Polyethers, complexes with

Recently, the presence of two different ions in a quadruplex channel has been reported. Four Ca2+ ions and three Na+ ions were detected, 3-4 A apart, in a four-stranded DNA G-quad-ruplex ion channel assembled from d(TG4T). Both the Ca2+ and the Na+ were found to be coordinated by six guanine-oxygens (608). It is interesting to compare these quadruplexes with species such as the stacked multimacrocyclic polyether complexes with spacers arranged to keep the mean planes roughly parallel and sufficiently far apart for each to bind a cation (609). 

In the presence of dilute surfactant concentrations above their cmc, the hydrophobically modified polyethers complex with the surfactants by intercalating their hydrophobic endgroups into the surfactant micelles (Fig. 32). Some of the remaining hydrophobes... 

STABILITY CONSTANTS OF CYCLIC POLYETHER COMPLEXES WITH UNIVERLENT CATIONS... 

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Polyethers. Antibiotics within this family contain a number of cycHc ether and ketal units and have a carboxyHc acid group. They form complexes with mono- and divalent cations that ate soluble ia aoapolar organic solvents. They iateract with bacterial cell membranes and allow cations to pass through the membranes causiag cell death. Because of this property they have been classified as ionophores. Monensia, lasalocid, and maduramicia are examples of polyethers that are used commercially as anticoccidial agents ia poultry and as growth promotants ia mmiaants. 

In 1975, Weber and Vogtle showed that open-chained polyethers, 2,6-pyridine-dimethanol, and ortho-xylene derivatives terminated in an 8-quinolinyl group could form stable, 1 1, crystalline complexes with a variety of metal salts. The podands were prepared from 8-hydroxyquinoline and the corresponding dihalides. A typical example is shown in Eq. (7.10). 

The strength of this bonding depends on the kind of ether. Simple ethers fonn relatively weak complexes with metal ions, but Charles J. Pedersen of Du Pont discovered that certain polyethers fonn much more stable complexes with metal ions than do simple ethers. 

Crown ether (Section 16.4) A cyclic polyether that, via ion-dipole attractive forces, forms stable complexes with metal ions. Such complexes, along with their accompanying anion, are soluble in nonpolar solvents. 

Complexes with chelating organic reagents such as salicylaldehyde and -diketonales were first prepared by N. V. Sidgwick and his students in 1925, and many more have since been characterized, Stability, as measured by equilibrium formation conslanis, is rather low and almost invariably decreases in the sequence Lj > Na > K. This situation changed drainalically in 1967 w hen C. J. Pedersen announced the synthesis of several macrocyclie polyethers which were shown to form stable complexes with... 

Cholanic acid also possesses the ability of transporting cations across a lipophilic membrane but the selectivity is not observed because it contains no recognition sites for specific cations. In the basic region, monensin forms a lipophilic complex with Na+, which is the counter ion of the carboxylate, by taking a pseudo-cyclic structure based on the effective coordination of the polyether moiety. The lipophilic complex taken up in the liquid membrane is transferred to the active region by diffusion. In the acidic region, the sodium cation is released by the neutralization reaction. The cycle is completed by the reverse transport of the free carboxylic ionophore. 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

Kuokkanen evaluated a series of constants. Kpeg for substituted diazonium ions with PEG 1000 and found a reaction constant (p = 1.12) comparable to those for complexation with the three crown ethers investigated by Nakazumi et al. (1983), p = 1.18-1.38). It is therefore likely that the host-guest interaction of diazonium ions with acyclic polyethers is basically similar to that with crown ethers. A dual substituent parameter analysis (DSP, see Sec. 8.3) for (Nakazumi et al., 1987)... 

Silver(I) complexes with mixed donor S, 0-ligands have been described with 1-hydroxypyridine-2-thione (261),1437,1438 cyclo-L-methionylglycine,1439 acyclic polyethers (262),1440-1442 dithio- and trithio-oxalate,1443 or chlorosulfate.1444... 

The formation of relatively ill-defined catalysts for epoxide/C02 copolymerization catalysts, arising from the treatment of ZnO with acid anhydrides or monoesters of dicarboxylic acids, has been described in a patent disclosure.968 Employing the perfluoroalkyl ester acid (342) renders the catalyst soluble in supercritical C02.969 At 110°C and 2,000 psi this catalyst mixture performs similarly to the zinc bisphenolates, producing a 96 4 ratio of polycarbonate polyether linkages, with a turnover of 440 g polymer/g [Zn] and a broad polydispersity (Mw/Mn>4). Related aluminum complexes have also been studied and (343) was found to be particularly active. However, selectivity is poor, with a ratio of 1 3.6 polycarbonate polyether.970... 

The macrocycle types discussed so far tend to form very stable complexes with transition metal ions and, as mentioned previously, have properties which often resemble those of the naturally occurring porphyrins and corrins. The complexation behaviour of these macrocycles contrasts in a number of ways with that of the second major category of cyclic ligands - the crown polyethers. 

This chapter has largely been concerned with two- and three-dimensional polyether ligand systems and their complexes with a range of metal ions. A variety of other related ligand systems are also known but in many instances these have been primarily used to form complexes with non-metallic guests. 

Crown polyethers. Macrocyclic effects involving complexes of crown polyethers have been well-recognized. As for the all-sulfur donor systems, the study of the macrocyclic effect tends to be more straightforward for complexes of cyclic polyethers especially when simple alkali and alkaline earth cations are involved (Haymore, Lamb, Izatt Christensen, 1982). The advantages include (i) the cyclic polyethers are weak, uncharged bases and metal complexation is not pH dependent (ii) these ligands readily form complexes with the alkali and alkaline earth cations... 

Macrocyclic compounds with ion-chelating properties occur naturally and often function as ionophores, translocating ions across biological membranes many of these compounds are small cyclic polypeptides. Some natural carboxylic polyethers are selective for Li+ and are, therefore, ionophores for Li+. Monensin, shown in Figure Id, is a natural ionophore for Na+ but it will also complex with Li+ and it has been shown to mediate the transport of Li+ across phospholipid bilayers [21]. It has been proposed that synthetic Li+-specific ionophores have a potential role as adjuvants in lithium therapy, the aim being to reduce the amount of... 

The macrocydic polyether dibenzo[24]crown-8 (DB24C8, Scheme 3) binds124,251 dialkylammonium-containing guests in solution, as well as in the solid state, to generate complexes with pseudorotaxane co-conformations that are stabilized primarily by [0—H-N+] and [O —H-C] hydrogen bonds. The com-plexation and decomplexation kinetics of these recognition events are influenced by the size of the... 

Potassium acetate complex with dicyclohexyl-18-crown-6-polyether, 52, 71 Potassium amide, 52, 75 Potassium azide, 50, 10 Potassium tert-butoxide, 52,... 

Similar complexes with non-cyclic polyethers that could not be obtained by Pedersen were recently described by Rasshofer and Vogtle (1978) and Suh and Saenger (1978). 

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