Helicates quadruple-stranded

Fig. 13 Three representations of the decanuclear, quadruple-stranded helicate 16 formed from the polydentate ligand 15 and silver(I) ions [137], The two sets of ligands 15 are shown in red and blue to illustrate the helical twist...

The spherical Cu(I) cation (d ° electronic configuration) is thus ideally suited for the formation of double-stranded helicates with LI and L4-L6, because of its lack of directional coordination bonds and its limited electrostatic factor z /R = 1.30 eu A (z is the charge of fhe cation and R is its ionic radius Shannon, 1976), which is compatible with CN = i (Figure 5). The demonstration of this novel concept in coordination chemistry, combined with the undeniable aesthetic appeal of helical strucfures, were at the origin of a considerable enthusiasm leading to the isolation and structural characterization of a plethora of binuclear double-, triple-, and quadruple-stranded helicates during the past two decades (for comprehensive reviews, see Albrecht, 2001 Constable, 1992, 1994, 1996 ... 

Firstly, the total number of helical strands n wrapped about the helical axis defined by the metal ions corresponds to single- n = 1), double- n = 2), triple- n = 3), and quadruple-stranded n = 4) helicates. Secondly, ligand strands containing similar successive binding units are termed homotopic,... 

Please note that the quadruple-stranded helicate shovm in Figure IIC is a typical example of a chiral helical object strictly produced by the wrapping of the spacers, while the square-planar Pd-N4 imits are achiral. 

FIGURE 11 Crystal structures of (A) double-stranded (redrawn after Piguet et al., 1989a), (B) triple-stranded (redrawn after Williams et al., 1991), and (C) quadruple-stranded (redrawn after McMorran and Steel, 1998) helicates highlighting the relationship between the helicities about the main axis and the perpendicular twofold axes. 

FIGURE 12 Stereochemical match between (A) bis-tridentate (redrawn after Piguet et al., 1992a) and (B) bis-bidentate segmental ligands with trivalent lanthanides to give triple-stranded, respectively, quadruple-stranded helicates (reproduced by permission from Dong et al., 2007, American Chemical Society). 

Representation of decanuclear quadruple-stranded helicate 5.18 (reproduced with permission from... 

Structure of the quadruple-stranded helicate [PF6CPd2(17)4] " encapsulating a PFg anion (reprinted with permission from reference... 

Figure 7 A dinuclear macrocycle reorganizes into a quadruple-stranded helicate in the presence of hexafluorophosphate anion (represented by a sphere). ...

While some homopolynucleotides such as poly A and poly U can sometimes occur as single-stranded helices, others can form quadruple strands incorporating an appropriate H bonding scheme (10.89b). 

Recently, Pikramenou and coworkers (Bassett et al., 2004) studied rare-earth complexes of the bis(jS-diketone) ligands l,3-bis(3-phertyl-3-oxopropanoyl)benzene and l,3-bis(3-phenyl-3-oxopropanoyl)-5-ethoxy-benzene. These complexes have a helical structure. Both triple-stranded neutral complexes of the type [R2L3], and quadruple-stranded anionic complexes of the type [R2L4] are formed, where L is the deprotonated form of the bis08-diketone) ligands. The solution structure of these complexes was studied in detail by NMR spectroscopy. 

Not all DNAs are double helices duplex DNA) Some types of viral DNA are single stranded and even a few DNA triple and quadruple helices are known... 

Further developments involve the investigation of the mechanism of formation of double- and triple helicates and of the effect of variations in ligand structure on their features, the determination of their physico-chemical (thermodynamic, kinetic, electrochemical, photochemical) properties, the exploration of the coordination chemistry of the ligand strands. For instance, it may be possible to obtain quadruple helical complexes with ions of high coordination number such as the lanthanides and linear ligands containing bipy or terpy units. Using cubic metal ions would also be of interest. 

A rare example of higher stranded helicates is found in the decanuclear quadruple-helicate 16. Lehn and coworkers reported the dynamic equilibrium... 

In (poly-G) , for example, which forms a gel-like structure, there are four bases linked together as indicated ( G Quartet ). This scheme allows four helices to run parallel to a four-fold fibre axis. Monovalent cations may be able to stabilise such arrangements by lying at the centre (10.89b). Another new variety of four-stranded DNA with intercalated bases was reported in 1995 [54]. Quadruple helices can be formed by telomeres (Chapter 11) and interaction with lipids may be important [55-58]. 

Besides hydrogen bonds and salt bridges, metal-ligand coordination is another well-known approach to constrnct well-defined supramolecular entities such as double, triple, and quadruple helical structures such as helicates, from otherwise flexible organic molecular strands. For a presentation of these metal-based architectures, the reader is referred to Self-Assembly of Coordination Chains and Helices, Self-Processes. 

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