Helical representative examples

Figure 2.5 Representative examples of two SET domain-containing structures, (a) Human SET7/9 in complex with SAH and a histone peptide cofactor (PDB code lo9s). The N-SET (shown in light blue), SET (p strands shown in yellow and helices in red) and C-SET (shown in magenta) domains in SET7/9 are indicated. The bound SAH molecule is shown in a stick model, together with the lysine side-chain of the bound histone peptide (the rest of the peptide is not shown).

Representative examples of linear, helical, and columnar assemblies are summarized in Fig. The growth... 

Representative examples of 1-D coordination polymer motifs are illustrated in Scheme 5. In these motifs, the helix remains quite rare in the context of coordination polymers, but there is additional interest because it is inherently chiral irrespective of the components. The inherent chirality of this architecture comes from spatial disposition rather than the presence of chiral atoms. [Ni(4,4 -bpy)(benzoate)2(MeOH)2] self-assembles as a helical architecture that is sustained by the linking of octahedral metal connectors with linear ligands (Scheme 5d). The helical chains pack such that they are staggered, but align in a parallel fashion. Therefore, the bulk crystal is polar as every helix in an individual crystal is of the same handedness. Other examples of 1-D coordination polymers are listed in Table j... 

We have seen the importance of the self-assembly of coordination chains with their neighbors to control not only the dimensionality of the resulting network but also its topology, stability, and properties. This concept is even more crucial in multistrand helices, where each strand interweaves with the other. Thus, in many cases, the formation and stabilization of these supramolecular systems are closely related to the presence of hydrogen bonds. A representative example of a simple double-stranded helical system stabilized by hydrogen bonds has been reported by Chen et al. The simplest unit in [Co(L3i)(phen)(H20)2] consists of a helical chain created by the connection of... 

As a representative example, the solid state structure of the corresponding ethylene-bridged complex is shown in Scheme 2.2 and exhibits the special role of one lithium counter ion that stabilizes the helicate in concert with two molecules of water as a template. Later, bisimine-bridged ligands were also used to examine this class of coordination compounds. In this case, the helicates can be formed by simply mixing a diamine, 2,3-dihydroxybenzaldehyde, a source of titanium(IV) ions, and an alkali metal carbonate as a base. This procedure in principle resembles a hierarchical process with an initial imine condensation followed by helicate self-assembly, although the order of events cannot be predicted definitively. 

A series of dinuclear triple-stranded lithium-bridged titanium helicates has been characterized by x-ray diffraction studies. Figure 2.2 shows a representative example of the structure of the anionic dinucelar complex [Li3 (l)6Ti2 ]-. Two... 

It should be noted that complex formation often allows stabilization of conformational and tautomeric forms, which are not characteristic of the parent polymers. A representative example is poly(ethylene glycol) (PEG), which has a helical structure with alternating trans- and gauche-CO, OC, and C-C bonds, respectively (T2G conformation, helix 2). PEG reacts with HgCl2 to form a complex of... 

The catalysts mentioned above are the tools used to build various polymeric architectures via ROMP, ranging from the simplest linear homopol3mier to intricate double helices. This chapter aims to delve into these architectures from the most basic to the most complex. In many cases, only representative examples are discussed and should only be used as a guide as the inclusion of all work from the field into a single chapter is not possible. Ultimately, this chapter aims to give its readers an appreciation and understanding of the stmctural opportunities already obtained through ROMP and a vision for its future. 

FIGURE 11 Ribbon representations of (a) myoglobin and (b) cytochrome B562. These are representative examples of all a-proteins which exhibit the two major ways of packing a-helices in proteins. 

Antiparallel tt-helix proteins are structures heavily dominated by a-helices. The simplest way to pack helices is in an antiparallel manner, and most of the proteins in this class consist of bundles of antiparallel helices. Many of these exhibit a slight (15°) left-handed twist of the helix bundle. Figure 6.29 shows a representative sample of antiparallel a-helix proteins. Many of these are regular, uniform structures, but in a few cases (uteroglobin, for example) one of the helices is tilted away from the bundle. Tobacco mosaic virus protein has small, highly... 

Because of the double helical nature of DNA molecules, their size can be represented in terms of the numbers of nucleotide base pairs they contain. For example, the E. coli chromosome consists of 4.64 X 10 base pairs (abbreviated bp) or 4.64 X 10 kilobase pairs (kbp). DNA is a threadlike molecule. The diameter of the DNA double helix is only 2 nm, but the length of the DNA molecule forming the E. coli chromosome is over 1.6 X 10 nm (1.6 mm). Because the long dimension of an E. coli cell is only 2000 nm (0.002 mm), its chromosome must be highly folded. Because of their long, threadlike nature, DNA molecules are easily sheared into shorter fragments during isolation procedures, and it is difficult to obtain intact chromosomes even from the simple cells of prokaryotes. 

The suggestion of a helical host molecule was originally put forward by Hanes 136) and then developed by Freudenberg and his colleagues 137). Chemical 138 140) and X-ray diffraction studies by Rundle et al.141 143) and by Bear 144,145) demonstrated that these ideas were correct, and revealed that the helical structure had an outer diameter of 13.0 A, an inner diameter of 5 A, and a pitch of 8.0 A with six glucose units per turn. The iodine atoms were arranged in a linear fashion with an average I-I separation of approximately 3.1 A. These early results have been reviewed 146, 147). They represent the first confirmed example of helical structure for a biopolymer. 

A last example of a dopant whose chirality has been investigated by the LC technique is represented by helicenes and related molecules. Once again, compounds very different spectroscopically, such as 46-49, and hence hardly comparable with chiroptical techniques, are very similar in shape and give helical twisting powers of the same sign and of comparable intensity80 the twisting powers of helicenes have been successfully calculated by the shape model of Ferrarini et al.73... 

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