Ion carrier

Quite recently, Okahara and his coworkers have extended their method to the formation of long chain N-alkylmonoazacrowns. It was expected that such compounds as N-decylmonoaza-18-crown-6 may be useful as new surfactants with complexing ability with metal salts, phase transfer catalysts and selective ion carriers . ... 

Recently, Shinkai and Manabe achieved the active transport of K+ using a new type of carrier 39 derived from diaza crown ether43, 44). The ionophore forms the zwitter-ionic species 39b, which is most lipophilic among other species (39a, 39c), at about neutral pH region, and it acts as effective ion carrier in the active transport... 

FIG. 17 Schematic illustration of the setup for a tip-dip experiment. First glycerol dialkyl nonitol tetraether lipid (GDNT) monolayers are compressed to the desired surface pressure (measured by a Wilhehny plate system). Subsequently a small patch of the monolayer is clamped by a glass micropipette and the S-layer protein is recrystallized. The lower picture shows the S-layer/GDNT membrane on the tip of the glass micropipette in more detail. The basic circuit for measurement of the electric features of the membrane and the current mediated by a hypothetical ion carrier is shown in the upper part of the schematic drawing. 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

It is now recognised that a wide range of organic molecules, collectively termed ionophores 185,186) or complexones 187), are able to facilitate ion (usually cation) transport. Two major mechanisms have been revealed for this process, namely the involvement of transmembrane ion carriers and transmembrane pores or channels (see Fig. 19). The majority of ionophores studied to date are natural antibiotics and their synthetic analogues which are, on a biological scale, comparatively small molecules lending themselves to study outside the biological system. In contrast far less is known about the molecular structures involved in normal transport processes. Such molecules are likely to be more complex or present in small amounts and may require... 

Fig. 19. Representation of the movement of metal ions (M + ) across a bilayer involving an ion carrier and a transmembrane channel...

Shamsipur, M. Mashhadizadeh, M. H. Highly efficient and selective membrane transport of silver(I) using hexathia-18-crown-6 as a specific ion carrier. Sep. Purif. Technol. 2000, 20, 147-153. 

It appears that Charlton et al.94,95 have discovered the first methods for reversible and continuous optical measurement of the clinically highly important alkali and earth alkali ions. In one approach94 they use plasticized poly(vinyl chloride) along with valinomycin as the ion carrier, and a detection scheme that was later refered to as co-extraction. In their system, potassium ion is extracted into plasticized PVC, and the same quantity of the anionic red dye erythrosine is co-extracted into it. The extracted erythrosine is quantified via absorbance or reflectance. 

Solvent polymeric membranes, conventionally prepared from a polymer that is highly plasticized with lipophilic organic esters or ethers, are the scope of the present chapter. Such membranes commonly contain various constituents such as an ionophore (or ion carrier), a highly selective complexing agent, and ionic additives (ion exchangers and lipophilic salts). The variety and chemical versatility of the available membrane components allow one to tune the membrane properties, ensuring the desired analytical characteristics. 

The dynamic pH response range of a membrane-based pH electrode can be tailored by the incorporation of different functional groups in the ion carriers. Yuan et al. [68]... 

With the more conductive liquids, the ion concentration becomes so great that ion concentration fluctuations on a statistical basis are likely to be small. However, charging can take place by three other mechanisms (1) mechanical disruption of any double layer of ions that may exist at the surface in times that are short compared with the relaxation time, with a predominance of the surface ions going to the portion of fluid coming from the surface (2) unequal ion mobility with the larger ions unable to return to the bulk of liquid as readily as the smaller and more mobile ones and (3) contaminating materials, such as dust or surfactants at the interfaces serving as ion carriers into one portion or the other of the ruptured liquid. 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

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]. 

This chapter is concerned with processes that lead to formation of ISE membrane potentials. The membrane potentials of electrodes with liquid membranes containing a dissolved ion-exchanger ion or a dissolved ionophore (ion carrier), and of electrodes with solid or glassy membranes will be considered. More complicated systems, for example ISEs with a gas gap and enzyme electrodes, will be discussed in chapters 4 and 9. 

This group of ISEs is based on the ion-selective character of the distribution equilibrium between water and the membrane phase. As was demonstrated in chapter 3, this ion-selectivity may be affected if an ion pair is formed in the membrane (section 3.2) and increased markedly if complexes are formed in the membrane between the test ion and special complexing agents, ion carriers or ionophores (section 3.3). 

In contrast to ISEs with neutral ion carriers in the membrane, not even qualitative rules have been formulated for the solvent effect on the behaviour of ISEs with ion-exchanger ions in a liquid membrane. A basic condition for the ion-exchanger ions is that they be strongly hydrophobic. It must hold for the standard Gibbs energy of transfer of the ion-exchanger ion X and the deter-minand Y that... 

A great number of ligands, such as the anions of ethylenediamine-NNN N -tetra-acetic acid (EDTA), described in detail by Schwarzen-bach and his school (29, 30), show a pronounced selectivity for alkaline earth and other metal cations (30). Because of the limited lipid solubility of these ligands and their complexes, such compounds are, however, not suited as ion carriers in lipophilic membranes (Fig. 2). The ability... 

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