Anion binding affinities

Figure 13.9 Electrochemical anion sensing by a dithiol SAM. Anion binding results in predictable cathodic perturbation of the SAM voltammetry. Anion binding affinities associated with such SAMs are, in some cases, orders of magnitude greater than those associated with the same receptors free in solution (reproduced by permission of The Royal Society of Chemistry).

Tongraung, Chantarasiri, and Tuntulani have reported two calix[4]arenes 15 and 16, one with urea appendages and a second with a crown strap in addition to the urea arms. These systems were found to bind tetrabutylammonium dihydrogen phosphate with a K, > 200 M 1 in DMSO-d6. In line with the authors expectations, the anion-binding affinities were then found to be enhanced upon the addition of sodium cation. For instance, in the presence of this cation, the association constant for calix[4]arene 16 increased to Ka = 1030 M-1 (as the PF6 salt DMSO-d6 solution) [55]. 

Reyheller C, Hay BP, Kubik S (2007) Influence of linker structure on the anion binding affinity of biscyclopeptides. New J Chem 31 2095-2102... 

The tren-based heteroditopic receptor 22, featuring a tripodal tetrahedral amide hydrogen-bond anion-recognition site in combination with benzo-15-crown-5 ether cation-binding moieties, has been found to cooperatively bind chloride, iodide, and perrhenate anions via co-bound crown ether-complexed sodium cations. It also can efficiently extract the radioactive sodium pertechnetate from simulated aqueous nuclear waste streams. The anion-binding affinity of 22 is considerably reduced in the absence of a co-bound cation <1999CC1253>... 

Good fluoride anion binding affinities were observed in the case of the two hexapyrrolic calix[4]pyrrole receptors 9 reported in 2014 by Anzenbacher and coworkers [24]. The incorporation of two additional carbonyl ester pyrrole units on either side of the macrocycle produced the cis- and traws-configurational isomers, 9a and 9b, respectively (cf. Fig. 12.7). In acetonitrile solution, a value of 1.0 X 10 for fluoride anion binding was estimated for both 9a and 9b on the... 

Frank M, Dieterich JM, Freye S, Mata RA, Clever GH (2013) Relative anion binding affinity in a series of interpenetrated coordination cages. Dalton Trans 42(45) 15906-15910... 

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). 

Fig. 11a,b. Binding affinities of IgG, IgM, and the antibody dendrimer (Gl) with the cationic porphyrin (TMPyP) (a) and those with the anionic porphyrin (TCPP) (b) estimated by ELISA... 

Other Ion Affinities Binding affinities for many different types of ions to neutrals are defined analogously to hydride affinity, as the 298 K enthalpy required to dissociate the complexed species. The ion can be a cation or an anion. Conversely, ion affinities can be described in terms of the dissociation. 

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