Triple-helical structures

Once again, it is possible to extend these ideas to the formation of complexes containing progressively more metal centres. As an example, consider the ligand 7.57. This contains a total of three didentate 2,2 -bipyridine-like domains. Upon reaction with nickel(n) salts, a trinuclear triple-helical complex, [Ni3(7.57)3]6+ 7.58, is formed, in which each of the six-co-ordinate nickel(n) centres is co-ordinated to a didentate metal-binding domain from each of three ligand threads. 

It should be stressed that the coding for the formation of these topologically complex molecules needs to be carefully controlled in order to obtain the desired structures. To illustrate this, consider ligand 7.59, which contains two didentate metal-binding domains. This might be expected to react with octahedral metal ions to give a triple-helical dinuclear complex. Reaction with iron(n) does indeed give a species of stoichiometry [Fe2(7.59)3]4+ however, the crystal structure reveals that an untwisted complex, 7.60, has been formed. 

7 The Three-Dimensional Template Effect, Supramolecular Chemistry and Molecular Topology 

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Collagen-like (ColQ) tailed forms or asymmetric multimers Characterized by triple helical structure of three collagenic subunits Q, each associated with... 

AH collagen types have a triple helical structure. In... 

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... 

The hope is that a triple helical structure will form around this oligonucleotide-binding site, inhibiting the expression of the downstream gene. 

Three pro-a chains assemble to form a triple helical structure (procollagen), which can now be transferred to the Goigi. Modification of oligosaccharide continues in the Golgi. 

Kishimoto T, Morihara Y, Osanai M, Ogata S-1, Kamitakahara M, Ohtsuki C, Tanihara M. Synthesis of poly(Pro-Hyp-Gly)n by direct polycondensation of (Pro-Hyp-Gly)n, where n = 1, 5, and 10, and stability of the triple-helical structure. Biopolymers 2005 79 163-172. 

Hydroxyproline and hydroxylysine Collagen contains hydroxy proline (hyp) and hydroxylysine (hyl), which are not present in most other proteins. These residues result from the hydroxylation of some of the proline and lysine residues after their incorporation into polypeptide chains (Figure 4.6). The hydroxylation is, thus, an example of posttranslational modification (see p. 440). Hydroxy proline is important in stabilizing the triple-helical structure of colla gen because it maximizes interchain hydrogen bond formation. 

An alternative approach is to synthesize highly specific hydrolysis-resistant DNA analogs that can form triple helical structures with DNA (Chapter 5). 

The remainder of Section 14 deals with triple helices and collagen mimetic structures (Sections 14.2.3.1 and 14.2.4). A description of routes is included which covers the synthesis of triple-helical structures using template (scaffolds) from a Lys-Lys dimer (Section 14.2.4.1), Glu-Glu dimer (Section 14.2.4.2), and appropriate Cys-Cys branches (Section 14.2.4.3). The syntheses were carried out using solid-phase techniques. A scaffold (template) using the Kemp triacid is also presented (Section 14.2.4.4). Lastly, a chemoselective ligation is presented which joins appropriately defined reaction sites (Section 14.2.4.5).[23]... 

Inouye, K., Sakakibara, S., and Prockop, D.J. (1976). Effects of the stereo-configuration of the hydroxyl group in 4-hydroxyproline on the triple-helical structures formed by homogenous peptides resembling collagen. Biochim. Biophys. Acta 420, 133-141. 

Kramer, R. Z., Bella, J., Mayville, P., Brodsky, B., and Berman, H. M. (1999). Sequence dependent conformational variations of collagen triple-helical structure. Nat. Struct- Biol. 6, 454—457. 

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