions results in further molecular organization to form the final toroidal product. The discussion draws parallels between the helical and toroidal structures here and secondary and tertiary structure in biological systems (482). Thermodynamic and kinetic intermediates have been characterized in the self-assembly of a di-iron triple stranded helicate with bis(2,2/-bipyridyl) ligands (483). 

De Mendoza reported the first example of anion-directed helix formation in 1996 [91]. The assembly of this helical structure relies, not only on electrostatic interactions between the anionic template and the positively charged strands, but also on hydrogen bonding. The tetraguanidinium strand 69 (see Scheme 34) self-assembles around a sulfate anion via hydrogen bonding to produce a double helical structure. The formation of this assembly and its anion-dependence was proposed on the basis of NMR and CD spectroscopic studies. 

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