Stoichiometric complexation

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... 

The method may also be applied to the analysis of silver halides by dissolution in excess of cyanide solution and back-titration with standard silver nitrate. It can also be utilised indirectly for the determination of several metals, notably nickel, cobalt, and zinc, which form stable stoichiometric complexes with cyanide ion. Thus if a Ni(II) salt in ammoniacal solution is heated with excess of cyanide ion, the [Ni(CN)4]2 ion is formed quantitatively since it is more stable than the [Ag(CN)2] ion, the excess of cyanide may be determined by the Liebig-Deniges method. The metal ion determinations are, however, more conveniently made by titration with EDTA see the following sections. 

The aromatic spacer group of the model receptors prevent the formation of intramolecular hydrogen bonds between the opposing carboxyls yet these functions are ideally positioned for intermolecular hydrogen bonds of the sort indicated in 32. The acridine derivatives do indeed form stoichiometric complexes with oxalic, malonic (and C-substituted malonic acids) as well as maleic and phthalic acids, Fumaric, succinic or glutaric acids did not form such complexes. Though protonation appears to be a necessary element in the recognition of these diacids, the receptor has more to... 

Not surprisingly, the diacid 13 and its diamide are waterlogged with 2-4 molecules of HzO from which they are difficult to liberate. Binding experiments in CHC13, a non-competing solvent, revealed that stoichiometric complexes, e.g. 48 were formed with diketopiperazines 40) (Kh 104) and amides such as malonamide. With structures of inadequate hydrogen bonding capacity, such as sarcosine anhydride, com-plexation does not occur. 

One of the classic examples in this series is the solubilization of p-amino-benzoic acid (PABA) by caffeine [51], for which the essential data are summarized in Fig. 9. The solubility of PABA in the absence of caffeine was reported to be 6.2 mg/ml, which could be increased to 7.7 mg/ml by the addition of at least 2 mg/ml of caffeine. The linear increase in PABA concentration as a function of caffeine concentration is consistent with the formation of a 1 1 stoichiometric complex. From the data obtained in the linear concentration region, a value of 48 L/mol was calculated for K. Further increases in the caffeine concentration up to 6 mg/ml had no effect on the PABA solubility. However, larger concentrations of caffeine led to a reduction in the dissolved... 

Antithrombin, already mentioned in the context of heparin, is the most abundantly occurring natural inhibitor of coagulation. It is a single-chain 432 amino acid glycoprotein displaying four oligosaccharide side chains and an approximate molecular mass of 58 kDa. It is present in plasma at concentrations of 150 pig ml 1 and is a potent inhibitor of thrombin (factor Ha), as well as of factors IXa and Xa. It inhibits thrombin by binding directly to it in a 1 1 stoichiometric complex. 

Although staphylokinase shows no significant homology with streptokinase, it induces a thrombolytic effect by a somewhat similar mechanism it also forms a 1 1 stoichiometric complex with plasminogen. The proposed mechanism by which staphylokinase induces plasminogen activation... 

A complete NMR approach has been employed to evaluate the complexation process of catechin A with p-CD and synthetic analogues.125 The analysis of the variation of the proton chemical shifts indicated the formation of a 1 1 stoichiometric complex. 2D-ROESY provided detailed spatial information of the complex while the binding constants were obtained by using diffusion-order spectroscopy (DOSY) techniques. 

Recently a porphyrin unit has been incorporated into an anion receptor (Fig. 46) (Beer et al., 1995g). H nmr titration experiments with this compound demonstrated the formation of 1 1 stoichiometric complexes with tetrabutylammonium halides, nitrate, hydrogensulfate and dihydrogenphosphate. 

In the case of weak interactions, which do not always lead to a stoichiometric complex, it is also possible to study the effect of the aromatic substances on the acceptor components, e.g. the solvent itself. 

Copper(II)-aminoglycoside complexes are often isolated as monomeric species over a wide pH range. The coordination complex is usually formed in 1 1 metal ligand stoichiometry, and 1 2 metal ligand stoichiometric complexes are rare. ... 

The complexation of sodium, potassium, and related cations by neutral multidentate molecules is, however, an uncommon phenomenon (30, 40), and stable stoichiometric complexes have been observed only in the last decade, and then only with biological material (3, 10—14, 30). This is one of the reasons why Pedersen (41) and Lehn (42, 43) recently studied macrocyclic polyethers (41) and macroheterobicyclic compounds (42, 43). Forming stable complexes in solution as well as in crystalline form with the salts of alkali and other metals, these ligands have aroused considerable interest (40, 44). 

Kabanov AV, Bronich TK, Kabanov VA, Yu K, Eisenberg A. Soluble stoichiometric complexes from poly(A-ethyl-4-vinylpyridinium) cations and poly(ethylene ox de)-block-polymethacrylate anions. Macromolecules 1996 29 8999. 

Phosphorus itself is not directly accessible to atomic absorption spectrometry, as its most characteristic atomic lines appear in the vicinity of 200 nm. Indirect methods are necessary if the technique is to be applied, and the procedures often require previous conversion of the phosphorus into phosphate. One possible approach is the formation of a well defined stoichiometric complex between phosphate and another ion, which may then be isolated and determined by atomic absorption spectrometry. 

Heat Capacities. The heat capacities (Cp in cal./mole-°C., not shown here) exhibit some unusual features, in that they show marked curvature changes at the characteristic mole ratios found thus far for each of the respective amides or pyrrolidones, again rendering it likely that definite stoichiometric complexes occur in the amide-water mixtures. 

While for the complexation with poly(sodium styrene sulfonate) or sodium cellulosesulfate 1 1 stoichiometry has been reported [150] a non-stoichiometric complex results with sodium carboxymethylcellulose [150]. Optimized conditions make it possible to create membranes with various properties using the PDADMAC/sodium cellulosesulfate system [166-168]. However, the symplex formation with PDADMAC or copolymers mostly results in flocculated precipitates [27,150,169]. Highly ordered mulilayer assemblies were prepared by alternate reaction of PDADMAC and various polyanions [170,171]. Recently, the efficiency and selectictivity of protein separation via PEL coacervation were examined using PDADMAC [172]. 

Kabanov, A.V., Bronich, T.K., Kabanov, V.A., Yu, K. and Eisenberg, A. (1996) Soluble stoichiometric complexes from poly(N-ethyl-4-vinylpyridinium) cations and polyethylene oxide)-Wock-polymethacrylate anions. Macromolecules, 29, 6797-6802. 

Stoichiometric complexes of the zinc(II)-cyclene monomer with tetraacetylriboflavin and creatinine were prepared and co-polymerised with ethylene glycol dimethyl acrylate. After exhaustive extraction of the material which removed impurities and the templates, a functional polymer was formed. Control polymers without template and with cobalt(II) instead of zinc(II) were also prepared. 

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