Alkali metal ions selectivity

Lariat crown ethers with two terminal pyrenyl sidearms connected to the same carbon 103 (/= 0, 1 m = 0-2 n = 0-2) or to two different carbon atoms 104 (m = 0-2 n 1,2) and 105 (m 0, 1) showed intramolecular excimer emission in the free state (Jt-Jt-stacking of the pyrene rings), whose intensity decreases with the increase of monomer emission intensity upon metal ion complexation <20020L2641, 2004JOC4403>. This response has been ascribed to the cooperative participation of one of the two sidearms in the complexation of the crown ring with the metal ion, which renders inoperative the Jt-Jt-stacking of aromatic rings. Most of these fluorophores show alkaline earth over alkali metal ion selectivities. 

The three aplasmomycins (12-14) were compared in terms of antibacterial activity (Table 6), alkali metal ion selectivity (Table 7) and transport ability. The results summarized below show that the antibacterial activity of aplasmomycin B (13) was nearly equal to that of aplasmomycin, while aplasmomycin C (14) showed a weaker activity. It was also noted that the ability to form complexes with other metals does not directly correspond with antibacterial activity. Cation selectivity decreased in the order Rb > K > Cs = Na > Li. The three aplasmomycins did not show any affinity towards divalent cations. 

There is a wide variety of ion-specific electrodes currently available and it would appear that new techniques will develop rapidly in the future. A promising area is the synthesis of series of cyclic polyethers by Pedersen (1967). These selectively bind alkali metal ions (Izatt et al, 1969). Such compounds hold promise for sensitive alkali-metal-ion-selective electrode membranes, and may well become the basis of some of the commercially available proprietary electrodes. 

Pseudocrown ethers, whose structures are maintained by coordination bonds instead of covalent bonds like typical crown ethers, are among the most suitable candidates for allosteric regulation of ion binding. A linear podand 2 possessing bipyridine moieties at the ends of the polyether chain was converted easily to the corresponding pseudocrown ether quantitatively by complexation with Cu+ (Scheme 1.1). The pseudocrown ether shows a positive allosteric effect on alkali metal ion selectivity in ion transport. The drastic conformational change from a linear to cyclic structure results in a significant macrocyclic effect favorable for ion selectivity. 

AMP-1 4.0 Microcrystalline ammonium molybdo-phosphate with cation exchange capacity of 1.2 mequiv/g. Selectively adsorbs larger alkali metal ions from smaller alkali metal ions, particularly cesium. 

Table 7. Selectivity orders for transport of alkali metal ions into toluene by crown ether carboxylic acids for several separation techniques...

Tabushi, I. Yamamura, K. Water Soluble Cyclophanes as Hosts and Catalysts, 113,145-182 (1983). Takagi, M., and Ueno, K. Crown Compounds as Alkali and Alkaline Earth Metal Ion Selective Chromogenic Reagents. 121, 39-65 (1984). 

Crown compounds as alkali and alkaline earth metal ion selective chromogenic reagents. M. Takagi and K. Ueno, Top. Curr. Chem., 1984,121, 39-65 (37),... 

The crown ether here was named by its decoverer Pedersen51 dicyclohexyl-18-crown-6(18 = number of atoms in the heterocyclic ring, 6 = number of oxygen atoms in the ring) its membrane shows an appreciably higher K+ selectivity with respect to the other alkali metal ions. There is still much research being carried out on the synthesis and practical use of crown ethers. 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

Other coordination modes of trans-diammac have been identified where one (154) or both (155) primary amines are free from the metal.721 725 An extension of this concept involves attachment of active functional groups such as crown ethers selectively at one primary amine to generate ditopic ligands capable of electrochemically sensing alkali metal ions through their inductive effect on the Co11111 redox potential. One example is provided by (156) further, the 15-crown-5 and 18-crown-6 analogs were also prepared.726... 

Pratt, L. R., Rempe, S. B., Topol, I. A., and Burt, S. K. (2000). Alkali metal ion hydration and energetics of selectivity by ion-channels. Biophys.J. 78, P2057-P2057. 

Polymer (184) has a network structure and was obtained by reaction of dibenzo-18-crown-6 with formaldehyde in formic acid. Amongst the alkali metal ions, it selectively captures K+ and Cs+ from methanol or methanol/water. A related polymeric product has been reported (as a gel) from the reaction of this crown with formaldehyde in chloroform using sulfuric acid as catalyst (Davydova, Baravanov, Apymova Prata, 1975). 

In general, the cryptands (213) show a stronger correlation between thermodynamic stability and match of the metal ion for the cavity. Thermodynamic data for complexation of the alkali metal ions with a number of cryptands is summarized in Table 6.2. The data for the smaller (less flexible) cryptands 2.1.1, 2.2.1, and 2.2.2 illustrate well the occurrence of peak selectivity. 

K+ channels selectively transport K+ across membranes, hyperpolarize cells, set membrane potentials and control the duration of action potentials, among a myriad of other functions. They use diverse forms of gating, but they all have very similar ion permeabilities. All K+ channels show a selectivity sequence of K+ Rb+ > Cs+, whereas the transport of the smallest alkali metal ions Na+ and Li+ is very slow—typically the permeability for K+ is at least 104 that of Na+. The determination of the X-ray structure of the K+-ion channel has allowed us to understand how it selectively filters completely dehydrated K+ ions, but not the smaller Na+ ions. Not only does this molecular filter select the ions to be transported, but also the electrostatic repulsion between K+ ions, which pass through this molecular filter in Indian file, provides the force to drive the K+ ions rapidly through the channel at a rate of 107-108 per second. (Reviewed in Doyle et al., 1998 MacKinnon, 2004.)... 

E-3 (Figure 10.26) is the first example of an ionophoric calixarene with appended fluorophores, demonstrating the interest in this new class of fluorescent sensors. The lower rim contains two pyrene units that can form excimers in the absence of cation. Addition of alkali metal ions affects the monomer versus excimer emission. According to the same principle, E-4 was designed for the recognition of Na+ the Na+/K+ selectivity, as measured by the ratio of stability constants of the complexes, was indeed found to be 154, while the affinity for Li+ was too low to be determined. 

The discovery of crown ethers and cryptands in the late sixties opened new possibilities of cation recognition with improvement of selectivity, especially for alkali metal ions for which there is a lack of selective chelators. Then, the idea of coupling these ionophores to chromophores or fluorophores, leading to so-called chromoionophores and fluoroionophores, respectively, emerged some years later l9) As only fluorescent probes are considered in this chapter, chromoionophores will not be described. 

A different direction in ion-selective electrode research is based on experiments with antibiotics that uncouple oxidative phosphorylation in mitochondria [59]. These substances act as ion carriers (ionophores) and produce ion-specific potentials at bilayer lipid membranes [72]. This function led Stefanac and Simon to obtain a new type of ion-selective electrode for alkali metal ions [92] and is also important in supporting the chemi-osmotic theory of oxidative phosphorylation [69]. The range of ionophores, in view of their selectivity for other ions, was broadened by new synthetic substances [1,61]. 

The 2-(AuC1)4 and 2-(PtCl2SMe2)4 complexes (see above), show extractability properties vs. alkali metal ions, with a greater affinity for than for other alkali metal ions [48]. No structural data were available and the nature of the binding in the formation of these complexes was not investigated. Similarly, the anionic complexes [2-Cu4(/t-Cl)4(/t3-Cl)] and [2-Ag4(/t-Cl)4(/t4-Cl)] have been shown to act as host for the selective binding of alkali metal cations and divalent metal ions like or Pb. Both complexes... 

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