Receptors of anions

At the present time, there is a striking contrast between the extensive development of fluorescent probes for cation recognition and the limited number of available probes for anions notwithstanding the great need for the latter. This is due to the difficulty of the design of selective anion receptors progress made in the relevant field of supra-molecular chemistry will certainly lead in the future to new selective fluorescent signaling receptors of anions. 

Enantioselective anion sensing is included in a number of reviews covering much broader subjects, namely receptors of anions in general [2-8] and chro-mophoric sensors of anions [9,10]. There are also more specialized reviews dealing with receptors for anions based on macrocyclic polyamines [11], amides [12],... 

From a supramolecular point of view, anionic species play a key role in many mineral or biological routes. Despite this, however, anion-centered self-assembly processes were little explored. Recently, anion coordination was addressed mainly through the design of e z/q-receptors on the contrary, the electron-poor organic halides work effectively as ew-receptors of anions. The ability described here of the halide anions to act as electron donors to carbon-bound halogens may develop as a general protocol for halide-centered supramolecular chemistry. 

Derivatives in the s- and p-Block Elements Supported by Poly(pyrazolyl)borato Ligation Models for Carbonic Anhydrase, Receptors for Anions, and the Study of Controlled Crystallographic Disorder Gerard Parkin... 

ALKYL, HYDRIDE, AND HYDROXIDE DERIVATIVES OF THE s- AND p-BLOCK ELEMENTS SUPPORTED BY POLY(PYRAZOLYL)BORATO LIGATION MODELS FOR CARBONIC ANHYDRASE, RECEPTORS FOR ANIONS, AND THE STUDY OF CONTROLLED CRYSTALLOGRAPHIC DISORDER... 

As illustrated in the previous sections, the uninegative poly(pyrazo-lyl)borato ligands provide a versatile system for coordination to cationic centers. However, in their triprotonated form, such ligands offer potential for acting as receptors for anions (Scheme 28). Indeed, since the coordination chemistry of anions (199) is significantly less developed than that of cations, protonated poly(pyrazolyl)borato ligands offer potential for exploring the chemistry of such systems. 

Although non-covalent interactions of anions are one of the most actively explored areas of supramolecular chemistry [15], the anion sensing and recognition have up to now relied primarily on electrostatic binding or hydrogen bonding to the receptor [16,54-61]. However, recent UV-Vis and NMR spectral studies clearly reveal that complex formation takes place in the solutions between halides and neutral olefinic and aromatic it-acceptors such as those in Fig. 3 [23,62],... 

Because of their interference with the interaction between the viral envelope gpl20 glycoprotein and the cellular CD4 receptor, poly anionic substances not only inhibit virus adsorption to the cells but also block syncytium (giant cell) formation between the HIV-infected (gpl20 + ) cells and uninfected (CD4 + ) cells. Since syncytium formation results in a selective destruction of the CD4+ cells, this syncytium formation may play an important role in the pathogenesis of AIDS (a hallmark of which is a progressive decline of the CD4+ cells). 

Ryeom, SW, Silverstein, RL, Scotto, A, and Sparrow, JR, 1996a. Binding of anionic phospholipids to retinal pigment epithelium may be mediated by the scavenger receptor CD36. J Biol Chem 271, 20536-20539. 

In contrast to the -conjugated probe architecture utilizing an ICT process, the number of anion probes that rely on the fluorophore-spacer-receptor design and an active PET process is abundant [72], Again, anthracene and naphthalene... 

The methods of anion detection based on fluorescence involve quenching, complex formation, redox reactions and substitution reactions (Fernandez-Gutierrez and Munoz de la Pena, 1985). This chapter will be restricted to anion molecular sensors based on collisional quenching (in general, they exhibit a poor selectivity) and on recognition by an anion receptor linked to a fluorophore (fluoroionophore). 

There a great need for selective anion sensors, but the number of available sensors is rather limited because of difficulties in their design. However, new selective sensors are expected because of the considerable progress made in the synthesis of anion receptors. 

Electrochemical and Optical Sensing of Anion Systems by Transition Metal Based Receptors, Coord. Chem. Rev. 205, 131—55. 

Electrochemical recognition of anionic guest species by redox-active receptor molecules... 

The addition of tetrabutylammonium chloride to H solutions of [68] and [69] in deuteriated acetonitrile resulted in remarkable nmr shifts of the respective proton signals of both receptors. Of particular note were the substantial downfield shifts of the amide protons (AS = 1.28 ppm for [68] and 1.52 ppm for [69]) on addition of one equivalent of chloride. These results suggest that a significant —CO—NH-Cr hydrogen-bonding interaction contributes to the overall anion complexation process. Subsequent nmr titration curves suggesting 1 1 stoichiometry with anion complexes of [68] and [69] were found in all cases. Negligible shifts were observed under identi-... 

In an effort to impart selectivity and enhance complex stability for this class of anion receptor novel ditopic biscobaltocenium receptor molecules (Fig. 42) have been synthesized and their coordination and electrochemical properties have been studied (Beer et al., 1993f, 1995h). The two positively charged metallocene centres linked by various alkyl, aryl and calix[4]arene spacers... 

Obtained in acetonitrile solution containing 0.2 mol dm-3 BuJNBF4 as supporting electrolyte. Solutions were 1 x 10-3 mol dm 3 in receptor and potentials were obtained with reference to an Ag/Ag electrode. Coulometric investigations suggest Et values represent a two-electron reduction process. b Cathodic shift in reduction potential produced by presence of anions (up to 4 equiv) added as their tetrabutylammonium salts. Precipitation of complex observed no CV could be obtained. Obtained in acetonitrile solution. 

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