Coordination helical structure

Lithium perchlorate is dissolved in PEO owing to the coordination of Li+ cation by oxygen atoms in the polymeric chain. The complex thus formed has the helical structure (Fig. 2.18), and exhibits ionic conductivities of up to 10 4 S/cm at 60°C. 

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

Pauling, L. and Corey, R. B. (1951), Atomic coordinates and structure factors for two helical configurations of polypeptide chains , Proceedings of the National Academy of Sciences (USA),... 

ABF was probed through the reversible unfolding of a short peptide, deca-L-alanine, in vacuo [52] (see Fig. 4.10). The reaction coordinate, , is the distance separating the first and the last Ca carbon atom of the peptide chain. was varied between 12 and 32 A, thereby allowing the peptide to sample the full range of conformations between the native a-helical structure and the extended structures. The force acting along was accrued in bins 0.1 A wide. 

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). 

The structural characterization of this assembly has revealed that chloride coordination (via hydrogen bonding to the protonated pyridyl groups of the strands) induce the strands to adopt a double-helical structure in the solid state. 

The Ag cations are coordinated to two sulfur atoms of different cavitands with Ag-S distances in the range 2.47-2.50 A. In the solid, efficient r-stack-ing of the P-phenyl groups with the picrate anions stabilizes the supramolecular complex (Fig. 10). The two cavitands are aligned along their common C4 axis and offset by about 45°, leading to a helical structure. The inner space is reduced by the occupancy of the sulfur atoms, and there is probably not enough room to accommodate small guests inside the cavity. 

Polymers invariably form helical structures, and the helix symmetry is denoted by u, indicating that there are u repeat units in V turns of the helix. The helix pitch is denoted by P and the molecular repeat distance is c = vP. X-ray diffraction patterns from non-crystalline specimens contain diffracted intensities restricted to layer lines that are spaced by 1/c. On a diffraction pattern from a polycrystalline specimen, diffraction signals, or Bragg reflections, occur only at discrete positions on the layer lines, the positions being related to the lateral dimensions of the unit cell of the crystal. The meridian (vertical axis) of the diffraction pattern is devoid of diffracted intensity unless the layer line number J, is a multiple of u, so that u can be determined straightforwardly. The diffracted intensities can be calculated using standard expressions (2), for model structures (i.e. given the atomic coordinates). 

The large molecule consists of a single peptide chain 35% P-sheet and 20% helical structure are found in the folded stmcture. The active site is a 1.2 nm deep conical cavity in the central pleated sheet, with a ion located at its bottom. Three histidine residues hold the Zn +, which also binds an HjO molecule. The active-site cavity is divided into hydrophilic and hydrophobic halves. The inhibitors of the enzyme replace the water on the Zn + ion and also block the fifth coordination site where COj should bind. 

The beautiful circular tetra- 454 [24], penta- 455 [25] and hexa-nuclear 456 [24] helical structures are built of the ligands each of which coordinates with three metal ions. Interestingly, the self-assembly of trisbipyridine ligand 457 to afford the penta- or hexanuclear complex is governed by the counterion. Namely,... 

Hatano et aL U7) studied the poly(L-lysine)(PLL)- and poly(DL-lysine)-Cu(II) complexes as catalysts in the oxidation of 3,4-dihydroxyphenylalanine. The catalytic activity of PLL-Cu was found to be greater than that of ethylenediamine-Cu. The oxidation was asymmetrically catalyzed by PLL-Cu the D-isomer of the substrate coordinated to PLL-Cu more strongly and its oxidation rate was greater than that of the L-enantiomer. The asymmetrical oxidation was ascribed to the helical structure of PLL-Cu in an aqueous solution of pH 10.5118. ... 

Figure 5-3 The double-helical structure of DNA. The structure shown is that of the B form and is based on coordinates of Amott and Hukins.10 The major and minor grooves, discussed on p. 213, are marked.

The structure of Cd2As3I involves As atoms in a helical structure, while the Cd atoms are five-coordinate to three As and two I atoms, with r(Cd—As) at 2.62-2.74 A and r(Cd—I) at 2.98-3.43 A.1092 A binuclear complex of Zn11 and Cu with histidine has also been reported.1093... 

This differential selectivity results from the differing numbers of donor atoms offered by the two dialdehydes upon which these structures are based. Phenanth-roline dicarbaldehyde readily lends itself to the construction of a set of homo-ligands bearing a number of donor atoms divisible by 4, matching the coordination preference of copper(I), as seen in the dicopper and tricopper helicate structures discussed earlier. 

A helical structure can be stabilized by introducing ligand units to the amino acid side-chains. On addition of appropriate metal ions the a-helix (Figure 1.3.3A) is formed by use of the metal as a cross-linking unit [5]. Attachment of metal binding sites to the end of well-chosen decapentapeptides and coordination of the random coil peptide to appropriate metal ions leads to induction of an a-helix... 

In structures like A-C rather large peptide domains are conformationally fixed by coordination to metals. Smaller domains can also be obtained, however (Figure 1.3.4) [11]. Extensive NMR studies of the metallomacrocycle D show that an a-helical structure is adopted by the Ac-His-Ala-Ala-Ala-His-NH2 pentapeptide on binding to the (en)Pd-fragment (en = ethylene diamine). The peptidic part of D represents one single turn of the helix [12]. 

There are many examples of compounds, usually short polymers, that spontaneously form double helical structures. A classic example involves a number of bipyridyl ligands joined by short, flexible spacers. In the presence of Cu(I), which has both an affinity for nitrogen-containing ligands and a preference for a tetrahedral disposition of donor atoms, discrete double helical complexes are formed [23], A similar phenomenon gives rise to a triple helical motif when a metal that preferentially adopts an octahedral coordination geometry is used [24],... 

This approach mimics familiar biological self-assembly phenomena such as protein folding [ 192], protein aggregation [ 192] and nucleotide pairing [ 188]. It incorporates features described in each of the above strategies (i.e., I—III), to give specialized nanoscopic structures, that can be precisely designed, usually with excellent control over CMDPs. Recent examples include so called structure directed synthesis by Stoddart [3a] (see Chapter 1 of this book) to produce toroidal bis-bipyridinium cyclophanes that are reminiscent of a molecular abacus , melamine-cyanuric acid lattices by Whitesides [193] and unique helical structures based on coordination of bipyridyl units to copper (II) ions by Lehn [194],... 

A more recent example of dynamic coordination chemistry controlled by anion recognition was demonstrated with the series of ligands 2, which formed linear binuclear triple helicates with Co2+ anions (Fig. 4) [16,17]. The asymmetric ligand 2a, functionalized at one end with an amide hydrogen-bond donor group, may form two types of helicate structures head-to-head-to-head (HHH) and head-to-head-to-tail (HHT) isomers. With the weak hydrogen-bond acceptor C104 as counteranion, a 3 1 mixture of HHH HHT isomers was observed. However, with the... 

The linking of j8-diketonates by bridges allows the formation of face-to-face complexes (11-XXVII) similar to those of face-to-face porphyrins (Section 9-12) small molecules may occupy the central hole. With trivalent ions such as Ti3+, V3+, Mn3+, or Fe3+, triple-helical structures consisting of two 6-coordinate M111 ions chelated by three bis(dike) ligands can be obtained.113... 

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