Binding anion

Anions have high free energies of solvation compared to cations of similar size and hence hosts for anions experience more competition from the surrounding medium. For example, the standard free energies of hydration, AG ydr , for F and K+ are -465 and -295 kj mol respectively. 

Generally, most anions exist in a narrow pH window. This can be problematic for hosts that contain, for example, polyammonium functionalities within the receptor, as the host may not be fully protonated in the pH range where the anion is deprotonated. 

Many anions are generally coordinatively saturated and they only bind via weak interactions, such as hydrogen bonding and van der Waals interactions. 

Bowman-James, K., Alfred Werner revisited The coordination chemistry of anions, Acc. Chem. Res., 2005, 38, 671-678. 

Early work showed that electrostatic interactions to a tetrahedral tertiary ammonium cryptand 2.44 result in a very effective host for iodide. This work was later extended to a series of zwitterionic, overall neutral cryptands (Section 2.7), containing either BH3 (as in 2.45) or a pendant carboxylate functionality. 

Much like cations, oxyanions adsorb strongly onto oxides, usually by a ligand exchange mechanism (Parfitt 1979 Sposito 1984 McBride 1999 Sparks 2002), which can be represented as follows 

The preceding examples consistently show that oxyanion adsorption on common oxides (of Al, Fe, Mn, Ti) leads to strong specific binding, commonly through bidentate covalent complexes and in some cases with monodentate binding as well. 

Among simple, non-O-containing anions, fluoride is one of the most active and important, and the adsorption of F has been investigated due to its environmental importance (Fan, Paiker, and Smith 2003 Mohapatra et al. 2009 and references therein). Fluoride has been found to enter the substrate crystalline lattice by solid diffusion and/or ion exchange mechanism (Fan, Paiker, and Smith 2003). 

FIGURE 9.21 Molecular models employed in the theoretical analysis of the interaction of chromate anions with ferrihydrite. Protonated (top) and nonprotonated (bottom) cluster models used to calculate infrared frequencies of chromate coordinated to iron oxide outer-sphere (a, d) monodentate (b, e) and bidentate (c, f). Some explicit water molecules were removed for clarity. Black, dark gray, light gray, and white spheres represent O, Fe, Cr, and H atoms, respectively. (Reprinted with permission from Johnston and Chrysochoou 2012, 5851-5858. Copyright 2011 American Chemical Society.) 

Amonette, J. E. 2002. Methods for determination of mineralogy and environmental availability. In Soil Mineralogy with Environmental Applications, ed. J. B. Dixon and D. G. Schulze, 153-197. Soil Science Society of America Book Series 7. Madison, WI Soil Science Society of America. 

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HairC are. Quaternary ammonium compounds are the active ingredients in hair conditioners (31—39,239). Quaternaries are highly substantive to human hair because the hair fiber has anionic binding sites at normal pH ranges. The use of quaternaries as hair conditioners can be broken down into creme rinses and shampoo conditioners. 

Anion binding with polyazamacroheterocyclic cations and neutral S,N- and B,0-macroheterocycles 99CLY546. 

Application of Cation and Anion Binding Properties of Macrocyclic Polyamines... 

Recently considerable attention has been directed at anion binding ligands. Macrobicyclic 27 29) and macrotricyclic amines 30,31) were topologically designed to host anions such as spherical Cl-, linear Nf 32). These anion substrates are incorporated into macrocyclic cavities lined with appropriate anion-binding sites capable of forming hydrogen bonds like those of protonated amines (see /, below). 

If electrostatic interactions play a major role in the strength of anion binding, then it is natural to expect that the higher the polyammonium cation charge is the stronger the affinity for anions will be. This was found to be case with [24]aneN6 and [32]aneNg which possess 6 and 8 protons, respectively, at pH 7 (see Table 4)46). 

In connection with the anion binding ligands, it should be added that tetraprotonated [18]aneN6 -4H+ in aqueous solutions interacts with the monoanions Cl- and NOf having log of 1.8 and 2.3 53). Interestingly, the anion binding was found... 

In addition to [18]aneN6, a newly synthesized [18]aneN6-hexaacetic acid (X) has also been tested 34,35). (X) was originally designed to combine the anion-binding function of [18]aneN6 with the cation-binding function of EDTA for more efficient dissolution of calculi. 

Chemical transformations at the macroeyclic chromophorc of expanded porphyrins are still not known. The complexation behavior of expanded porphyrins is very different from that of nonexpanded porphinoid macrocycles. The coordination hole of the expanded porphyrins is often too big for the complexation of a single metal ion, so in fact two metal ions can be chelated. With some expanded porphyrins, anion binding is observable, a striking difference to the nonexpanded porphyrins. The complexation behavior and the host-guest chemistry of expanded porphyrins is a rapidly growing field of research. The work in this field has been reviewed. Ie f... 

This scheme shows that cyanide sourced from smoking or otherwise (see 6.3.7) may determine the metabolism of chrysotherapeutic agents. [Au(CN)2] anions bind to serum albumin predominantly by the formation of adducts without the displacement of cyanide [94]. The ions bind tightly to albumin independent of the oxidative state of Cys-34. The equilibrium constant values for [Au(CN)2] binding to serum albumin are similar to values for other gold complexes that bind to albumin. This indicates that albumin can act as a carrier for transporting [Au(CN)2] in the bloodstream. 

Bivalent inhibitors of thrombin have been synthesized to bind the anion-binding exosite and active (catalytic) site of thrombin simultaneously. By coupling the carboxy terminal fragment of hirudin to a tripeptide (D-Phe-Pro-Arg) by including a spacer molecule, both the anion exosite and the catalytic site are blocked. An example of such a molecule is Hirulog, which has 20 amino acids and has a Kj of 2 nM (61). Its ability to block the active site has been questioned, since thrombin has been shown to cleave the Arg-Pro bond of Hirulog slowly in vivo (58). In addition to hirudin and hirudin-like compounds, three other classes of site-directed thrombin inhibitors deserve mention. 

For (184)—(186), since these are neutral ligands the uptake of a metal cation is also accompanied by anion binding such that electro-neutrality is maintained. Thus resins of this type may sometimes be useful for separation of either cations or anions. 

Fig. 4. Alignment of amino acid residues surrounding the binding sites of iGluRl-7. Function lock, interdomain hydrogen bond Pkt, amphiphilic or hydrophobic residues bordering the pocket -NH3+, COO, and distal anion bind the respective charged moieties of glutamate Wat, binds to a binding-site water molecule in iGluR2.

Beer, Paul D. and Smith, David K., Anion Binding and Recognition by Inorganic... 

Animal uptake, of herbicides, 23 310 Animal viruses, 3 135-136 Animal waxes, 26 203, 206 Anion binding, in supramolecular chemistry, 24 43-47 Anion-exchange membranes, 15 836 Anion exchangers, organic fouling of, 24 416... 

See also Supramolecular architectures Supramolecular assemblies anion binding in, 24 43-47 binding neutral molecules in, 24 47-49 bottom-up nanoscale fabrication in, 24 61 cation binding in, 24 40-43 current and anticipated applications for, 24 52... 

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