Helicates cyclic

Figure 10.27 The structure of [Cd12(10.20)18](BF4)24 showing (a) polyhedral metal cage and the four encapsulated BF4 anions (b) view down one of the Cd6 pseudohexagonai faces, emphasising the cyclic helical array of ligands and the presence of an anion in the centre of the face (reproduced with permission from [22] 2006 American Chemical Society).

Figure 10.79 X-ray molecular structures of cyclic helicates (a) pentanuclear species formed from 10.124 and templated by Cl 87 (b) hexanuclear species formed from 10.126 and templated by C104 (reproduced by permission of The Royal Society of Chemistry).

Argent, S. P., Adams, H., Riis-Johannessen, T., etal, Coordination chemistry of tetradentate N-donor ligands containing two pyrazolyl-pyridine units separated by a 1,8-naphthyl spacer Dodecanuclear and tetranuclear coordination cages and cyclic helicates. Inorg. Chem. 2006, 45, 3905-3919. 

Adding metal salts demonstrated a weak but clear effect on t a in the presence and absence of metal ions, which results from folding the chain around the metal ion. A schematic representation indicating the conformation change from a linear to a more cyclic (helical) structure is shown in Figure 6.6. 

An extension of the above study has produced a further example of the expanding cyclic helicate category mentioned previously. Interaction of 92 with silver(I) results in the spontaneous assembly of a circular, six-fold, single-helical species. The X-ray structure of this novel product is shown in Figure 6.34. The six silver ions are related by a crystallographic C -axis, with each being tetrahedrally-co-ordinated by four almost equidistant nitrogen donors located on two different... 

Figure 154. Penta- and hexameric cyclic helicates formed through host—guest templation. The crystal structure of the pentameric complex is shown.

The history of the creation of the first smart polymeric system dates back to the works of Kuhn and Katchalsky (88, 89). They demonstrated that collagen fibers changed dimension reversibly on transition from cyclic helices to random coils when immersed cyclically between salt solution and water. This was referred to as a mechanochemical system capable of transforming chemical energy to mechanical work. 

In cyclic nucleotide-regulated channels, this domain serves as a high-affinity binding site for 3-5 cyclic monophosphates. The CNBD of channels has a significant sequence similarity to the CNBD of most other classes of eukaryotic cyclic nucleotide receptors and to the CNBD of the prokaryotic catabolite activator protein (CAP). The primary sequence of CNBDs consists of approximately 120 amino acid residues forming three a-helices (oA-aC) and eight (3-strands ( 31- 38). 

Fig. 2.3 Model of the 2i- and 3i-helical structures proposed for PHB chains with ideal torsion angle values. The 2i-helix was determined by fiber X-ray diffraction of PHB [49-51] while the 3i-helical fold was constructed by using preferred dihedral angles found along the backbone in crystal structures of cyclic oligomers 9 ( oligolides ) [37, 43, 45]...

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