Selectivity for Anions

Macrocyclic polyamines that can be fully or nearly fully protonated in the neutral pH range appear to be the best ligands for the biologically important carboxylate and adenosine phosphate anions because the formation of these anions occurs in these pH regions. Lehn and Dietrich and their coworkers have synthesized macrocyclic polyamines [24]N(,(53) and [32]N8(54), based on propylene units (Dietrich et al., 1981), and macrocycles with mixed nitrogen-oxygen donor atoms connected by ethylene units, [27]Nft03(55) (Dietrich et al., 1981) and [24]N(,02(56) (Hosseini and Lehn, 1987 Hosseini et al., 1983). These macrocycles are similar to those found in natural systems. 

Both types of protonated macrocycles were found to form stable and selective complexes with both inorganic [i.e., SO , Co(CN)6, and Fe(CN)h ] and organic (i.e. carboxylate and nucleotide) polyanions in aqueous solution in the neutral pH range. Since selectivity in these systems depends on electrostatic and geometric effects, modification of macrocyclic cavity shape and size should allow one to control the selectivity sequence (Dietrich et al.. 1981). The interactions of 53 and 54 in their less than fully protonated forms with carboxylate and nucleotide anions have been investigated recently (Hosseini and Lehn, 1988). 

Macrocyclic penta- and hexaamines [15]N5(27) and [18]N,(22), based on ethylene units, are selective in their triprotonated forms at neutral pH for polycarboxylate anions that occur in the catabolic tricarboxylic acid cycle in 

Large polyazacycloalkanes produce highly charged protonated species in the neutral pH range. Complexation of such large polyazacycloalkanes as 

Ditopic macrocyclic polyamines containing 1,3-diamine and longer units, 

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Before the appearance of analytically useful ISEs with liquid membranes, Sollner and Shean [200] obtained a liquid ion-exchange membrane with marked selectivity for anions. The first ISE with a liquid membrane was the calcium electrode described by Ross [179] with Ca " -dialkylphenylphosphate in dioctylphenylphosphonate. 

Let me put it to Professor Lehn that biology can do better than he can with small molecules. From uroporphyrin biology makes (1) copper porphyrins (turacin feathers), (2) heme (iron) porphyrins, (3) magnesium chlorins, (4) cobalt corrins (B12). These compounds are formed with very little confusion between metals and partners. I cannot give selectivity factors, but they are very big. How is this managed Is it not better than man can do Maximum size selectivity for anions is not yet understood as few anion-binding sites in biology are known. 

To accommodate the rapid diffusion that is often needed to supply food, water, and inorganic ions to cells, membranes contain a variety of small pores and channels. The pores may be nonspecific or they may be selective for anions or cations or for some other chemical characteristics. They may be permanently open or sometimes closed and referred to as gated. 

The kinetic data reveal a complex dependence on the anion concentration and the hydrogen-ion concentration and have been interpreted on the basis of ion-pair and ion-triplet formation. The uncatalyzed path ( 0)has been shown to involve (NH3)5Co(OH)Co(NH3)55+ (= M5+)and the ion pair M x Y4 +, and it was proposed that the ion pair M x Y4+ scavenges Y- from solution and not from the second coordination sphere (357). It was shown that the reactive intermediates are quite selective for anions (as well as being selective for the N terminus of NCS-, the ratio for N-bound S-bound being approximately 4), and this has been interpreted as arising from a genuine, coordinately unsaturated intermediate. The acid-catalyzed path has been interpreted in terms of the formation of protonated unaggregated reactant, MH6+, and small concentrations of the protonated ion pairs and ion triplets MH x Y5+ and MH x Y24+ (355, 356). 

A few other types of anion sensors have been mentioned recently in the literature. Tetrathiofulvalene microcrystals immobilised at a platinum electrode displayed electrochemical properties that were affected by the presence of anions in solution, with some selectivity for anions such as bromide [ 145]. A flow-injection analysis system using anion-exchange columns for separation and polyaniline electrodes as detectors could detect dichromate down to 0.004 ppb and could be used for seawater samples [146]. 

Analytical Properties Substrate has 38 chiral centers and 7 aromatic rings surrounding 4 cavities (A, B, C, D), making this the most structurally complex of the macrocyclic glycopeptides substrate has a relative molecular mass of 2066 this phase can be used in normal, reverse, and polar organic phase separations selective for anionic chiral species with polar organic mobile phases, it can be used for a-hydroxy acids, profens, and N-blocked amino acids in normal phase mode, it can be used for imides, hydantoins, and N-blocked amino acids in reverse phase, it can be used for a-hydroxy and halogenated acids, substituted aliphatic acids, profens, N-blocked amino acids, hydantoins, and peptides Reference 47, 48... 

Mobile and stationary phases need to harmonize with each other a certain eluent will not necessarily interact well with a given ion exchanger. Typical eluents for separations with electronic suppression are diluted phthalic or benzoic acid, perhaps with a low amount of acetone or methanol (to influence the selectivity) for anions, and nitric, oxalic, tartaric, citric or dipicolinic acid (the latter one for complex formation)... 

Acid-cataiyzed condensation of 3,4-difluoropyrrole with acetone produces octailuorinated calix[4]pyrrole that shows enhanced affinity and selectivity for anionic species.Compound 12, for example, proved to be a better anion receptor than 11. binding fluoride, chloride. 

Because of the better understanding of the processes taking place at the surface of stationary phases, the number of ion-exchange materials for ion chromatography has increased significantly over recent years. As a result, the multitude of commercially available columns with their different selectivities for anion and cation separations and their determination is almost confusing. This entry is a review of stationary phases used in modem ion chromatography. 

Research of synthetic transporters selective for anions is less developed than that devoted to cation transporters. Nevertheless, in the last few years this area has received much attention. Chloride is the most abundant anion in physiological solutions and hence chloride transporters are the most studied. The transmembrane transport of other physiologically relevant anions such as bicarbonate was characterized more recently and there are examples of systems promoting phosphate and carboxylate transport. ... 

Table 5. Commercially available ISEs with selectivity for anions...

Little attention has been paid to modulation of enzymic activity by interaction with inorganic ions, with the possible exception of enzymes utilizing and producing PO4. The selectivity for anions by biological systems has been recently reviewed (Wright and Diamond, 1977). 

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