Simultaneous binding, cations anions

Ion bridging is a specific type of Coulombic interaction involving the simultaneous binding of polyvalent cations (e.g., Ca, Fe, Cu ) to two different anionic functional groups on biopolymer molecules. This type of ionic interaction is commonly involved in associative self-assembly of biopolymers. As a consequence it is also an important contributory factor in the flocculation (via bridging or depletion) of colloidal particles or emulsion droplets in aqueous media containing adsorbed or non-adsorbed biopolymers (Dickinson and McClements, 1995). 

Appropriately strapped calixarenes have also been shown to complex anions and cations simultaneously. Such a system is the ditopic, calixarene-based receptor 11 (Scheme 9.4) reported by Tumcharern and coworkers [51[. Receptor 11, in which the strap incorporates two different binding motifs (urea and amide), showed selectivity for the tetrahedral phenylphosphinate anion (PPhH02) over simple Y-shaped anions, such as acetate. After initial screening studies, quantitative analyses were carried out using NMR spectroscopic methods. Specifically, titrations were carried out in CD3CN using the TBA salts of three promising anions, namely acetate, phenylphosphinate, and diphenylphosphate. Standard analyses of the... 

Dipolar electrostatic interactions have also been manipulated for the purposes of anion binding. Macrocyclic receptor 10 was shown to be capable of binding halide anions through interactions with the positive ends of the S = O and P = O dipoles (54). Evidence was also provided for the simultaneous binding of primary alkyl ammonium cations (to the oxygen atoms) and halide anions (to the dipoles). This topic of simultaneous cation and anion recognition is of considerable current interest. Further examples will be encountered during this chapter. 

The ditopic receptor 119 has been shown to bind both inorganic anions and alkali metal cations simultaneously (233). Selectivity was observed for HS04-over CP and the binding of alkali metal cations in the crown ethers was proven to substantially enhance the magnitude of anion binding (through electrostatic interaction). 

Another interesting development is the use of bicyclic guanidinium receptors as heteroditopic systems for the enantioselective recognition of amino acids. In this case the guanidinium fragment was combined via a flexible linker with a crown ether moiety in order to promote the simultaneous binding of the carboxylate anion and the ammonium cation belonging to a zwitterionic amino acid [45]. 

Multi-crown dendrimers (Fig. 3.7-12) were found to exhibit good solubihty in solvents of low polarity, making them attractive as extractants for separation processes. The combination of protonated tertiary amine groups as anion binding sites together with cation-active crown ether moieties represents an approach for the simultaneous binding of cations and anions. Extraction studies performed with sodium pertechnetate and mercury(II) chloride have shown that the guest molecules are mainly bound in the interior of the protonated polyamine skeleton [18]. 

Pyrrole- and Polypyrrole-Based Anion Receptors, p. 1176 Simultaneous Binding of Cations and Anions, p. 1291... 

Inelastic Neutron Scattering, p. Ill Isostructurality of Inclusion Compounds, p. 767 The Lock and Key Principle, p. 809 Macrocycle Synthesis, p. 830 Molecular Wires, p. 925 Self-Assembling Capsules, p. 1231 Simultaneous Binding of Cations and Anions, p. 129i Simultaneous Binding of Cations and Neutral Molecules, p. 1295... 

A variation of the receptor polarization effect is the binding strategy shown by schematic Complex 9. In this case the binding of an ion to one face of an appropriately designed receptor polarizes (and perhaps preorganizes) the receptor so that it can bind the counterion to the opposite face with increased affinity. Two examples that use this strategy are the macrocyclic phosphine oxide 10, which can simultaneously bind a monoalkylammonium cation and Cf J " and the cyclopeptide 11, which simultaneously binds an alkyltrimethylammonium cation and tosylate anion " (Chart 2). 

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