Ionophore ligands

Fig i Mechanisms of ion-selective optode response in aqueous samples containing a cationic analytes, M+, or b anionic analytes, X-. L represents the ionophore ligand, C the chromoionophore and Y" and Z+ the corresponding ionic additives (counterions)... 

The avermectins have been shown to increase chloride conductance in invertebrate electrophysiological preparations (J, JA,25) and modulate the binding of several GABA receptor-ionophore ligands (9, 10), but the molecular mechanisms underlying their neurotoxicity remain poorly defined. Recently, Abalis e t al. (1 ) reported an avermectin Independent stimulation of chloride uptake in rat brain vesicles tnat... 

Table 3. Ion specific ratios for ionophoric ligands determined in several solvents... 

Table 5. Stability constants of complex formation between murexide and several open chain ionophoric ligands with alkali cations...

Fig. 12. Model representation of the loading of a macrocyclic ionophore (monactin) with Na+. The solvated metal ion approaches the carrier, the conformation of which permits its polar groups to be in close contact with the solvent. The uptake of the metal ion then occurs under stepwise substitution of the solvent molecules by the polar groups of the ionophoric ligand. In its final conformational state the carrier completely encloses the metal ion with its non-polar groups being accumulated at the surface...

Figure 10 Monovalent ion coordination to monensin and narasin. (a) Structures of the ionophore ligands and an X-ray image of the sodium complex of monensin. (b) Solution conformations for the Na, K+, and Rb bound form of monensin. The terminal ligand changes to accommodate larger ligands, (c) Solution conformations for the Na+, K+, and Rb+ bound form of narasin. The ligands stay the same, but the core expands to accommodate the larger ions. Carboxylate binding is not observed for either monensin or narasin (reproduced with permission from Martinek et a/. ).

Synthetic ionophore ligands, mostly based on cyclic and branched polyethers, have been widely used in the last decade to study carrier facilitated cation transport across membranes. It has recently been observed that the organometallic ligand (C H )Co[P0(0C2H )2]2 Iso has... 

At one extreme, a derivative free of coordination by water or anions has been prepared by the reaction of europium metal with NOBF4 in dry acetonitrile (Albin et al., 1983). The resulting Eu(CH3CN) (Y) species (Y = BF4, PF ) reacts further with two equivalent ionophoric ligands to give bis(ligand) complexes. 

Heterocycles as ligands in ionophores for potentiometric and optical sensors 98CRV1593. 

In this review, recent development of active transport of ions accross the liquid membranes using the synthetic ionophores such as crown ethers and other acyclic ligands, which selectively complex with cations based on the ion-dipole interaction, was surveyed,... 

Lee, S. S. Yoon, I. Park, K.-M. Jung, J. H. Lindoy, L. F. Nezhadali, A. Rounaghi, G. Competitive bulk membrane transport and solvent extraction of transition and post transition metal ions using mixed-donor acyclic ligands as ionophores. J. Chem. Soc.-Dalton Trans. 2002, 2180-2184. 

Lachowicz, E., Rozanska, B., Teixidor, F., Meliani, H., Barboiu, M. and Hovnanian, N. (2002) Comparison of sulphur and sulphur-oxygen ligands as ionophores for liquid-liquid extraction and facilitated transport. Journal of Membrane Science, 210, 279—290. 

Polymers with low glass transition do not require plasticizers. However, these compounds are often unpolar (Table 4) and, consequently, they are unsatisfactory solvents for polar ligands, ionophores, dyes and analytes. 

Most indicator chemistry is adapted to aqueous solution (for titration in water). Therefore, the molecules are water-soluble and if dissolved in lipophilic polymers, they are washed out immediately. In order to make dyes, ionophores and ligands soluble in polymers and to avoid leaching of the components into the sample solution, they have to be made lipophilic6. 

L neutral ligand C neutral chromo-ionophore R- lipophilic anionic site I analyte cation... 

L neutral ligand C- charged chromo-ionophore R+ lipophilic cationic site X- analyte anion... 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

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