Structures anion selectivity

The transmembrane potential derived from a concentration gradient is calculable by means of the Nemst equation. If K+ were the only permeable ion then the membrane potential would be given by Eq. 1. With an ion activity (concentration) gradient for K+ of 10 1 from one side to the other of the membrane at 20 °C, the membrane potential that develops on addition of Valinomycin approaches a limiting value of 58 mV87). This is what is calculated from Eq. 1 and indicates that cation over anion selectivity is essentially total. As the conformation of Valinomycin in nonpolar solvents in the absence of cation is similar to that of the cation complex 105), it is quite understandable that anions have no location for interaction. One could with the Valinomycin structure construct a conformation in which a polar core were formed with six peptide N—H moieties directed inward in place of the C—O moieties but... 

Dutzler R, Campbell EB, Cadene M et al (2002) X-ray structure of the C1C chloride channel at 3.0 A resolution molecular basis of anion selectivity. Nature 415 287—294... 

FIGURE 5-14 Structures of some chemical species useful for designing anion-selective electrodes (a) Mn(III) porphyrin (b) vitamin Bi2 derivative (c) tri-n-octyltin chloride (d) lipophilic polyamine macrocyclic compound. 

Typically, functional porins are homotrimers, which assemble from monomers and then integrate into the outer membrane. The general porins, water-filled diffusion pores, allow the passage of hydrophilic molecules up to a size of approximately 600 Daltons. They do not show particular substrate specificity, but display some selectivity for either anions or cations, and some discrimination with respect to the size of the solutes. The first published crystal structure of a bacterial porin was that of R. capsulatus [48]. Together with the atomic structures of two proteins from E. coli, the phosphate limitation-induced anion-selective PhoE porin and the osmotically regulated cation-selective OmpF porin, a common scheme was found [49]. Each monomer consists of 16 (3-strands spanning the outer membrane and forming a barrel-like structure. 

Vancomycin has been used widely for enantiomeric resolution as it is very effective for the enantiorecognition of anionic compounds, particularly those containing carboxylic groups in their structure. The selectivity toward these... 

In this paper, we report the development of ISEs that have been designed by using molecular recognition principles. Specific examples include the development of polymer membrane anion-selective electrodes based on hydrophobic vitamin B12 derivatives and a cobalt porphyrin. The selectivity patterns observed with these electrodes can be related to differences in the structure of the various ionophores, and to properties of the polymer film. 

In summary, it has been demonstrated that ISEs can be designed by employing molecular recognition principles. In particular, the feasibility of using hydrophobic vitamin B12 derivatives and electropolymerized porphyrin films in the development of polymer membrane anion-selective electrodes has been demonstrated. The studies indicated that the changes in the selectivity of these ISEs can be explained by the difference in structure of the ionophores. In addition, it was shown that by electropolymerization of a cobalt porphyrin, anion-selective electrodes can be prepared that have extended lifetimes compared with PVC-based ISEs, which use a similar compound as the ionophore. 

Zeth, K., Diederichs, K., Welte, W., and Engelhardt, H. (2000). Crystal structure of Omp32, the anion-selective porin from Comamonas acidovorans, in complex with a periplasmic peptide at 2.1 A resolution. Structures, 981-992. 

Scheme 2 Structures of selected anionic copper halides...

The reaction can be conducted either in THF or in an aromatic solvent, where it requires the presence of one equivalent of crown ether. The crown ether adduct of the product 4a was subjected to X-ray diffraction analysis and represents the first fully characterized vinyloligosilyl anion [6]. The crystal structure and selected bond lengths and bond angles are given in Fig. 1 and Table 1, respectively. 

Multiple metal ion coordination of anions, or cascade binding, has been an area of intense recent investigation for coordination chemists. The role of the metals in these receptors is threefold. They provide the source of interaction with anions through the formation of coordinate bonds. They fulfill a structural role, the separation of the metal ions being used to impart anion selectivity or different modes of anion binding, and in some cases they have taken a functional role. In particular, the use of this type of receptor for biocatalytic mimicking and rate enhancement has already been established and could prove particularly fruitful. 

Cobaltocenium based systems have therefore proved very versatile in the field of anion coordination chemistry. The role of the cobaltocenium unit is to enhance interaction with the bound anion and to function as a sensing unit. The structure of the receptor can then be controlled by simple organic manipulations of the Cp groups. In this manner, novel anion selectivity can readily be incorporated into these receptors. 

Lewis acidic hosts (Section V.A) illustrated important theoretical concepts such as the chelate effect and binding cooperativity, which have now been shown to exist for anion as well as cation binding. This work has also resulted in the crystallographic determination of eye-catching solid state receptor-anion complexes, while heteroelement NMR has allowed an accurate means of probing the solution phase structure of these complexes. Already, multinuclear tin systems are being built into functioning anion selective electrodes. 

It is the aim of this chapter is to present the efforts made worldwide for the development of chemical sensors based on the unique chemical recognition capabilities of organotin structures. In particular, we will examine in a time-based flowchart the progress of the design and application of Sn(IV)-based ionophores and their application in the development of anion selective chemical potentiometric sensors. 

From all anion selective channels, the Cl channel (CLC) family is of key importance, as the Gl ion is the most abundant physiological anion. In general, similar to the K" " channels, the CLCs oppose excitability and help stabilize the resting membrane potential. There are three major groups of the CLC family with diverse structures and functions ... 

Fig. 12.17 The structures of selected borate anions trigonal planar and tetrahedral B atoms are present, and each tetrahedral B carries a negative charge. The [B405(011)4] anion occurs in the minerals borax and kernite. In the pyroborate ion, [B205]", the B—O—B bond angle depends on the cation present, e.g. ZB—O—B = 153° in C02B2O5, and 131.5° in Mg2B205.

Table 16.4 Structures of selected interhalogens and derived anions and cations. Each is consistent with VSEPR theory.

The combination of piperazine with trimesoyl chloride in composite membrane form was named NS-300. Trimesoyl chloride leads to a crosslinked polyamide structure. Apparently, however, considerable formation of hydrolyzed carboxylate groups also occurs. This is evidenced by the anion selectivity of the membrane, demonstrated by the sequential salt rejection data in Table 5.4 for a series of salts on test with a single set of membrane specimens. 

---